U.S. patent application number 13/355120 was filed with the patent office on 2012-05-10 for microcoil magnetic resonance detectors.
Invention is credited to Victor Esch, Eiichi Fukushima, Andrew F. McDowell, Meghan Norvell, Laurel Sillerud.
Application Number | 20120112744 13/355120 |
Document ID | / |
Family ID | 40344590 |
Filed Date | 2012-05-10 |
United States Patent
Application |
20120112744 |
Kind Code |
A1 |
McDowell; Andrew F. ; et
al. |
May 10, 2012 |
MICROCOIL MAGNETIC RESONANCE DETECTORS
Abstract
The present invention provides microcoil magnetic resonance
based modules, detection devices, and methods for their use.
Inventors: |
McDowell; Andrew F.;
(Albuquerque, NM) ; Fukushima; Eiichi;
(Albuquerque, NM) ; Esch; Victor; (Albuquerque,
NM) ; Norvell; Meghan; (Albuquerque, NM) ;
Sillerud; Laurel; (Albuquerque, NM) |
Family ID: |
40344590 |
Appl. No.: |
13/355120 |
Filed: |
January 20, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12257238 |
Oct 23, 2008 |
8143896 |
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13355120 |
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60981948 |
Oct 23, 2007 |
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61099975 |
Sep 25, 2008 |
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Current U.S.
Class: |
324/309 |
Current CPC
Class: |
G01R 33/4808 20130101;
G01R 33/465 20130101; G01N 24/088 20130101; G01N 33/54333 20130101;
G01N 24/08 20130101; G01R 33/302 20130101; G01R 33/307
20130101 |
Class at
Publication: |
324/309 |
International
Class: |
G01R 33/465 20060101
G01R033/465 |
Claims
1-32. (canceled)
33. A method for detecting a target entity from a sample, the
method comprising: introducing magnetic beads having a
target-specific binding moiety to a sample comprising a target
entity to form target/bead complexes, wherein the magnetic beads
comprise at least 60% by weight magnetic material and the beads
bind the target in an amount sufficient to allow for specific
detection of the target over background; separating the complexes
from remaining components of the sample; and detecting the target
entity.
34. The method of claim 33, wherein the magnetic material is a type
selected from the group consisting of: ferromagnetic material,
paramagnetic material, ferri-magnetic material, diamagnetic
material, and superparamagnetic material.
35. The method of claim 33, wherein the target-specific binding
moiety is an antibody.
36. The method of claim 33, wherein the magnetic material is
selected from the group consisting of: iron, iron oxide, Fe:Pt,
gadolinium metal, gadolinium oxide, iron nitride, and a combination
thereof.
37. The method of claim 36, wherein the iron oxide is
Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4.
38. The method of claim 36, wherein the iron nitride is
Fe.sub.2N.
39. The method of claim 33, wherein the target entity is a
bacteria.
40. The method of claim 33, wherein the sample is a body fluid.
41. The method of claim 40, wherein the body fluid is selected from
the group consisting of: blood, urine, saliva, semen, vaginal
secretions, tears, amniotic fluid, and cerebral spinal.
42. The method of claim 33, wherein the sample is a swab.
43. The method of claim 33, wherein the swab is a skin swab or
wound swab.
44. A method for detecting a bacteria in a body fluid sample, the
method comprising: introducing magnetic beads having a
bacteria-specific binding moiety to a sample comprising a bacteria
to form bacteria/bead complexes, wherein the magnetic beads
comprise at least 60% by weight magnetic material and the beads
bind the bacteria in an amount sufficient to allow for specific
detection of the bacteria over background; separating the complexes
from remaining components of the sample; and detecting the
bacteria.
45. The method of claim 44, wherein the magnetic material is a type
selected from the group consisting of: ferromagnetic material,
paramagnetic material, ferri-magnetic material, diamagnetic
material, and superparamagnetic material.
46. The method of claim 44, wherein the bacteria-specific binding
moiety is an antibody.
47. The method of claim 44, wherein the magnetic material is
selected from the group consisting of: iron, iron oxide, Fe:Pt,
gadolinium metal, gadolinium oxide, iron nitride, and a combination
thereof.
48. The method of claim 47, wherein the iron oxide is
Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4.
49. The method of claim 47, wherein the iron nitride is
Fe.sub.2N.
50. The method of claim 44, wherein the body fluid is selected from
the group consisting of blood, urine, saliva, semen, vaginal
secretions, tears, amniotic fluid, and cerebral spinal.
51. The method of claim 50, wherein the body fluid is blood.
Description
CROSS REFERENCE
[0001] This application claims priority to U.S. Provisional Patent
Application Ser. Nos. 60/981,948 filed Oct. 23, 2007 and 61/099,975
filed Sep. 25, 2008, both hereby incorporated by reference in their
entirety.
FIELD OF INVENTION
[0002] The present invention relates to magnetic resonance and
detection of labeled targets.
BACKGROUND OF THE INVENTION
[0003] A variety of experiments in Nuclear Magnetic Resonance (NMR)
could benefit from miniaturization of the detector coil. When
samples are mass-limited, reducing the detection volume to match
the sample size offers enhanced Signal-to-Noise-Ratio (SNR)
performance. While some progress has been made in developing
portable microcoil-based NMR systems, devices that provide improved
SNR, throughput, capabilities, and other benefits would be of great
value to the art.
SUMMARY OF THE INVENTION
[0004] In a first aspect, the present invention provides modules,
comprising:
[0005] (a) a microcoil possessing an inner diameter of between 25
microns and 550 microns;
[0006] (b) a conduit disposed proximate to the microcoil, wherein
the conduit is in fluid communication with a sample reservoir;
[0007] (c) an affinity column in fluid communication with the
conduit and the sample reservoir; and;
[0008] (d) a connector for connecting the module to a magnetic
resonance detector.
[0009] In a second aspect, the present invention provides modules
comprising:
[0010] (a) a plurality of microcoils each possessing an inner
diameter of between 25 microns and 550 microns;
[0011] (b) a conduit disposed proximate to each microcoil, wherein
the conduit is in fluid communication with a sample reservoir;
and
[0012] (c) a connector for connecting the module to a magnetic
resonance detector.
[0013] In a third aspect, the present invention provides microcoils
comprising an inner diameter of between 25 microns and 550 microns,
wherein the microcoil is an effective magnetic resonance
transmitter or receiver coil; and wherein the microcoil is within
or surrounds an affinity column.
[0014] In a fourth aspect, the present invention provides detection
devices comprising
[0015] (a) a permanent magnet possessing a field strength of less
than or equal to 4 Tesla;
[0016] (b) a microcoil disposed proximate to a magnetic field
generated by the permanent magnet, wherein the microcoil possesses
an inner diameter of between 25 microns and 550 microns;
[0017] (c) a conduit disposed proximate to the microcoil, wherein
the conduit is in fluid communication with a sample reservoir;
[0018] (d) an affinity column in fluid communication with the
conduit and the sample reservoir.
[0019] In a fifth aspect, the present invention provides detection
devices comprising
[0020] (a) a permanent magnet possessing a field strength of less
than or equal to 4 Tesla;
[0021] (b) a plurality of microcoils each possessing an inner
diameter of between 25 microns and 550 microns, wherein each
microcoils is an effective magnetic resonance transmitter or
receiver coil; and
[0022] (c) a conduit disposed proximate to each microcoil, wherein
the conduit is in fluid communication with a sample reservoir.
[0023] In a sixth aspect, the present invention provides magnetic
resonance detectors, comprising
[0024] (a) a housing comprising a conduit guide;
[0025] (b) a permanent magnet possessing a field strength of less
than or equal to 4 Tesla within the housing; and
[0026] (b) the module of any embodiment of any of aspect of the
invention, wherein the connector connects the module to the housing
via the conduit guide.
[0027] In a seventh aspect, the present invention provides methods
for detecting a target is a sample fluid, comprising:
[0028] (a) flowing a fluid containing one or more magnetically
labeled targets through the conduit of the detection device of any
embodiment of any aspect of the invention;
[0029] (b) energizing the microcoil at a frequency that permits
detection of a magnetic resonance within the sample fluid; and
[0030] (c) processing a signal received from the microcoil to
detect the magnetically labeled targets in the sample fluid.
[0031] In an eighth aspect, the present invention provides methods
for detecting a target in a sample fluid, comprising:
[0032] (a) introducing a sample fluid into a sample reservoir,
wherein the sample reservoir is in fluid communication with a
conduit;
[0033] (b) flowing the sample fluid into an affinity column in the
conduit, wherein the affinity column comprises one or more capture
agents that bind to one or more targets of interest;
[0034] (c) flowing a fluid comprising magnetic particles into the
affinity column, wherein the magnetic particles are capable of
binding selectively to the one or more targets of interest bound to
the affinity column via the one or more capture agents, and wherein
binding of the magnetic particles to the targets produces magnetic
particle-target complexes;
[0035] (d) washing the affinity column to reduce the number unbound
magnetic particles;
[0036] (e) eluting bound magnetic particle-target complexes from
the affinity column;
[0037] (f) flowing the fluid comprising the magnetic
particle-target complexes through a conduit disposed proximate to a
microcoil, wherein the microcoil possesses an inner diameter of
between 25 microns and 550 microns, wherein the microcoil is an
effective magnetic resonance transmitter or receiver coil; and
[0038] (g) energizing the microcoil at a frequency that permits
detection of a magnetic resonance within the sample fluid; and
[0039] (h) processing a signal received from the microcoil to
detect magnetic particle-target complexes in the sample fluid.
[0040] In a ninth aspect, the present invention provides methods
for detecting a target in a sample fluid, comprising:
[0041] (a) introducing a sample fluid into a sample reservoir,
wherein the sample reservoir is in fluid communication with a
conduit, wherein the conduit is disposed proximate to a microcoil,
wherein the microcoil possesses an inner diameter of between 25
microns and 550 microns, wherein the microcoil is an effective
magnetic resonance transmitter or receiver coil;
[0042] (b) flowing the sample fluid into an affinity column in the
conduit, wherein the affinity column comprises two or more layers,
wherein each layer comprises one or more capture agents that bind
to one or more targets of interest in the sample fluid, wherein
each layer in the affinity column is capable of binding to
molecules distinct from other layers; and wherein the affinity
column is located at least partially within the microcoil;
[0043] (c) flowing a fluid comprising magnetic particles into the
affinity column, wherein the magnetic particles are capable of
binding selectively to the one or more targets of interest, and
wherein binding of the magnetic particles to the targets produces
magnetic particle-target complexes;
[0044] (d) energizing the microcoil at a frequency that permits
detection of a magnetic resonance within the affinity column;
and
[0045] (e) processing a signal received from the microcoil to
detect magnetic particle-target complexes in one or more layers of
the affinity column.
[0046] In a tenth aspect, the present invention comprises methods
for detecting a sample in a target fluid, comprising:
[0047] (a) mixing a sample fluid with magnetic particles capable of
binding to the one or more targets of interest in the sample fluid,
wherein binding of magnetic particles to targets produces magnetic
particle-target complexes
[0048] (b) introducing the sample fluid into a sample reservoir,
wherein the sample reservoir is in fluid communication with a
conduit, wherein the conduit is disposed proximate to a microcoil,
wherein the microcoil possesses an inner diameter of between 25
microns and 550 microns;
[0049] (c) flowing the sample fluid into an affinity column in the
conduit, wherein the affinity column comprises two or more layers,
wherein each layer comprises one or more capture agents that bind
to one or more targets of interest, wherein each layer in the
affinity column is capable of binding to molecules distinct from
other layers; and wherein the affinity column is located at least
partially within the microcoil;
[0050] (d) energizing the microcoil at a frequency that permits
detection of a magnetic resonance within the affinity column;
and
[0051] (e) processing a signal received from the microcoil to
detect magnetic particle-target complexes in one or more layers of
the affinity column.
[0052] In an eleventh aspect, the present invention provides
methods for detecting a target in a sample fluid, comprising:
[0053] (a) introducing a sample fluid into a sample reservoir,
wherein the sample reservoir is in fluid communication with a
conduit, wherein the sample fluid comprises magnetic particles
capable of binding selectively to one or more targets of interest
in the sample fluid, and wherein binding of magnetic particles to
targets produces magnetic particle-target complexes;
[0054] (b) flowing the sample fluid through the conduit disposed
proximate to a microcoil, wherein the microcoil possesses an inner
diameter of between 25 microns and 550 microns;
[0055] (c) energizing the microcoil at a frequency that permits
detection of a magnetic resonance within the sample fluid;
[0056] (d) processing a signal received from the microcoil to
detect magnetic particle-target complexes in the sample fluid;
[0057] (e) flowing a portion of the flowing fluid in which the
labeled entity was detected through a conduit disposed proximate to
a secondary microcoil;
[0058] (f) energizing the secondary microcoil at a frequency that
permits detection of a magnetic resonance within the sample fluid;
and
[0059] (g) processing a signal received from the secondary
microcoil to determine a further property of the magnetic
particle-target complexes.
[0060] In a twelfth aspect, the present invention provides methods
for detecting a target in a sample fluid, comprising:
[0061] (a) introducing a sample fluid into a sample reservoir,
wherein the sample reservoir is in fluid communication with a
conduit, wherein the sample fluid comprises magnetic particles
capable of binding selectively to one or more targets of interest
in the sample fluid, and wherein binding of magnetic particles to
targets produces magnetic particle-target complexes;
[0062] (b) flowing the sample fluid through the conduit disposed
proximate to a microcoil, wherein the microcoil possesses an inner
diameter of between 25 microns and 550 microns;
[0063] (c) energizing the microcoil at a frequency that permits
detection of a magnetic resonance within the sample fluid;
[0064] (d) processing a signal received from the microcoil to
detect magnetic particle-target complexes in the sample fluid;
[0065] (e) diverting a portion of the flowing fluid in which the
magnetic particle-target complexes were detected into a
sequestration chamber to produce a concentrated target
solution.
[0066] In a thirteenth aspect, the present invention provides
methods for detecting a target in a sample fluid, comprising:
[0067] (a) introducing a sample fluid into a sample reservoir,
wherein the sample reservoir is in fluid communication with a
conduit, wherein the sample fluid comprises two or more targets of
interest, and wherein the sample fluid comprises magnetic particles
differentially bound to the two or more targets of interest to
create at least a first magnetic particle-target complex and a
second magnetic particle-target complex;
[0068] (b) flowing the sample fluid through the conduit disposed
proximate to a microcoil, wherein the microcoil possesses an inner
diameter of between 25 microns and 550 microns;
[0069] (c) energizing the microcoil at a frequency that permits
detection of a magnetic resonance within the sample fluid; and
[0070] (d) processing a signal received from the microcoil to
differentially detect the at least first magnetic particle-target
complex and the second magnetic particle-target complex in the
flowing fluid.
[0071] All aspects and embodiments of the methods of the present
invention can be carried out using the modules, microcoils, and
detection devices of any embodiment of any aspect of the invention,
of the invention.
BRIEF DESCRIPTION OF THE FIGURES
[0072] FIG. 1 depicts a portion of a detector in accordance with an
embodiment of the present invention.
[0073] FIG. 2 depicts a cross-sectional view of a detector in
accordance with an embodiment of the present invention.
[0074] FIG. 3 depicts three example microcoil constructions.
[0075] FIG. 4 depicts a portion of a detector comprising a conduit
that contains multiple conduit branches.
[0076] FIG. 5 depicts a portion of a detector comprising additional
fluidic components coupled to a conduit.
[0077] FIG. 6 depicts a portion of a detector comprising three
affinity columns arranged to permit multiplexing of a fluid
sample.
[0078] FIGS. 7a-7c depict schematic diagrams of electrical
connections between a tuning circuit and a microcoil.
[0079] FIG. 8 depicts an example module.
[0080] FIG. 9 is a time series of images generated in accordance
with a method of the present invention.
[0081] FIG. 10 is a contour plot depicting the full time course of
a detection experiment conducted in accordance with a method of the
present invention.
[0082] FIG. 11 is a graphical representation of the movement of an
entity through a conduit developed in accordance with a method of
the present invention.
[0083] FIG. 12 is a graphical representation of a layered affinity
column for use with the invention.
[0084] FIG. 13 is a graphical representation of a layered affinity
column integrated with a microcoil for use with the invention.
[0085] FIG. 14 is a schematic depicting a basic embodiment of an
electrical LC resonating circuit according to the present
disclosure, in which a tuning inductor is connected in series with
a miniaturized or "micro" sample coil.
[0086] FIG. 14A is a schematic depicting the circuit shown in FIG.
14, except with the single sample coil replaced by a plurality of
coils connected in series.
[0087] FIG. 14B is a schematic depicting the circuit shown in FIG.
14, except with the single sample coil replaced by a plurality of
coils connected in parallel.
[0088] FIG. 15 is a schematic diagram indicating how an electrical
circuit according to the present disclosure can be implemented in a
magnetic resonance experiment.
[0089] FIG. 16 is a schematic diagram depicting another embodiment
of an electrical circuit according to the present disclosure, in
which the miniaturized sample coil is connected in parallel with
the tuning inductor via wires or cables of a length corresponding
substantially to a quarter wavelength of the resonant
frequency.
[0090] FIG. 17 is a schematic diagram, similar to FIG. 16, of yet
another alternative embodiment of a circuit according to the
present disclosure, showing the transmission line connected to and
across a partial segment of the tuning inductor.
[0091] FIG. 18 is a schematic diagram, similar to FIG. 16, of yet
another alternative embodiment of a circuit according to the
present disclosure, showing the connection of an impedance
transformer to the transmission line, between the sample coil and
the tuning inductor.
[0092] FIG. 19 is a schematic depicting a basic embodiment of an
electrical LC series resonating circuit according to the present
disclosure, in which a tuning inductor is connected in series with
a miniaturized or "micro" sample coil.
[0093] FIG. 20 is a schematic diagram elaborating somewhat on FIG.
14.
[0094] FIG. 21 is a schematic diagram elaborating somewhat on FIG.
16.
DETAILED DESCRIPTION OF THE INVENTION
[0095] In a first aspect, the present invention provides modules,
comprising:
[0096] (a) a microcoil possessing an inner diameter of between 25
microns and 550 microns;
[0097] (b) a conduit disposed proximate to the microcoil, wherein
the conduit is in fluid communication with a sample reservoir;
[0098] (c) an affinity column in fluid communication with the
conduit and the sample reservoir; and;
[0099] (d) a connector for connecting the module to a magnetic
resonance detector.
[0100] In a second aspect, the present invention provides modules
comprising:
[0101] (a) a plurality of microcoils each possessing an inner
diameter of between 25 microns and 550 microns;
[0102] (b) a conduit disposed proximate to each microcoil, wherein
the conduit is in fluid communication with a sample reservoir;
and
[0103] (c) a connector for connecting the module to a magnetic
resonance detector.
[0104] In a third aspect, the present invention provides microcoils
comprising an inner diameter of between 25 microns and 550 microns,
wherein the microcoil is an effective magnetic resonance
transmitter or receiver coil; and wherein the microcoil is within
or surrounds an affinity column.
[0105] In a fourth aspect, the present invention provides detection
devices comprising
[0106] (a) a permanent magnet possessing a field strength of less
than or equal to 4 Tesla;
[0107] (b) a microcoil disposed proximate to a magnetic field
generated by the permanent magnet, wherein the microcoil possesses
an inner diameter of between 25 microns and 550 microns;
[0108] (c) a conduit disposed proximate to the microcoil, wherein
the conduit is in fluid communication with a sample reservoir;
[0109] (d) an affinity column in fluid communication with the
conduit and the sample reservoir.
[0110] In a fifth aspect, the present invention provides detection
devices comprising
[0111] (a) a permanent magnet possessing a field strength of less
than or equal to 4 Tesla;
[0112] (b) a plurality of microcoils each possessing an inner
diameter of between 25 microns and 550 microns, wherein each
microcoils is an effective magnetic resonance transmitter or
receiver coil; and
[0113] (c) a conduit disposed proximate to each microcoil, wherein
the conduit is in fluid communication with a sample reservoir.
[0114] In a sixth aspect, the present invention provides magnetic
resonance detectors, comprising
[0115] (a) a housing comprising a conduit guide;
[0116] (b) a permanent magnet possessing a field strength of less
than or equal to 4 Tesla within the housing; and
[0117] (b) the module of any embodiment of any of aspect of the
invention, wherein the connector connects the module to the housing
via the conduit guide.
[0118] The modules, microcoils, detection devices and magnetic
resonance detectors of the various aspects and embodiments of the
present invention provide, for example, increased functionality and
detection modes, improved sensitivity and specificity, time and
cost savings, and a variety of other benefits including those
described below.
[0119] All of the various embodiments for different components of
the modules, microcoils, and detection devices of the first through
sixth aspects of the invention are capable of use together; thus,
any embodiment disclosed for one aspect can be combined with any
embodiment for another aspect, as will be understood by those of
skill in the art.
[0120] The modules of the invention can be used, for example, to
couple disposable components of any embodiment detectors of the
invention to permanent detector components by connecting the module
to the detector via, for example, the conduit guide as discussed
herein. In one example, all fluidic components of the detector are
located on the module, thus reducing the probability that portions
of a fluid sample will leak into the permanent detector components.
Multiple modules may be used with a single detector, for example,
to reduce the probability of contamination between detection
experiments. In one embodiment, different sample fluids can be
assigned their own modules which thus do not come in contact with
other test samples. Removable modules also permit the conduit and
microcoil characteristics to be adjusted based on the sample fluid
used, or other aspects of a detection experiment. For example, a
longer microcoil may be used in one experiment. In another example,
a larger-diameter conduit may be used.
[0121] To provide structural support or make the module easier to
handle, the module may be disposed on a surface, such as a card or
board for example, or the removable module may be disposed in a
housing. However, no support or housing means are necessary. For
example, the module may comprise a section of a conduit with a
solenoidal microcoil wrapped around a portion of the conduit. An
exemplary module is provided in FIG. 8.
[0122] In an example detector, the module can slide into the
detector on conduit guides that place the conduit and the microcoil
in a uniform region the field generated by the permanent magnet.
The microcoil can be mounted directly on the conduit, and
electrical leads extending from the microcoil can extend to
electrical contact pads or connector on an edge of the module. Any
other fluidic channels that are used in the course of the detection
experiment can also be contained on the module. The conduit guides
also place the module in a selected alignment with the magnetic
gradient coil and a vacuum fluidic drive.
[0123] One embodiment of the detector includes a test box and a
disposable module. The box may comprise the magnet, detection
circuitry and interface, fluidic driver and controls, user
interface, result printer, interface to the clinical data base,
module ID reader, conduit guide, master processor system and
software, and other associated power supplies and supporting
electronics.
[0124] As used herein, an "affinity column" comprises any means to
trap a target entity capable of separating biochemical mixtures
based on specific interactions with a target, including but not
limited to antigen-antibody interactions; enzyme-substrate
interactions; and receptor-ligands interactions. The affinity
column for use in the devices and methods of the invention may
comprise the "stationary phase" (the specific substance, or resin,
used to separate analytes) within the conduit, may comprise a
separate affinity column placed in the conduit (the stationary
phase and column hardware), or any other suitable variations
thereof.
[0125] An affinity column may be used with the modules of the
invention to trap a target entity and permit the target entity to
be labeled with a labeling bead, such as a magnetic label. Any
affinity column capable of separating biochemical mixtures based on
specific interactions with a target can be used. For example, one
or more affinity columns may comprise a capture agent to immobilize
a target entity in a sample fluid as a way to concentrate the fluid
prior to flowing the fluid through a detection zone of the conduit.
A "capture agent" is any molecule capable of selectively binding,
or being derivatized to selectively bind, a target of interest in
the sample fluid. Suitable capture agents include, but are not
limited to, proteins, nucleic acids, antibodies, lectins, enzymes,
mono-, or poly-saccharides. The capture agent can be attached to
the column packing material using a linker which binds to the
column through a reversible binding reaction. For example, a
hexa-histidine linker will bind to Ni or Co ions attached to a Ni-,
or Co-nitriloacetate-agarose column matrix. The binding of the
linker to the column can be reversed through competition with a
release agent, such as histidine or imidazole, in the case of a
hexa-his linker. This linker is covalently attached to the
recognition molecule via standard chemical methods.
[0126] This use of affinity columns provides a "pre-filter" for the
detection device, and can be used to both concentrate the targets
and remove excess unbound labeling beads. For larger samples, or
samples suspected of having low concentrations of targeted
entities, the use of a concentrating mechanism such as an affinity
column may facilitate a reduction in the sample volume. For
example, an environmental sample of 50 milliliters may be suspected
of containing low concentrations of a target entity. To speed up
the detection process, an affinity column with a labeled capture
agent capable of selectively binding to the target entity may be
used to isolate and hold the target entity while the remainder of
the sample is washed away. After the extraneous portion of the
sample is removed, and carrier fluid can be passed through the
affinity column to carry the previously trapped target entities
into the sensitive volume of the detector.
[0127] Alternatively, unlabeled target entities may be trapped by a
capture agent on a solid phase, such as in an affinity column or
other column known from chromatography. Once attached to the
affinity column and immobilized, a solution of labeled beads with
attached antibodies selective for the target entities may be
introduced so that all of the attached targets are labeled with
beads. The excess beads that do not label any target may then be
washed out of the column. The targets, with their labels attached,
may then be eluted from the column and this eluant can then be
processed by the NMR detector.
[0128] In another embodiment, the affinity column comprises two or
more layers (2, 3, 4, 5, 6, 7, 8, 9, 10, or more layers), and
wherein each layer comprises a different capture agent, which can
be used to trap different targets in different regions of space in
the column, which can be detected differentially using multiple NMR
detectors or MRI techniques, as described in more detail below. In
one embodiment, overall magnetic resonance signal changes are first
detected, and then MRI techniques are used to identify which of the
affinity column layers is the source of the change.
[0129] In another embodiment, the microcoil is integrated into the
affinity column (ie: within or surrounds the affinity column).
Affinity columns can be made very small and typically contain
substances that can produce NMR signals. Thus, a microcoil can be
built into (or around) a column so that it can directly detect
targets trapped inside; for example, the microcoil can be wound
around the affinity column, or embedded in its walls. A stationary
magnetic target can be detected many times, confirming the positive
detection. Different protocols can be applied to a target of known
location, improving detection accuracy and precision, allowing more
definitive detection, differential detection, etc. The integrated
detector coil eliminates the need for a separate elution step,
which reduces time, target damage, stripping of magnetic labels
from the targets, dilution of targets into a larger elution volume,
spreading of targets through diffusive processes into a larger
region of sample volume, etc.
[0130] The column material itself can be chosen so as to contribute
to the NMR signal, including that part of the signal affected by
the presence of a magnetic target. For example, the material can be
a gel, or any other material that is stationary but also provide a
liquid-like NMR signal. In one embodiment, the material can contain
hydrogen or fluorine atoms sufficient to produce a detectible NMR
signal. The material may also be a solid material containing
aluminum or any other atom that produces a sufficiently liquid-like
NMR signal, for example from the central transition of its
quadrupole-perturbed NMR spectrum. The column material can be
modified ("liquefied") so that it contributes to the detected NMR
signal. For example, the column material may be initially a solid
that produces no usable NMR signal, but can be transformed so that
it produces a usable liquid-like signal through the action of
chemicals or the changing of the temperature. In the extreme case,
the column material may be dissolved through the use of an
appropriate solvent. The column material can be chosen so that it
does not alter the magnetic field being applied to the sample space
(i.e., the column's stationary phase can be chosen so that it has
the same magnetic susceptibility as the fluid). For example, the
column material can be a glass, ceramic, or gel that contains an
appropriate admixture of impurities, such as paramagnetic ions (Cu,
Mn, Gd), so that the materials overall magnetic properties match
those of the sample fluid being processed. The impurities to be
mixed and their concentration can be chosen so that the presence of
the column material does not degrade the homogeneity of the
magnetic field in the column.
[0131] In another embodiment, the solid phase of the affinity
column can chosen so that it can be dissolved and the column
contents passed through the detector, eliminating the need for
elution and allowing more of the column itself to contribute to the
NMR signal. For example, the dissolving can be achieved through the
use of a solvent chosen to dissolve the solid phase, or by changing
the temperature of the column in a way that renders the solid phase
a liquid (such as raising the temperature to melt the solid phase).
The dissolution need not be complete and the solid phase may simple
break apart in to pieces that are small enough to pass through the
NMR detection coil.
[0132] In another embodiment, a plurality of different affinity
columns can be used, in parallel, or more preferably in series, in
order to attach a particular pathogen type in each separate
column.
[0133] The detectors of the invention comprise a microcoil disposed
proximate to the magnetic field generated by the permanent magnet,
wherein the microcoil possesses an inner diameter between 25
microns and 550 microns. The microcoil is an effective magnetic
resonance transmitter or receiver coil. In various embodiments, the
inner diameter of the microcoil can be between 25-500, 25-450,
25-400, 25-350, 25-300, 25-250, 25-200; 50-550, 50-500, 50-450,
50-400, 50-350, 50-300, 50-250, 50-200, 100-550, 100-500, 100-450,
100-400, 100-350, 100-300, 100-250, 148-648, 170-555, and 100-200
microns. In an example detector, the microcoil is solenoidal in
shape and can be wound around a section of a conduit that holds the
volume of fluid during a detection experiment. However, other coil
shapes may be used, including but not limited to planar coils,
rectangular coils, saddle coils, and meanderline coils. For
example, the cylindrical axis a flat or a solenoidal microcoil may
be capable of being oriented perpendicularly to the axis of a
conduit, and the coil may be filled with a material, for example
but not limited to, ferrites, to enhance the sensitivity of the
coil. Further, the microcoil may be formed through other
construction techniques, including but not limited to depositing
the coil material on a surface or etching the coil. The length of
the microcoil can be selected such that the length of the microcoil
is coextensive with the uniform region of the magnetic field
generated by the permanent magnet of the detection devices of the
invention. Other lengths may also be selected. In various
embodiments, the length of the microcoil is between 25 .mu.m-5 cm,
50 .mu.m-5 cm, 75 .mu.m-5 cm, 100 .mu.m-5 cm, 100 .mu.m-4 cm, 100
.mu.m-3 cm, 100 .mu.m-2 cm, 100 .mu.m-1.5 cm, 100 .mu.m-1 cm, 1.5
mm-1.5 cm, 2.0 mm-1.5 cm, 3 mm-1.5 cm, 4 mm-1.5 cm, 5 mm-1.5, 6
mm-1.5 cm, 7 mm-2 cm, 8 mm-1.5 cm, and 9 mm-1 cm. Generally, the
strength of the signal produced by a microcoil increases with the
length of the coil. Some detectors may include a multiplicity of
differently-sized microcoils. In an example detector, a first
microcoil with a larger inner diameter is used to conduct an
initial analysis of a sample. If the presence of an entity in a
fluid is detected by the first microcoil, the fluid may be diverted
to be analyzed at a higher sensitivity by a second microcoil with a
smaller inner diameter.
[0134] As used herein, the microcoil being "disposed proximate to
the magnetic field" means that at least a portion of the microcoil
is located within the magnetic field produced by the permanent
magnet. In an example detector, the entire coiled section of a
solenoidal microcoil is placed within a magnetic field and oriented
such that the magnetic field observed at all points on the coiled
section of the solenoidal microcoil is uniform. However, any
orientation of the microcoil within the magnetic field may be used,
for example an orientation that aligns the microcoil with a
non-zero component of the magnetic field in order to employ MRI
techniques. For example, the microcoil may be angled with respect
to the direction of the magnetic field, and portions of the
microcoil, such as the ends or the electrical leads from the
microcoil, may extend beyond the magnetic field.
[0135] In one embodiment, the microcoil is a closely wound
microcoil, with a total length according to any embodiment of the
invention, including ranges between 100 .mu.m and 1500 .mu.m, 100
.mu.m and 1100 .mu.m, or any range in between. In another
embodiment, the microcoil or closely wound microcoil is wound
around a capillary with an outer diameter of and/or an inner
diameter as discussed below. In another embodiment, the microcoil
comprises wire with a diameter less than or equal to 2.5 times a
skin-depth of the wire material.
[0136] The microcoil is capable of being energized at a frequency
that permits detection of a magnetic resonance within a volume of
fluid within the conduit based on the strength of the magnetic
field generated by the permanent magnet. The frequency that permits
detection of a magnetic resonance within a volume of fluid varies
with the strength of the magnetic field such that f=.gamma.'B,
where f is the frequency, B is the magnetic field strength, and
.gamma.' is a proportionality constant based on the nuclei examined
within the fluid. For example, .gamma.' for the nuclei of hydrogen
atoms is approximately 42.6 MHz/Tesla. In various examples, the
frequency that permits detection of a magnetic resonance with in a
volume of fluid in the conduit is between 1-100, 10-100, 20-100,
30-100, 40-100, 50-100, 60-100, 70-100, 80-100, 90-100, 20-85,
30-85, 40-85, 50-85, 60-85, 70-85, 80-85, 20-65, 30-65, 40-65,
60-65, and 35-45 MHz.
[0137] It is possible to provide for additional throughput by using
two, or more, microcoil sections in parallel in the modules and/or
detectors of the invention, in order to enhance throughput of
sample fluids. This may be optimized by the use of MRI techniques,
such that the signal from a particular coil may be discerned by the
characteristics of the signal, for example frequency content of
frequency characteristic. For example, magnetic field gradients
(discussed below), applied at the appropriate angle oblique to the
microcoils, separate the signals from each coil into its own unique
frequency range, allowing simultaneous signal acquisition from all
coils. Alternatively, or additionally, each coil may be
electrically connected individually, and the signal processed in a
separate electronic channel.
[0138] In any embodiment of any aspect of the present invention,
the microcoil may be part of a resonant circuit, comprising:
[0139] (a) the microcoil;
[0140] (b) an auxiliary inductor coil electrically connected in
series to the microcoil; and
[0141] (c) a tuning capacitor electrically connected to the
microcoil and the auxiliary inductor coil to form a resonant
circuit.
[0142] In various preferred embodiments, the microcoil is a closely
wound microcoil and/or comprises wire with a diameter less than or
equal to 2.5 times a skin-depth of the wire material.
[0143] Resonant circuits of all aspects and embodiments of the
present invention allow for the design and production of improved
microcoil NMR devices that provide, for example, improved SNR and
line width performance as well as increased functionality as
described in detail herein, and thus greatly improve detection
capabilities, and also allow further reductions in sample volume
and further miniaturization of microcoil NMR devices than was
possible in prior methods and devices. In various embodiment of the
resonant circuits, the microcoil or closely wound microcoil has a
total length as disclosed in any of the embodiments herein In
another embodiment of the resonant circuits, the microcoil or
closely wound microcoil is wound around a capillary with an outer
diameter and/or an inner diameter as disclosed in any of the
embodiments herein. In a further embodiment of the detector aspects
of the invention, the auxiliary or tuning inductor comprises wire
of a larger diameter than a diameter of the microcoil or closely
wound microcoil wire. In a still further embodiment of the resonant
circuits of the invention, the auxiliary or tuning inductor coil
has a radius of 0.3 cm to 0.6 cm. In another embodiment, the
resonant circuits are mounted on a mechanical support; when mounted
on a mechanical support, the resonant circuit may be installed in
one or more shielded probe bodies. In one embodiment, the one or
more shielded probe bodies comprise two shielded probe bodies,
wherein a first shielded probe body shields the closely wound
microcoil, wherein a second shielded body shields the shielded
probe body and/or shields the auxiliary inductor and the capacitor.
Each of these embodiments can be used in combination with other
embodiments of all the aspects of the invention, as well as further
aspects of the invention that involve use of the resonant
circuits.
[0144] Any conduit capable of receiving a fluid sample may be used,
including but not limited to a capillary tube. In one embodiment,
the conduit is hollow and cylindrical in shape, with the inner and
outer diameters sized to accommodate picoliter-microliter volumes
within the sensitive volume of the detector. The inner diameter of
the conduit may be selected based on the properties of the fluid
used in a detection experiment, as well as other parameters of a
detection experiment such as the strength of the signal produced by
a fluid, the flow rate of the fluid, and the desired resolution of
the experiment, for example. The conduit has an inner diameter
between 25 and 550 microns. In various embodiments, the conduit
inner diameter can be between 25-500, 25-450, 25-400, 25-350,
25-300, 25-250, 25-200; 50-550, 50-500, 50-450, 50-400, 50-350,
50-300, 50-250, 50-200, 100-550, 100-500, 100-450, 100-400,
100-350, 100-300, 100-250, 170-550, and 100-200 microns. The outer
diameters can be any that may suitably be used with conduits of the
inner diameters disclosed herein. Further, in embodiments where the
microcoil is used to retain the fluid (microcoil forms the
conduit), the wall thickness of the conduit may be reduced to zero.
The efficiency of the detector may be improved by reducing the
difference between the inner and outer diameters of the conduit. An
example conduit is a capillary tube with an inner diameter of 100
microns and an outer diameter of 170 microns. Conduits conforming
to different shapes may also be used, including but not limited to
elliptical conduits. Further, the conduit may comprise multiple
sections, and may include removable sections. Removable sections,
for example, may facilitate a reduction in the probability of a
contamination of a sample or may permit the device to be more
readily cleaned or repaired. The conduit itself may be disposed on
the conduit guide, either directly placed thereon or indirectly
with another component serving to guide the conduit into the
conduit guide. In one embodiment, the conduit is disposed on a
module, discussed in more detail herein, that the conduit guide is
capable of receiving.
[0145] The conduit can receive fluid from any suitable component,
including but not limited to a reservoir for providing fluid to the
conduit. Such a reservoir can be on board the device or on the
module discussed herein. The reservoir can simply be a component of
the conduit in which a valve is placed to control flow from the
reservoir to the portion of the conduit used for analysis.
[0146] As used herein, the conduit being "disposed proximate to the
microcoil" means any position from which a signal transmitted from
the microcoil can reach the conduit and the corresponding energy
released from the fluid in the sensitive volume of the conduit can
induce an electrical current in the microcoil. In an example
embodiment, a solenoidal microcoil is wrapped around the conduit,
such that the axis of the microcoil is parallel with the axis of
the conduit. In another example, a planar coil is located
immediately adjacent to the conduit and oriented such that the
center axis of the planar coil is perpendicular with the center
axis of the conduit.
[0147] Further, the conduit may comprise a plurality (ie: 2, 3, 4,
5, 6, 7, 8, 9, 10, 50, 100, or more) of branches capable of
receiving a volume of fluid. The plurality of branches may be
disposed proximate to a plurality of microcoils. In one embodiment,
each branch is disposed proximate to a separate microcoil, where
each branch and each microcoil may be the same or have different
sizes as deemed appropriate for the specific use. For example, the
plurality of branches may permit the division of a fluid sample
into multiple subsamples, or may permit assaying of more than one
fluid sample at a time. In a further embodiment, one or more
branches of the conduit are used in other fluidic processes. For
example, one or more branches may be fluidically coupled to one or
more affinity columns. In experiments that use labeling beads to
aide in the identification of entities in a fluid, different labels
may be added to each of the subsamples. The plurality of branches
may also be coupled to valves, sequestration chambers, and/or other
fluidic structures as suitable for a given purpose. Such fluidic
components can be "on board" the detector, or may be provided via a
removable module, such as one that can be connected to the conduit
guide.
[0148] The detectors of any embodiment of any aspect of the present
invention may comprise a conduit guide capable of receiving a
conduit for receiving fluid, wherein the conduit guide is capable
of disposing the conduit proximate to the microcoil and proximate
to the magnetic field generated by the permanent magnet (and
proximate to the magnetic gradient when the detector is in use).
The conduit guide may comprise any means for orienting the conduit,
including but not limited to mounting brackets, mechanical guides,
and couplings. The conduit guide can be made from any material or
materials that are capable of establishing and maintaining the
position of the conduit guide. For example, metals, plastics,
composite materials, ceramics, and multi-layer materials can be
used individually or in combination to form the conduit guide. In
an example detector, the conduit guide also aligns the axis of the
portion of the conduit that holds the sample fluid (sensitive
volume) during an experiment with the direction of the magnetic
gradient produced by the magnetic gradient generator. In another
example detector, the conduit guide is adjustable to permit an
operator to select a position of the conduit relative to the
magnetic gradient that minimizes a frequency shift in a signal
emitted by the sensitive volume when the magnetic gradient is
energized. In embodiments that utilize a coil as a magnetic
gradient generator, the conduit guide may align the long axis of a
cylindrical conduit with the center of the gradient coil. However,
the conduit guide may dispose the conduit in other positions and
orientations as deemed appropriate by an operator.
[0149] The module comprises a connector for connecting the module
to a detector, such as those disclosed herein. Any connector
capable of coupling the module to a detector may be used. For
example, any of the conduit guides described as part of the
detector or method aspects of the present invention may be used.
Further, any mechanical coupling capable of securing the module in
place may be used. For example, a connector where threaded screws
or bolts on the module were coupled to corresponding threaded holes
on the detector may be used. Other example connectors include
mechanical clips, snap fittings, mortise-and-tenon connections,
pins, socket fittings, and compression fittings.
[0150] The module may further comprise electrical contacts capable
of establishing an electrical connection between the microcoil or
the module and the detector of any embodiment of the invention. The
electrical connection between the removable module and the detector
may permit the microcoil to interface with a tuning circuit, signal
processor, or any other circuitry on a detector, such as those
disclosed herein. In removable modules that contain electronics
such as signal generators, signal processors, tuning circuits and
other electronics on the removable module, the electrical
connection between the removable module and the detector may permit
any of the electronic components on the removable module to
interface with electrical components within the detector. For
example, the electrical connection can be used to supply power to
the removable module or allow the removable module to connect to a
user interface. Any means of establishing an electrical connection
may be used. For example, wire leads coupled to the microcoil may
extend from the module. In embodiments where the microcoil is
electrically connected to the module, conductive traces may
establish a connection between the microcoil and an electrical
coupling on the module, which may be inserted into a receptacle on
a detector.
[0151] The module may further comprise a fluidic drive fluidically
coupled to the conduit. Any suitable fluidic drive may be used with
the module. The modules and detectors of any embodiment of any
aspect of the invention may further comprise a fluidic drive,
capable of being fluidically coupled to the conduit. The fluidic
drive may permit the purposeful diving of a fluid in the conduit.
Typically, the fluidic drive operates by applying a change in the
pressure on one end of the conduit. For example, a vacuum may be
attached to one end of the conduit to draw the fluid through a
portion of the conduit. A positive displacement pump, such as a
syringe pump may also be used to establish fluid flow. The fluid
drive may also use air pressure or gravity to drive the fluid. Any
device that is capable of imparting a flow to a fluid in the
conduit may be used. The fluidic drive can be "on board" the
detector, or may be provided via a removable module, such as one
that can be connected to the conduit guide (described in more
detail herein).
[0152] Other fluidic components that may be fluidically coupled to
the conduit, such as valves, sequestration chambers, and affinity
columns may also be included on the removable module and/or the
detector. Any valve that is capable of being fluidically coupled to
a portion of the conduit may be used with the modules of the
invention. A valve may allow the flow of a fluid in the conduit to
be controlled. For example, in modules with conduits that contain a
plurality of branches, one or more valves may be used to sequence
the flow of a fluid through the plurality of branches. A
sequestration chamber (which may be, for example, a well on a
microplate, a separate conduit branch, a reservoir, etc.) may be
used to hold a portion of a fluid used in a detection experiment.
For example, a valve may be used to divert a portion of the fluid
into a sequestration chamber if an entity is detected in the fluid.
Any volume sequestration chamber may be used. For example, a small
volume of a few nanoliters may be enough to allow for a subsequent
microscopic evaluation of the fluid in which target is detected. In
another example, several microliters, or even the entire volume of
a sample fluid may be held in the sequestration chamber.
[0153] The detectors of the invention comprise a permanent magnet.
The permanent magnet possesses a field strength, wherein the field
strength is less than or equal to 4 Tesla. In various embodiments,
the field strength may be between 0.1-4, 0.1-3.8, 0.1-3.6, 0.1-3.4,
0.1-3.2, 0.1-3.0, 0.1-2.8, 0.1-2.6, 0.1-2.4, 0.1-2.2, 0.1-2,
0.1-1.9, 0.1-1.8, 0.1-1.7, 0.1-1.6, 0.1-1.5, 0.1-1.4, 0.1-1.3,
0.1-1.2, 0.1-1.1, 0.1-1.0, 0.25-4.0, 0.25-3.5, 0.25-3.0, 0.25-2.5,
0.25-2, 0.25-1.9, 0.25-1.8, 0.25-1.7, 0.25-1.6, 0.25-1.5, 0.25-1.4,
0.25-1.3, 0.25-1.2, 0.25-1.1, 0.25-1.0, 0.5-2, 0.5-1.9, 0.5-1.8,
0.5-1.7, 0.5-1.6, 0.5-1.5, 0.5-1.4, 0.5-1.3, 0.5-1.2, 0.5-1.1, and
0.5-1.0 Tesla. The permanent magnet may be constructed out of a
single magnet, or a plurality of permanent magnets may be combined.
Further, any materials used in the construction of permanent
magnets may be used to form the permanent magnet for the detector.
For example, iron, other ferrous and non-ferrous alloys, ceramic
magnetic materials, rare earth magnets including SmCo and NdFeB,
and other magnetic materials may be used. The permanent magnet may
also be formed into any shape. For example, magnets (or
combinations of magnets) with curved, rectangular, cylindrical, or
other profiles may be used. In an example detector, a dipole magnet
with steel pole pieces is used as the permanent magnet. However,
other magnets, such as Halbach magnets may also be used. In an
example detector, the magnetic field produced by the permanent
magnet is uniform. However, permanent magnets that form magnetic
fields possessing gradients, such as the static gradients sometimes
encountered when constructing magnets, may also be used in the
detector. The slope of the gradient that may be present from the
permanent magnet may range between 0 G/cm and 1.0 G/cm. In
embodiments where the permanent magnet possesses a gradient, the
gradient generated by the magnetic gradient generator differs in
strength of type (for example, pulsed vs. static). The detector may
also comprise a multiplicity of permanent magnets with a
multiplicity of magnetic fields. In an example detector, two
permanent magnets may be used, the first with a magnetic field
strength of 2.0 Tesla, and the second with a magnetic field
strength of 1.0 Tesla. The use of different magnetic field
strengths during the course of a detection experiment may
facilitate the detection of a variety of different entities with a
fluid sample. Further, since the resonant frequency of the nuclei
in a fluid varies with the field strength, the use of detectors
with a multiplicity of field strengths may further reduce the
likelihood of false positive or false negative detections by
analyzing a fluid at more than one field strength and
frequency.
[0154] In one further embodiment of any other embodiment of the
invention, the module/detection device further comprises an NMR
imaging system. Such imaging systems are known to those of skill in
the art. In another embodiment, the module/detection device further
comprises a matching capacitor in electrical connection to the
resonant circuit, which connects the resonant circuit to the
detection electronics of the device (ie: the NMR detection
system).
[0155] In an embodiment of all aspects of embodiments of the
invention, the detection devices disclosed herein may also comprise
magnetic gradient generators and other components, such as a signal
processor, to form a microcoil NMR-MRI system. In this embodiment,
the microcoil(s) is/are disposed proximate to the magnetic
gradient. The detectors of this embodiment comprise a magnetic
gradient generator. The gradient is used to differentiate between a
plurality of signal detection volumes within a microcoil. The
gradient may also be used to compensate for a gradient in the
magnetic field generated by the permanent magnet. Any magnetic
gradient generator capable of applying a magnetic gradient to the
magnetic field generated by the permanent magnet may be used,
including but not limited to permanent magnets, superconducting
electromagnets, or gradient coils. In an example detector, the
magnetic gradient is linear, though any gradient may be used,
including, but not limited to non-linear gradients. The gradient
generator may generate gradients with slopes between 0.01 G/cm and
1.0 G/cm. In various embodiments, the slope of a localized area of
the gradient generated by the magnetic gradient generator is
between 0.01-1.0, 0.015-1.0, 0.02-1.0, 0.02-0.9, 0.02-0.8 0.02-0.7,
0.02-0.65, and 0.02-0.6 G/cm. As discussed herein, in embodiments
where the permanent magnet possesses a gradient, the gradient
generated by the magnetic gradient generator differs in strength of
type (for example, pulsed vs. static). A static gradient is used in
an example detector because static gradients are particularly
compatible with miniaturized NMR and MRI platforms, and are
relatively simple compared to other gradients. However, other
gradient types may be used, including but not limited to pulsed
gradients and combinations of pulsed and static gradients. The
strength of the gradient used in a detector determines the spatial
resolution of the detector. Increasing the strength of the magnetic
gradient permits the detector to identify magnetic resonances in
smaller sections of the microcoil. In an example detector, a
magnetic gradient of 0.07 G/mm is used in conjunction with a
microcoil that is 1.1 mm in length. Using the weakest gradient that
still provides the desired spatial resolution may facilitate more
narrow detection bandwidths and improved signal-to-noise
characteristics of the detector. In an example detector, a magnetic
gradient is selected based on the T.sub.2* of the sample fluid,
without changing the center frequency of the energy emitted by the
sample fluid during a detection experiment is used. In an example
detector, a first magnetic gradient of approximately 0.14 G/mm may
be applied during a first analysis of a sample. If the results of
the analysis are inconclusive, or if improved signal-to-noise
characteristics are desired, a second gradient of 0.07 G/mm may be
applied during a second analysis of the same sample. The detectors
of this embodiment may further comprise a signal processor
electrically coupled to the microcoil and capable of identifying a
plurality of frequency components and a plurality of magnitude
components within the signal received from the microcoil, and
capable of correlating the plurality of magnitude components and
plurality of frequency components to a presence or absence of an
entity in a volume of fluid at a plurality of locations along an
axial length of the conduit. The signal processor may use any
method for identifying frequency components and magnitude
components within a signal. In an example detector, the signal
processor is capable of performing a fast Fourier transformation on
the signal received from the microcoil to identify a plurality of
frequency components and a plurality of magnitude components within
the signal from the microcoil.
[0156] As used herein, the microcoil being "disposed proximate to
the magnetic gradient" means that at least a portion of the
microcoil is placed within the magnetic gradient generated by the
magnetic gradient generator. In an example detector, the microcoil
is located within the magnetic gradient and oriented such that the
axis of the microcoil is aligned to be parallel to the direction of
a linear magnetic gradient. However, any orientation of the
microcoil with respect to the gradient that aligns the microcoil
with a non-zero component of the magnetic gradient may be used. As
will be understood by those of skill in the art based on the
teachings herein, disposition of the microcoil proximate to the
gradient and proximate to the magnetic field are separate variables
in design of the detectors of the invention.
[0157] The modules and detectors of any embodiment of any aspect of
the invention may further comprise a tuning circuit electrically
coupled to the microcoil. The tuning circuit comprises a tuning
coil capable of having an inductance at least two times larger than
the inductance of the microcoil, and a capacitor coupled to the
tuning coil to form a resonant circuit. In various embodiments, the
tuning coil may have an inductance that is 3, 4, 5, 6, 7, 8, 9, 10,
20, 25, 50, 100, 250, 500, or 1000 times larger than the inductance
of the microcoil. The tuning circuit comprises a tuning coil that
possesses an inductance of at least 2 nH, and the tuning coil is
coupled to a capacitor to form a resonant circuit. In various
embodiments, the inductor may have an inductance between 2 nH-1
.mu.H, 10 nH-1 .mu.H, 50 nH-1 .mu.H, 100 nH-1 .mu.H, 200 nH-1 .mu.H
and 500 nH-1 .mu.H. The tuning coil may be coupled to the microcoil
to form a series or a parallel connection with the microcoil. Such
tuning coils can be "on board" the detector, may be provided via a
removable module, such as one that can be connected to the conduit
guide, or a combination thereof. Any method of coupling the
microcoil to the tuning coil may be used, including but not limited
to a transmission line between the microcoil and the tuning coil.
An example tuning circuit is disclosed in U.S. Pat. No. 7,405,567,
incorporated herein by reference. Such tuning coils can be "on
board" the detector, may be provided via a removable module, such
as one that can be connected to the conduit guide, or a combination
thereof.
[0158] The tuning coil, which may also be referred to as an
auxiliary inductor or a tuning inductor may be designed in
accordance with a method comprising:
[0159] (a) preparing a microcoil with wire of a first diameter;
[0160] (b) determining an RF resistance of the microcoil;
[0161] (c) winding an auxiliary inductor coil with wire of a second
diameter, where the second diameter is greater than the first
diameter, and wherein a radius of the auxiliary inductor coil is
determined using the formula:
r coil = 25 l wire kd wire 46 .pi. ##EQU00001##
where l.sub.wire is the wire length, d.sub.wire is the wire
diameter, and kd.sub.wire is the turn-to-turn wire spacing.
[0162] The detectors of any embodiment of any aspect of the present
invention may further comprise a signal processor electrically
coupled to the microcoil and capable of identifying a plurality of
frequency components and a plurality of magnitude components within
the signal received from the microcoil, and capable of correlating
the plurality of magnitude components and plurality of frequency
components to a presence or absence of an entity in a volume of
fluid at a plurality of locations along an axial length of the
conduit. The signal processor may use any method for identifying
frequency components and magnitude components within a signal. In
an example detector, the signal processor is capable of performing
a fast Fourier transformation on the signal received from the
microcoil to identify a plurality of frequency components and a
plurality of magnitude components within the signal from the
microcoil. The signal processor may comprise the computer programs
disclosed herein.
[0163] In a further embodiment, the modules/detector may comprise
physical computer readable storage media, for automatically
carrying out the methods of the invention on a detector, such as
those disclosed herein. As used herein the term "computer readable
medium" includes magnetic disks, optical disks, organic memory, and
any other volatile (e.g., Random Access Memory ("RAM")) or
non-volatile (e.g., Read-Only Memory ("ROM")) mass storage system
readable by the CPU. The computer readable medium includes
cooperating or interconnected computer readable medium, which exist
exclusively on the processing system or be distributed among
multiple interconnected processing systems that may be local or
remote to the processing system.
[0164] In a seventh aspect, the present invention provides methods
for detecting a target is a sample fluid, comprising:
[0165] (a) flowing a fluid containing one or more magnetically
labeled targets through the conduit of the detection device of any
embodiment of any aspect of the invention;
[0166] (b) energizing the microcoil at a frequency that permits
detection of a magnetic resonance within the sample fluid; and
[0167] (c) processing a signal received from the microcoil to
detect the magnetically labeled targets in the sample fluid.
[0168] The methods of the present invention allow direct detection
of single cells, molecules, etc. in a given sample volume. The
methods comprise moving a sample fluid through a detector and
determining if an object of interest goes through, which are
counted. Prior art detectors are sensitive to minimum
concentrations of objects. It is especially unusual to use NMR to
detect single objects, as traditional NMR detection techniques are
low-sensitivity and require target numbers many times larger than a
single cell or molecule.
[0169] The methods of the invention comprise energizing the
microcoil at a frequency that permits detection of a magnetic
resonance within the fluid. The microcoil is used to transmit a
pulsed electromagnetic signal at a selected resonant frequency
towards the fluid in the conduit. The frequency of the transmitted
signal is selected based on the properties of the particular nuclei
that are being examined in the detection experiment and the
strength of the magnetic field. The microcoil is also used to
detect the energy that is absorbed or released by the nuclei in the
magnetic field in response to the transmission of the resonant
frequency signal. This energy induces an electrical current in the
microcoil that corresponds with the energy absorbed or released by
the nuclei in the flowing fluid. The presence of an entity within
the fluid causes a change in energy that can be detected by the
microcoil.
[0170] Such forms of contrast can arise also from any other
interactions of the labeled entity and the surrounding fluid. The
general form of this interaction is due to the mismatch in magnetic
property called susceptibility, between the entity and the carrier
solution. Thus, in principle, the device being disclosed can also
be sensitive to almost any distinct object including even bubbles.
However, this effect scales with the strength of the magnetic field
so it is not a large effect in our relatively low field permanent
magnets especially compared to the magnetic particles that are
chosen because of their strong magnetic strength, referred to as
magnetic moments.
[0171] When the distortion in the uniform magnetic field in the
vicinity of the object is a result of ferromagnetism or
"super-paramagnetism" of the object itself, the object is
responding non-linearly to the uniform magnetic field, and hence it
is not described as having a magnetic susceptibility. Instead, the
object is described as having a magnetic moment, which may or may
not be due to a saturation magnetization of the object. This
magnetic particle disrupts the field homogeneity in its
neighborhood, and this disruption may be detected by NMR
methods.
[0172] The object disturbing the magnetic field in its vicinity may
be much too small to be detected directly by NMR. The field
disturbance may be effective over a volume very much larger than
the object itself. If the volume of material affected by the object
is large enough, then the presence of the object can be detected by
NMR. In this sense, the magnetic field disturbance serves to
amplify the NMR signal indicating the presence of the object.
[0173] The most basic form of NMR contrast is to detect whether
something is present in the sample region or not. Using the methods
of the present invention when a dilute solution containing labeled
entities, for example, bacteria, is passed through the NMR
detector, the labeled entities can be detected individually. The
magnetic beads that label specific targets affect the NMR signal
from the surrounding fluid adversely. Therefore, if the "sphere of
influence" of the magnetic particles is comparable to the volume
being examined, i.e., the "sphere" should span a significant
fraction of the tube through which the solution is flowing, the NMR
signal that arises from the total cross-section of the sample tube
are attenuated. Protocols for excitation and detection may also be
devised to highlight other properties of the sample fluids. Fluids
may differ in how long they produce signals in response to an
excitation, for example. These differences in "relaxation time" may
be exploited to make a differential detection of the two (or more)
fluids in a mixture. There are several different relaxation times,
each appropriate to a particular method for exciting and detecting
the atomic nuclei, and each related to different chemical or
physical properties (or combination of properties). The most
commonly discussed relation times are denoted T.sub.1, T.sub.2, and
T.sub.2*. Many other parameters, including the diffusion tensor,
flow velocity, elastic modulus, chemical environment, and
temperature, can be encoded into the detected signal in an NMR
experiment. It is also possible to combine the detection of more
than one physical parameter into the same NMR experiment. It is
common, for example, to combine both T2 detection and spatial
location detection into a "T2-weighted image." In this case, the
detected signal is affected by both the amount of material located
at every position in space and the T2 of the material located
there.
[0174] In an eighth aspect, the present invention provides methods
for detecting a target in a sample fluid, comprising:
[0175] (a) introducing a sample fluid into a sample reservoir,
wherein the sample reservoir is in fluid communication with a
conduit;
[0176] (b) flowing the sample fluid into an affinity column in the
conduit, wherein the affinity column comprises one or more capture
agents that bind to one or more targets of interest;
[0177] (c) flowing a fluid comprising magnetic particles into the
affinity column, wherein the magnetic particles are capable of
binding selectively to the one or more targets of interest bound to
the affinity column via the one or more capture agents, and wherein
binding of the magnetic particles to the targets produces magnetic
particle-target complexes;
[0178] (d) washing the affinity column to reduce the number unbound
magnetic particles;
[0179] (e) eluting bound magnetic particle-target complexes from
the affinity column;
[0180] (f) flowing the fluid comprising the magnetic
particle-target complexes through a conduit disposed proximate to a
microcoil, wherein the microcoil possesses an inner diameter of
between 25 microns and 550 microns, wherein the microcoil is an
effective magnetic resonance transmitter or receiver coil; and
[0181] (g) energizing the microcoil at a frequency that permits
detection of a magnetic resonance within the sample fluid; and
[0182] (h) processing a signal received from the microcoil to
detect magnetic particle-target complexes in the sample fluid.
[0183] The use of affinity columns as a filtering step before NMR
detection can be used to concentrate the targets and/or remove the
excess unbound labels. The sample fluid is pumped through a column
in which a stationary phase of an affinity column is placed. The
stationary phase is functionalized with one or more capture agents
to preferentially trap the targets of interest. The sample fluid
may be circulated through the column numerous times if useful in
trapping all the targets. Once the column is loaded with one or
more target types, a fluid containing the magnetic labels is flowed
into the column. These labels are chosen to selectively bind only
to the target(s). The labeling fluid may be recirculated. Once the
targets are labeled, the column is flushed to remove all magnetic
labels that are not attached to targets. Then the column is eluted
in a manner that released the bond between the targets and the
stationary phase, without disturbing the bond between the targets
and the magnetic labels. The elutant thus will contain labeled
targets, a reduced number of excess magnetic labels, and the total
volume is substantially less than the original sample (ie: the
targets will have been concentrated.) The elutant will then be
passed though the NMR detector, which will count the magnetically
labeled targets. Even if all the excess beads are not removed, a
reduction in their numbers is very useful. In another embodiment,
the stationary phase of the column can be liquefied and could pass
through the NMR detector. For example, a solvent appropriate for
the column's solid phase material would dissolve the solid phase.
The stationary phase can be released (via the removal or induced
failure of the frit on the output side of the column) and pass
through the NMR detector. For example, if the solid phase is made
up very fine particles that can pass through the detector, they can
be held in place in the column by a grate-like structure, called a
frit, which constrains them mechanically to stay within the column.
Once the column is loaded, the frit is released, and the now
unconstrained particles flow through the detector, along with any
targets bound thereto.
[0184] Concentration of target entities on the affinity column
allows processing of larger amounts of biological, or other fluids
of interest, such as environmental water, industrial wastes,
process water, or water used to wash foodstuffs, such as
vegetables. The column can accommodate much higher flow rates than
are used in the detection step. It may not be otherwise practical
to process large sample fluid volumes directly through the
microcoil because it is understood that the high sensitivity of the
method partly derives from the use of very small volume microcoils.
The coil volume may be measured in, for example, nanoliters
(10.sup.-9 Liter). However, with the use of an affinity column with
a volume of, for example, microliters (10.sup.-6 Liter) one
thousand times as much fluid can be processed in order to
concentrate and trap targets in the same amount of time required to
process the microcoil volume. This may serve to shorten the
analysis cycle markedly; for example in this case by 1000-fold. It
also may reduce the amount of non-specific material introduced into
the NMR detector so that the rate of false-positive results may be
reduced. The use of an affinity column may increase the sensitivity
of the detection method. For example, if one is wishing to detect a
single bacterial cell in 10.sup.-2 Liter of whole human blood, if
the flow rate is limited, for example to 10.sup.-6 Liter per
minute, it would require 10,000 seconds, or almost 3 hours to
process a single blood sample. However, with a 10.sup.-5 Liter
affinity column in place, the flow may be increased to 10.sup.-3
Liter per minute with the result that the blood sample may be
loaded onto the column in only 10 seconds. Then, the column can be
eluted, and only the fraction known to contain the void-volume peak
(containing the target of interest) need be sent into the NMR coil.
The column also serves to provide a surface onto which the specific
biochemistry needed to assemble the unique target complex can take
place. Material specifically eluted from the column by the release
agent (imidazole, glucose, pH, ionic strength, etc.) then enters
the detection zone and provides a specific signal characteristic of
the target sought.
[0185] In a ninth aspect, the present invention provides methods
for detecting a target is a sample fluid, comprising:
[0186] (a) introducing a sample fluid into a sample reservoir,
wherein the sample reservoir is in fluid communication with a
conduit, wherein the conduit is disposed proximate to a microcoil,
wherein the microcoil possesses an inner diameter of between 25
microns and 550 microns, wherein the microcoil is an effective
magnetic resonance transmitter or receiver coil;
[0187] (b) flowing the sample fluid into an affinity column in the
conduit, wherein the affinity column comprises two or more layers,
wherein each layer comprises one or more capture agents that bind
to one or more targets of interest in the sample fluid, wherein
each layer in the affinity column is capable of binding to
molecules distinct from other layers; and wherein the affinity
column is located at least partially within the microcoil;
[0188] (c) flowing a fluid comprising magnetic particles into the
affinity column, wherein the magnetic particles are capable of
binding selectively to the one or more targets of interest, and
wherein binding of the magnetic particles to the targets produces
magnetic particle-target complexes;
[0189] (d) energizing the microcoil at a frequency that permits
detection of a magnetic resonance within the affinity column;
and
[0190] (e) processing a signal received from the microcoil to
detect magnetic particle-target complexes in one or more layers of
the affinity column.
[0191] In a tenth aspect, the present invention comprises methods
for detecting a sample in a target fluid, comprising:
[0192] (a) mixing a sample fluid with magnetic particles capable of
binding to the one or more targets of interest in the sample fluid,
wherein binding of magnetic particles to targets produces magnetic
particle-target complexes
[0193] (b) introducing the sample fluid into a sample reservoir,
wherein the sample reservoir is in fluid communication with a
conduit, wherein the conduit is disposed proximate to a microcoil,
wherein the microcoil possesses an inner diameter of between 25
microns and 550 microns;
[0194] (c) flowing the sample fluid into an affinity column in the
conduit, wherein the affinity column comprises two or more layers,
wherein each layer comprises one or more capture agents that bind
to one or more targets of interest, wherein each layer in the
affinity column is capable of binding to molecules distinct from
other layers; and wherein the affinity column is located at least
partially within the microcoil;
[0195] (d) energizing the microcoil at a frequency that permits
detection of a magnetic resonance within the affinity column;
and
[0196] (e) processing a signal received from the microcoil to
detect magnetic particle-target complexes in one or more layers of
the affinity column.
[0197] In these aspects, the methods identify target entities by
location within a multi-layer affinity column. By trapping
different targets in different regions of space, they can be
differentially detected using multiple NMR detectors or MRI
techniques. For example, a change in overall signal can first be
detected, followed by the use of MRI methods to identify which of
the layers is the source of the signal change. In one embodiment,
the microcoil is integrated with the affinity column. Affinity
columns can be made very small and typically contain substances
that can produce NMR signals. Hence, a microcoil can be built into
(or around) a column and used to directly detect targets trapped
inside. The coil can be wound around the column, or embedded in its
walls.
[0198] In one embodiment, a fluid containing magnetically labeled
targets can be made to flow through such a column, with the column
functionalized with capture agents to preferentially trap the
targets. This trapping can be chemical and/or physical in nature.
The magnetism of the target will affect the detected NMR signal;
many different effects are possible as described herein, and the
NMR signal detection protocol is optimized for each one
accordingly. In one embodiment, NMR detections might proceed while
the fluid is flowing through the affinity column, or the fluid flow
might be stopped once the sample has been loaded onto the affinity
column. The NMR protocol (or data analysis procedure) can be
designed to distinguish between stationary magnetic targets and
moving magnetic particles. For example, the distinction can be
based on two detections of position of the target, where if the
position changed, the target is be identified as moving. In another
example, a flow-compensated NMR pulse sequence (such as those used
in MRI angiography) is used to distinguish between signals detected
from moving and non-moving samples. Techniques for optimizing the
detection of NMR signals from moving sample fluids may be employed,
for example, by using flow compensating magnetic field gradient
wave forms, or by rapidly repeated data acquisitions together with
small flip angles optimized to give optimal SNR. The column
material itself can be chosen so as to contribute to the NMR
signal, including that part of the signal affected by the presence
of a magnetic target. The column material can be modified
("liquefied") so that it contributes to the detected NMR signal.
The column material can be chosen so that it does not alter the
magnetic field being applied to the sample space (i.e., the
column's stationary phase can be chosen so that it has the same
magnetic susceptibility as the fluid). In another embodiment, the
affinity column's solid phase can be dissolved so that the column's
contents pass through the detection zone, eliminating the need for
elution and allows more of the column itself to contribute to the
NMR signal (for column-integrated detectors).
[0199] The use of these methods provides many advantages, in that
stationary magnetic target can be detected many times, confirming
the positive detection. Different protocols can be applied to a
target of known location, improving detection accuracy and
precision, allowing more definitive detection, differential
detection, etc. The integrated detector coil eliminates the need
for a separate elution step, which may take too much time, damage
the targets, strip the magnetic labels from the targets, cause the
targets to be diluted into a larger elution volume, cause the
targets to spread through diffusive processes into a larger region
of sample volume, etc.
[0200] In an eleventh aspect, the present invention provides
methods for detecting a target in a sample fluid, comprising:
[0201] (a) introducing a sample fluid into a sample reservoir,
wherein the sample reservoir is in fluid communication with a
conduit, wherein the sample fluid comprises magnetic particles
capable of binding selectively to one or more targets of interest
in the sample fluid, and wherein binding of magnetic particles to
targets produces magnetic particle-target complexes;
[0202] (b) flowing the sample fluid through the conduit disposed
proximate to a microcoil, wherein the microcoil possesses an inner
diameter of between 25 microns and 550 microns;
[0203] (c) energizing the microcoil at a frequency that permits
detection of a magnetic resonance within the sample fluid;
[0204] (d) processing a signal received from the microcoil to
detect magnetic particle-target complexes in the sample fluid;
[0205] (e) flowing a portion of the flowing fluid in which the
labeled entity was detected through a conduit disposed proximate to
a secondary microcoil;
[0206] (f) energizing the secondary microcoil at a frequency that
permits detection of a magnetic resonance within the sample fluid;
and
[0207] (g) processing a signal received from the secondary
microcoil to determine a further property of the magnetic
particle-target complexes.
[0208] In order to enhance signal to noise (S/N), it is possible to
have a two step process, whereby a fast detection process with
poorer S/N is used to determine the presence of any pathogen, and
then a slower detection process, with better S/N is used to
determine a signal level with good discrete discrimination between
magnetic bead levels used. The methods of this aspect of the
invention use an initial detection step to trigger a secondary
measurement in the same device. This can be done with or without
sequestration. The "secondary microcoil" can be a different
microcoil or the same microcoil where the sample fluid has been
recirculated to it for measurement of a secondary property of the
magnetic particle-target complex (ie: something other than
detection). In one embodiment, the device is operated in a high
through-put mode initially, with a relatively large volume of
sample being processed. The initial detection process is optimized
for sample throughput, making it difficult to learn much more than
the simple presence of a target. Once a detection event has
occurred, however, we can shift the focus of the device from merely
detecting things to learning more about them, such as their
identity. This is possible without changing the label on the
targets, but by acquiring data in ways that give more resolution,
have more precision, etc. These other data collection techniques
are slower than the high-through put mode. The second measurements
can be done, for example, in a separate microcoil. This separate
coil may have characteristics that optimize signal to noise in
addition to the reduced fluid flow velocity in the coil. The
separate coil and detection process may employ different NMR
excitation and detection techniques capable of making measurements
or achieving measurement accuracy distinct from those possible in
the initial detector coil.
[0209] In one embodiment, the separate microcoil is placed along
the conduit flow path downstream from the initial microcoil. The
fluid flow rate might be stopped or slowed, or the geometry of the
flow can be changed (larger conduit diameter, e.g.) in a manner
appropriate for enhanced, reduced-speed detections. The second step
can also be done in the first microcoil, with the target brought
back into that coil by flowing the sample in the opposition
direction. The second step could occur in a "by line," a parallel
flow path in which the fluid moves more slowly, or is temporarily
stopped. The second step may comprise an NMR experiment protocol
that is distinct from the initial protocol so that it highlights a
different aspect of the target's signal.
[0210] Alternatively, the second "high S/R" detection process may
be applied after the entire sample fluid has been processed in the
initial step.
[0211] These second steps may comprise any useful measurement,
including but not limited to precise measurements of relaxation
times, and spectroscopic identification of objects or molecules
including the identification of properties or features of the
labels that were not discernable during the rare entity detection
phase of the device's operation.
[0212] In a twelfth aspect, the present invention provides methods
for detecting a target in a sample fluid, comprising:
[0213] (a) introducing a sample fluid into a sample reservoir,
wherein the sample reservoir is in fluid communication with a
conduit, wherein the sample fluid comprises magnetic particles
capable of binding selectively to one or more targets of interest
in the sample fluid, and wherein binding of magnetic particles to
targets produces magnetic particle-target complexes;
[0214] (b) flowing the sample fluid through the conduit disposed
proximate to a microcoil, wherein the microcoil possesses an inner
diameter of between 25 microns and 550 microns;
[0215] (c) energizing the microcoil at a frequency that permits
detection of a magnetic resonance within the sample fluid;
[0216] (d) processing a signal received from the microcoil to
detect magnetic particle-target complexes in the sample fluid;
[0217] (e) diverting a portion of the flowing fluid in which the
magnetic particle-target complexes were detected into a
sequestration chamber to produce a concentrated target
solution.
[0218] Since the methods of the present invention determine where
in space a target is, an identified target can then be sequestered
for further analysis. The detection volume can be very small (a few
nanoliters, typically), and this volume can be confined in a very
narrow conduit, and the sample fluid being measured can be made to
flow through this conduit. We can, for example, use a detection
event to trigger microfluidic valves that will collect very small,
isolated sections of the flowing sample in a sequestration chamber,
so that the collected sections contain the objects that were seen
by the NMR detector. Nearly all of the background fluid flows
through the conduit without being sequestered. The end result is
the sequestration of the objects of interest into a very small
total volume. That is, a dramatic enhancement of concentration of
the targets, making further processing (either in the NMR device or
in some subsequent device or procedure) much faster, easier, and
more effective. Further in-device processing might be used to
differentially identify targets, suppress false positives
(especially those due to locally high background bead
concentrations, eg.), etc. In one embodiment, sequestration can be
allowed to complete during an initial detection run, with a
subsequent processing of the sequestered sample, or the processing
the sequestered material might commence immediately. In a further
embodiment, sequestration can be used to reduce false positives
from high background concentrations, by running the sequestered
sample through the detection zone again (same or different
microcoil).
[0219] In a thirteenth aspect, the present invention provides
methods for detecting a target in a sample fluid, comprising:
[0220] (a) introducing a sample fluid into a sample reservoir,
wherein the sample reservoir is in fluid communication with a
conduit, wherein the sample fluid comprises two or more targets of
interest, and wherein the sample fluid comprises magnetic particles
differentially bound to the two or more targets of interest to
create at least a first magnetic particle-target complex and a
second magnetic particle-target complex;
[0221] (b) flowing the sample fluid through the conduit disposed
proximate to a microcoil, wherein the microcoil possesses an inner
diameter of between 25 microns and 550 microns;
[0222] (c) energizing the microcoil at a frequency that permits
detection of a magnetic resonance within the sample fluid; and
[0223] (d) processing a signal received from the microcoil to
differentially detect the at least first magnetic particle-target
complex and the second magnetic particle-target complex in the
flowing fluid.
[0224] In this aspect, the methods permit detection and
classification/identification in a single data acquisition step. We
can differentially detect two or more unique targets if we can
attach different amounts of magnetic material to each. All such
labeled targets are detected in detection devices of the invention
because they will all cause a decrease in the detected NMR signal.
The more magnetic targets will cause a larger decrease in signal.
We can determine the degree of signal decrease (or other
modification of the signal) and use this information to determine
which kind of target was detected.
[0225] This method requires different levels of magnetic labeling
on the targets to be differentiated. These levels of labeling can
be achieved in various ways, including but not limited to: [0226]
(a) Attaching the same number of beads (on average) of the same
size to the unique targets, where the beads for target A have more
magnetic strength than the beads for target B. Different strength
beads may be made from different magnetic materials, or contain
different amounts of the same material (e.g. 60% Fe3O4 vs. 90%
Fe3O4); [0227] (b) Attaching different sized beads to the different
unique targets. This results in differential labeling because
different numbers of beads are attached per unit target surface
area, and the magnetic moment per bead is different for the
different sized beads (e.g., beads containing the same fractional
amount of the same magnetic material will differ in magnetic
strength due to the total magnetic content); [0228] (c) Attaching a
maximal number of the same beads to all of the unique targets,
where the targets themselves differ in total surface area. The
unique targets differ in total magnetic moment because the total
number of beads that can be accommodated; [0229] (d) Use the same
beads for all targets where the particular capture agent chosen,
such antibodies, have different affinities for the different unique
targets, resulting in a different (average) number of beads
attached to each type of target.
[0230] In one embodiment, the level of labeling is measured in a
separate step from the initial detection. This second step might
occur in the same fluid stream (e.g., in a second microcoil and
detection volume optimized for level detections, or by slowing down
the flow temporarily to improve level-precision, etc.) or may be in
a parallel stream or a parallel stop-flow experiment. The level of
labeling determination might be done in a completely separate
apparatus, using sequestration techniques in the initial detector.
A different experimental magnetic resonance protocol might be
employed to differentiate between the different types or degrees of
labeling. We have generated data showing the different detection of
clusters of small numbers of beads. This data demonstrate
extraction of signal intensity differences using the methods and
devices of the invention, and the ability to discern multiple
levels of "labeling."
[0231] The following further embodiments can be used with any
embodiment/aspect of the invention.
[0232] To achieve stronger, more-sensitive, faster, or otherwise
improved detection, the microcoil detector may be immersed in a
material that matches the susceptibility of the metal used to wind
the coil. Alternatively, or in addition, the wire of the coil may
be chosen or treated such that its magnetic susceptibility matches
its surroundings. The material forming the tube on which the coil
is wound, or in which the coil is wound, and which contains the
fluid sample may be chosen to match either the susceptibility of
the fluid, or of the wire, or in any other way to best facilitate
detection.
[0233] In one embodiment, the processing comprises processing a
plurality of signals received from the microcoil over time (ie: two
or more time points), wherein the processing comprises identifying
a plurality of frequency components and a plurality of magnitude
components within each signal received from the microcoil, and
correlating the plurality of magnitude components and plurality of
frequency components within each signal to a presence or absence of
a labeled entity in the flowing fluid at a plurality of locations
along an axial length of the microcoil. In this embodiment, signals
are processed from two or more time points (ie: 2, 3, 4, 5, 6, 7,
8, 9, 10, or more), permitting correlation of signal data from two
or more different time points to identify an appropriate
correlation between signals, confirming that the signal is not
caused by background or other electrical disturbances. For example,
a first signal is detected in one location and a second signal is
detected at a second time point at an appropriate distance from the
first signal, based on fluid velocity and other factors. Those of
skill in the art will understand, based on the teachings herein
that appropriate spacing between time points will depend on a
variety of factors, including but not limited to microcoil size,
fluid velocity and viscosity, conduit size, etc.
[0234] The methods of the present invention can be used, for
example, in MRI and/or NMR detection of one or more labeled
molecules ("entity") in a fluid; the methods disclosed herein
provide dramatic reduction in susceptibility to false positive and
false negative detections. The methods permit the detection of a
labeled entity in a flowing fluid, and also permit multiplexing of
the detection process, providing the reliability of repeated single
NMR experiments in a single experiment. The methods also permit the
presence and movement of a labeled entity within a sample volume to
be observed and recorded. Entities that can be detected using the
devices and methods of the invention, include, but are not limited
to, cells (such as bacteria, fungi, other parasites, cancer cells,
etc.), viruses, proteins (including antibodies), prions, nucleic
acids, carbohydrates, lipids, small molecules, antibiotics, toxins,
etc.
[0235] In various embodiments, detection via the methods of the
invention can comprise detection, identification, and/or
quantitation of entities in a sample fluid. Any sample fluid of
interest can be used, including but not limited to bodily fluid
samples (blood, urine, saliva, semen, vaginal secretions, tears,
amniotic fluid, cerebral spinal fluid, etc.), swab samples from
skin, wound, or other body sites; semi-solid samples, such as fecal
samples (processed by appropriate sample dilution in a liquid),
environmental water, industrial wastes, process water, liquid
foodstuffs (including but not limited to milk, juice, drinking
water, soda, etc.) or water used to wash foodstuffs such as
vegetables and fruit. The size of the sample volume may vary
depending on the properties of the fluid being analyzed, the
desired flow rate through the sensitive volume, and the sizes of
the conduit. The sensitive volume of the conduit (ie: the volume
being analyzed) can vary depending on the size of the sample
volume, the size of the conduit, the flow rate, the size of the
microcoil, and other factors as determined by a user. In various
embodiments, the sensitive volume ranges between 100 picoliters and
50 microliters. In various other embodiments, the sensitive volume
is ranges between 4-7, 2-10, 1-15, 0.5-20, 0.4-50, 0.3-75, 0.2-100,
0.1-150, 0.05-300, 0.04-500, 0.03-1000, 0.02-2000, 0.01-5000
nanoliters. In an example detector, less than a nanoliter of fluid
is contained in the sensitive volume of the detector at any given
time. However, nanoliter, microliter, and even milliliter-sized
samples may be used depending on the overall capacity of the
detector. For larger samples, or samples suspected of having low
concentrations of targeted entities, the use of a concentrating
mechanism such as an affinity column may facilitate a reduction in
the sample volume. For example, an environmental sample of 50
milliliters may be suspected of containing low concentrations of a
target entity. To speed up the detection process, an affinity
column with a moiety capable of selectively binding to the target
entity may be used to isolate and hold the target entity while the
remainder of the sample is washed away. After the extraneous
portion of the sample is removed, and carrier fluid can be passed
through the affinity column to carry the previously trapped target
entities into the sensitive volume of the detector.
[0236] Any suitable method for labeling target entities can be
employed. In various embodiments of the methods of the invention
can comprise labeling a target entity with a detection enhancing
label using specific attachment chemistry to influence the magnetic
resonance properties of the sample fluid. Any label that can be
used to specifically target an entity of interest can be used,
including but not limited to magnetic beads derivatized for binding
to an entity of interest. In an example use of a label, a magnetic
bead labels a pathogen by using antibodies bound to the magnetic
bead that are selective for target antigens on the pathogen. The
magnetic bead alters the magnetic resonance of the fluid
surrounding the bead, causing the fluid surround the bead to behave
differently than the fluid would in the absence of the magnetic
bead. Any bead that causes the target entity or the fluid
surrounding the target entity to exhibit different magnetic
resonant behavior than the remaining fluid in a sample can be used
as a label. For example, magnetic and non-magnetic beads can be
used. In another example use of a label, a plurality of different
beads with known magnetic resonance profiles and the ability to
attach to specific target entities are added to a fluid sample. If
one of the known magnetic resonance profiles is detected during an
experiment, then the corresponding entity is present within the
fluid. By using a multiplicity of different beads during a single
experiment, a single experiment can be used to detect a
multiplicity of entities within the fluid. Beads may also be
internalized by the target entity, either singly or in
multiplicity, for example by eukaryote cells.
[0237] The use of multiple layers of beads results in an increased
magnetic moment. By increasing the effective area of the target
cell or molecule by attaching a first layer of beads (or single
bead) or first layer of molecules, and attaching the magnetic
labels to this first layer, the target object is effectively larger
and has more surface area. Hence, it can bind more of the magnetic
labels that we use for detection. The first layer need not be
magnetic. The methods for attachment between target and first bead,
and between first bead and magnetic label, can be chosen to enhance
the sensitivity and specificity of the labeling.
[0238] In another embodiment, two (or more) magnetic materials with
different chi are used to differentially label targets, so that in
low fields, one will have a higher magnetic moment than the other
because it is more highly magnetized. Paramagnetic materials
respond linearly to the application of an applied field. In
particular, they become magnetized in direct proportion to the
strength of the applied field, with the proportionality constant
often called "chi", the magnetic susceptibility. This linear
response cannot go on forever, at some high value of the field, the
magnetism of the material no longer increases. The material is said
to saturate, and the magnetic moment that it has in this state
(Msat) is the largest it can have. Of interest here are two
materials with different chi, so that in low fields, one will have
a higher magnetic moment than the other because it is more highly
magnetized. However, if this material with a higher chi value has a
lower Msat value, then as the applied magnetic field is increased
(by choice of a different or subsequent magnet, say) then the
second material will eventually have a higher magnetic moment. The
strength of the magnetic moment can be assessed in our detection
process, and we will be able to tell which is stronger in any given
magnetic field. Then at one field strength bead A has a larger
moment, while at another strength bead B has a larger moment.
[0239] The label may also be one that modifies signal by
displacement of the medium surrounding the object(s), such as
through attachment of glass or plastic beads. Alternatively a label
may be used that enhances or changes the signal, such as by
attachment of a material whose properties are measured directly, as
opposed to the signal from the medium. The bead could consist of
iron, iron oxide (Fe.sub.2O.sub.3, Fe.sub.3O.sub.4), Fe:Pt,
gadolinium metal or gadolinium oxide, iron nitride (Fe.sub.4N), or
other ferromagnetic, paramagnetic, or super-paramagnetic material
encapsulated in some manner, for example, encapsulated in glass or
in a plastic. The bead could also be made up almost entirely of the
magnetic material, either coated or uncoated. A coating might be
applied to guarantee inertness in the fluid of interest, to
stabilize the magnetic particle against degradation during use or
storage, or to allow the attachment of other materials or coatings
in order to optimize some behavior, such as attachment of
antibodies for the facilitation of attaching to the target
entity.
[0240] Magnetic material may be chosen to provide the best possible
magnetic moment, or provide the best overall signal performance. As
some materials saturate in an applied field, another material with
a lower magnetic moment might allow for a better overall signal due
to lack of saturation. The magnetic material can be ferro-magnetic,
ferri-magnetic, para-magnetic, super-paramagnetic, or diamagnetic.
If ferro-magnetic, the beads may be magnetized prior to their use
in the device, or they may be demagnetized initially and become
permanently magnetized as they pass through the magnet.
[0241] Iron oxide beads may also be of a particular size, i.e.
nanoparticles, such that the beads are super-paramagnetic, with
advantages that the beads are not magnetized when there is no
strong applied field, while on the other hand developing a very
large magnetic moment when subjected to a magnetic field. The lack
of a permanent magnetic moment can keep the beads from aggregating
in the absence of an applied magnetic field.
[0242] The effect on the surrounding medium of beads with smaller
magnetic moments can be different from the effect of beads with
larger magnetic moments because the size of the volume of
surrounding fluid affected by the bead is roughly proportional to
the magnetic moment of the beads. This can be used to facilitate
differentiation of targets to which beads preferentially attach. To
achieve differences in magnetic moment of bead labels, beads
containing different densities of magnetic material, beads of
different sizes, beads containing different materials, or any
combination of these or other factors may be employed.
Differentiation may also be achieved by variations in the
multiplicity of the labeling, for example, with larger targets
carrying more beads on their surface.
[0243] In an example of one embodiment, two species of bacteria are
present in a fluid, and each are specifically labeled with beads
with the associated antibody targeting the specific species of
bacteria. The amount of iron oxide in the beads of the label for,
in one example, bacteria 2 (B2) is larger than that of the label
for bacteria 1 (B1). Including the effect of the average binding
density of the bead labels to the target bacteria, it is possible
to differentiate the signals from the two species of bacteria based
on the differences in the net magnetic moment, and therefore the
amount of fluid medium the targeted bacteria affects as it flows
through the detection coil. The detection events are manifested in
a reduction in the signal from the medium, with the target with
larger net magnetic moment producing a larger decrease in
signal.
[0244] It may be advantageous to provide additional labeling to the
sample, for example to increase the signal associated with a
detection event, wherein the initial label bead itself is
subsequently labeled by additional beads. These secondary beads may
be appropriate for creating additional signal or entity
differentiation. For example, secondary beads containing a larger
fraction of magnetic material may be constructed to bind to primary
beads which recognize a particular entity, while other secondary
beads, containing a smaller fraction of magnetic material may be
constructed to bind to primary beads which recognize a separate
entity. Differentiation of one labeled entity from the other is
accomplished by an analysis of the resulting signal from the
medium. Secondary beads can be used to alter the signal in some
other way, for example adding ferromagnetic secondary beads to the
initial beads for a particular bacterial species, and paramagnetic
secondary beads to the initial beads for another bacterial species,
and in this way differentiate between two or more bacterial
species.
[0245] The methods may be used for rare entity detection or for
entity concentration measurement device. In rare entity mode, the
detector will generally have only one target entity in its
sensitive volume at any one time. In this mode, the goal of the
device is to find the rare target in a relatively large volume of
fluid that is flowing through the detector volume. In concentration
mode, the detector will have many targets in the sensitive volume
and the methods can be used to characterize the concentration in
relative or absolute terms.
[0246] Signal differentiation may also be achieved through control
of the magnetic moment of the bead that is attached to a particular
target. This may be achieved through control of the amount of
magnetic material in a bead, the size of beads at a given
concentration, the type of magnetic material used to produce
different magnetic moments, the number of beads attached, and so
on. Thus it is possible to provide, for example, multiply labeled
species or strains of pathogens and have them be uniquely
identified by characteristics of the NMR signal, such as
amplitude.
[0247] Differentiation between targets may be achieved via
measurements of secondary effects of the labels on the detected
signal. For example, the detection of the presence of a target may
be achieved by making one kind of measurement, for example
T.sub.2*, while the differentiation between different targets so
detected may be achieved via the measurement of a second NMR
property, for example T.sub.1. The differentiation may be enhanced
through the use of more than one kind of bead that share the same
(or nearly the same) values of some detection properties while
differing in other properties. These different beads may label
distinct targets, or some combination of beads might label each
target, with the combination being distinct from one target to the
other.
[0248] In addition to a magnetic label, further entity
differentiation can be achieved through the attachment of other
material to the target, such as fluorescent, optical-absorption, or
acoustic labels. An example of an acoustic label is a structure
with a characteristic ultrasonic signature. Fluorescent labels
include fluorophores (fluorescein, rhodamine) and quantum dots.
This additional labeling may be provided to, for example, do sample
detection and identification either in conjunction with or as a
separate step, before or after the detection according to the
methods of the invention. For example the detection event can
trigger the isolation, or sequestration of the detected bacteria
into a separate fluid path, wherein it can be further interrogated,
for example optically using fluorescence or absorption. The
additional labels may be attached before or after the detection
step, or may be integrated into a single object as a "multi-modal"
label.
[0249] The sample fluid in which labeled target are processed may
be changed or modified to enhance signal. Useful changes may be
made to the hydrogen density, viscosity, or other chemical or
physical properties of the fluid. The changes can be achieved by
replacement of one fluid by another, by the addition of solid or
fluid components to the matrix fluid, by changing the temperature
or pressure of the fluid, etc. These changes may affect the
T.sub.1, T.sub.2, or T.sub.2* of the fluid in a beneficial way, for
example, by yielding higher detection signals and allowing faster
processing. The T.sub.1 of the fluid may be reduced through the
introduction of Magnevist or any other T.sub.1 contrast agent.
Shorter T.sub.1s are beneficial in that the detection measurement
can be performed more rapidly, and repeated at higher rate.
[0250] The labeling of individual target entities can rely on any
suitable labeling technique, including but not limited to
antibody-based techniques. Antibodies are protein molecules which
recognize particular chemical sites, called epitopes, on other
molecules. Antibodies can be attached, either chemically, or
biochemically, to the surfaces of primary or secondary magnetic
beads. A biochemical example is a bead coated with streptavidin, a
protein which binds very tightly to biotin. Then, a biotinylated
antibody for a particular entity will bind to a streptavidin-coated
bead. A solution of antibody-labeled beads is then mixed with the
sample fluid in which the target entity (such as a pathogen) may be
present, for example bacteria in blood. Based on the type of
antibody coating of the bead, a bead, or many beads, will attach to
the target bacteria. The blood containing the labeled bacteria then
flows through the detection coil, allowing the presence of the
labeled bacteria to be ascertained.
[0251] Alternatively, the target entities may be trapped on a solid
phase, such as in an affinity column or other chromatography
column. Once attached to the affinity column and immobilized, the
solution of label beads with attached antibodies may be introduced
so that all of the attached targets are labeled with beads. The
excess beads that do not label any target may then be washed out of
the column. The targets, with their labels attached, may then be
eluted from the column and this eluant can then be processed by the
NMR detector.
[0252] If many beads attach to a single bacterium the effect of the
labeled bacterium on the signal from the medium can be easily
observed against the background signal the medium in the presence
of isolated single beads. Alternatively, when only a single bead is
attached to a bacterium, the unbound beads are preferably removed.
The removal of the excess beads may be accomplished by any suitable
means. In one example, antibody-beads are incubated with the
clinical sample; unattached antibody-beads are then washed through
a one-way valve, such as a silicon with a pore size on nm scale
(ie: too small for any target pathogens). A second valve would then
open and a second wash would send only bead-antibody-complexes and
blood components into the detector. Alternatively, the removal of
the excess beads may be accomplished by a filter column.
[0253] The fluid flow may be stopped during the detection, or the
flow may be continuous. For example, the method may comprise
detecting the presence of a target entity during fluid flow,
followed by a second detection, with fluid flow or with fluid flow
stopped, to determine the identity of the detected target entity.
Either a single microcoil can be used, with control of the flow
(including reversing the flow to bring the target back into the
coil), or a secondary coil (placed downstream from the primary
microcoil, or in a branch into which the target is steered) can be
used. The target entities to be detected may pass through the
detector and into an sequestration chamber without being
substantially modified. Living organisms or cells retain viability.
The output fluid in the sequestration chamber is available for
further testing or repeat testing in the device, in a similar
device optimized for a different measurement, or any other device
or process. Sequestered target entities can be sequestered into a
very small volume to enable the rapid location of the targets on a
microscope slide or the rapid processing of only the small volume
in subsequent processes or devices. The collected, concentrated
targets may be further analyzed in the detector, either in the coil
used to detect them in the first place, or in a separate detection
circuit or method.
[0254] The magnetic field may be produced by a permanent magnet,
such as the permanent magnets described as part of the detector
aspect of the present invention. Any of the magnetic gradient
generators that may be used in the detector aspect of the invention
may also be used to apply a magnetic gradient to a magnetic field
according to the methods of this aspect. Any of the microcoils,
conduits, modules, detectors, and combinations thereof disclosed
herein may also be used according to the methods of the invention.
Further multiplexing of the methods can be achieved by use of
conduits comprising multiple branches, which can be coupled with
the use of multiple microcoils to provide a variety of detection
zones, and further coupled with microfluidics to permit, for
example, (i) selective flow of a fluid sample of interest to
multiple detection zones for separate detection assays (ie:
different fluid flows; different field strengths; different
magnetic gradients; combinations thereof; etc.); and (ii) selective
flow of multiple fluid samples of interest to separate detection
zones for sample multiplexing. Other variations is apparent to
those of skill in the art based on the teachings herein.
[0255] Any method of causing the fluid to flow may be used. For
example, any of the fluidic drives disclosed herein may be used to
impart a flow on the fluid. The flow rate used can be any rate
desirable for a given application. As used herein, "flow rate"
refers to the amount of fluid that moves through the sensitive
volume of the detector over time. In one embodiment, the flow rate
of the fluid may be between 0.01 microliters per minute and 2.0
microliters per minute. In various other embodiments, the flow rate
can be between 0.01-500, 0.01-450, 0.01-400, 0.01-350, 0.01-300,
0.01-250, 0.01-200, 0.01-150, 0.01-100, 0.01-50, 0.01-10, 0.01-8,
0.01-6, 0.01-4, 0.01-3, 0.01-2, 0.01-1.75, 0.01-1.50, 0.01-1.25,
0.01-1.0, 0.01-0.9, 0.01-0.8, 0.01-0.7, 0.01-0.6, 0.01-0.5,
0.5-2.5, 0.5-2.25, 0.5-2.0, 0.5-1.9, 0.5-1.8, 0.5-1.7, 0.5-1.6,
0.5-1.5, 0.5-1.4, 0.5-1.3, 0.5-1.2, 0.5-1.0, 150-250, 175-225,
180-220, 185-215, 190-210, 195-205, 198-202 and 199-201 microliters
per minute depending on the properties of the fluid and the
particular experiment that uses the methods of the present
invention. Lower flow rates and even static fluids may also be
used. Any flow rate that permits the detection of a magnetic
resonance within the fluid may be used. The flow rates outside the
sensitive volume of the detector may also vary. For example, fluid
that is held in sequestration chamber or held behind a valve may be
static. For fluid flowing through an affinity column, or through a
portion of the conduit that distributes fluid samples to other
conduit branches, the flow rates may be higher. Further, the flow
rate through the sensitive volume of the detector may be varied.
For example, a valve placed before the sensitive volume may be used
to increase or decrease the flow rate during the course of a
detection experiment. In an example method, a detection experiment
is primarily conducted at 2 microliters per minute. However, if a
signal received from the microcoil indicates that an entity might
be present in the fluid, a valve may be activated to slow the flow
rate through the sensitive volume to 1.0 microliters per minute. In
another example method, a detection experiment may commence at flow
rate of 0.5 microliters per minute. If, after a predetermined
period of time no entities have been detected, a valve may be
activated to increase the flow rate through the sensitive volume of
the detector to complete the detection experiment more quickly.
[0256] When a flowing fluid enters the magnetic field, the magnetic
field causes some of the nuclei in the fluid to align with the
field. The magnetic gradient causes the nuclei within the fluid to
resonate at different frequencies depending on their location
relative to the magnetic field and the magnetic field gradient. By
altering the frequencies at which the nuclei in the fluid resonate,
the magnetic gradient establishes the spatial resolution of the
detector, which permits the detection of an entity within an
identifiable section of the microcoil. In an example implementation
of the methods of the invention, the conduit is positioned such
that a long axis of the conduit is parallel with the direction of
the gradient. However, the conduit can be disposed elsewhere within
the field of the magnetic gradient generator provided there is
sufficient gradient component along the conduit to spatially
resolve magnetic resonance events along the conduit.
[0257] The methods also comprise processing a signal received from
the microcoil to detect an entity in the fluid. The combination of
the magnetic gradient (if present) and the magnetic field causes
the field strength to vary along the length of the conduit, which
in turn causes the nuclei within the fluid to resonate at different
frequencies depending on the location of the fluid, permitting
identification of numerous contiguous signal producing volume
elements along the microcoil. Consequently, the microcoil receives
signals from the fluid at a plurality of frequencies, each
corresponding to the location of the fluid. By correlating the
frequency and magnitude components of the signals received by the
microcoil to the corresponding locations within the conduit,
measurements identifying the magnetic resonant behavior of the
fluid throughout the length of a portion of the conduit can be
obtained.
[0258] These measurements can be converted into a variety of
different representations of the movement of an entity though the
fluid, including but not limited to data array of frequency, and
position, vs time. The construction of a data array from the signal
received from the microcoil can be accomplished using a variety of
techniques, including but not limited to a Fourier transformation.
In an example implementation of the method, a Fast Fourier
Transform technique is used. Baseline correction techniques and
phase adjustments may also be used to augment the signal
processing. The signal processor may display the signal received
from the microcoil in processed or unprocessed form on a user
interface such as a monitor or screen.
[0259] Other data manipulations may also be particularly useful for
reducing the susceptibility to false positive and false negative
detections. If the data is acquired in a manner that allows subsets
of the signal detected by the microcoil to be assigned to different
positions within the conduit, a device implementing the method can
become self-calibrating. Further, the determination that an entity
is present or absent in a portion of the conduit may be based on
detecting a change in a magnitude component relative to a plurality
of other magnitude components. Comparing relative magnitudes may
reduce the probability that a global increase or decrease in a
processed signal that is unrelated to the presence or absence of an
entity in the fluid would trigger a detection event.
[0260] The processing of a signal may also include comparing data
from successive signal acquisitions. In embodiments that impose a
unidirectional flow on the fluid, an entity in the fluid may pass
through the detector from one end of the conduit to the other. By
comparing successive data acquisitions, the passage of an entity
through the conduit may be tracked. Further, in embodiments that
compare successive data acquisitions, data analysis techniques such
as correlation analyses may be used to reject false detections due
to electrical noise or other random fluctuations, because it is
unlikely that electrical noise or other random fluctuations would
mimic the appearance of an entity moving through a fluid in
multiple data acquisitions.
[0261] The processing of a signal may also include accumulating
signal peaks or pertinent signal features, including but not
limited to signal minima associated with a target, of a data array
representation of the signal received from a microcoil.
Accumulating signal peaks may allow multiple scans to be correlated
and combined to show the movement of an entity through the fluid. A
characteristic velocity of a signal peak that is related to a
radial position in a conduit of an entity can be used to accumulate
signal peaks across successive scans.
[0262] Motion correction methods may also be utilized to further
enhance the accuracy of the data acquired. In embodiments where the
fluid flow is relatively slow or static, spin-echo or
gradient-recalled echo images may be formed. In embodiments using
pulsed field gradients, the signal-to-noise ratio and accuracy of
the images may be further improved. Multi-dimensional imaging may
also be implemented with embodiments that use slow or static flows.
Embodiments that implement echo-based techniques may allow a
plurality of effects of a labeled entity's presence in a fluid to
be measured and imaged.
[0263] In embodiments with multiple branches and multiple
microcoils, various methods may be used to identify the magnetic
resonance behaviors within a given microcoil. For example, the
microcoils may be angled with respect to the magnetic gradient such
that each spatial element of each microcoil resonates at a
different range of frequencies. In another example, electrical
switching can be used to selectively monitor a microcoil for a
period of time. In another example, each microcoil may be coupled
to a dedicated signal processor.
[0264] In any embodiment of any aspect of the methods of the
invention where a resonant circuit is used, the methods may
comprise resonating the resonant circuit at low frequency, and
detecting resonance signals from the sample. "Low" frequency is the
frequency appropriate for detecting the desired resonance in the
magnetic field of the magnet chosen for the device.
[0265] The methods of the invention may further comprise
electrically coupling the microcoil to a tuning circuit, wherein
the tuning circuit comprises a tuning coil having an inductance at
least two times the inductance of the microcoil and a capacitor
coupled to the tuning coil to form a resonant circuit. Any of the
tuning circuits disclosed herein may be used as the tuning
circuit.
[0266] Further, any of the methods of the present invention may be
implemented by a computer program for use with a detector, such as
those disclosed herein. The computer program may be implemented in
software or in hardware, or a combination of both hardware and
software.
[0267] In a further aspect, the present invention provides physical
computer readable storage media, for automatically carrying out the
methods of the invention on a detector, such as those disclosed
herein. As used herein the term "computer readable medium" includes
magnetic disks, optical disks, organic memory, and any other
volatile (e.g., Random Access Memory ("RAM")) or non-volatile
(e.g., Read-Only Memory ("ROM")) mass storage system readable by
the CPU. The computer readable medium includes cooperating or
interconnected computer readable medium, which exist exclusively on
the processing system or be distributed among multiple
interconnected processing systems that may be local or remote to
the processing system.
EXAMPLES
[0268] Referring now to the figures, FIG. 1 depicts a portion of an
exemplary microcoil MRI detector 100. A microcoil 102 is
solenoidally shaped and wrapped around a conduit 104. The portion
the conduit 104 that is within the microcoil 102 is the sensitive
volume of the detector 100. The axis of conduit 104 is aligned with
a magnetic gradient 106. In a detection experiment, a fluid flowing
in direction 108 passes through the conduit 104 and the microcoil
102. The microcoil 102 is electrically coupled to both a tuning
circuit 110, and a signal processor 112.
[0269] FIG. 2 depicts a cross-sectional view of a portion of an
exemplary MRI detector 100. The microcoil 102 is wrapped around the
conduit 104 and disposed in a gap in a magnet 206. Magnet pole
faces 208 and 210 possess opposite polarity, and a uniform magnetic
field 212 is established across the gap in magnet 206.
[0270] FIG. 3 depicts three different example microcoil
constructions 302, 304, and 306. The first microcoil 302 is a
solenoidal coil wrapped around a conduit 308. The second microcoil
304 is a flat coil, positioned adjacent to the conduit and oriented
to place the axis of the second microcoil 304 perpendicularly to
the axis of conduit 308. The third microcoil 306 is a meanderline
coil place adjacent to the conduit 308.
[0271] FIG. 4 depicts a portion of an example detector 400 wherein
a conduit 402 contains three conduit branches 404a-404c. Microcoils
406a-406c are solenoidal in shape and are wrapped around conduit
branches 404a-404c. A fluid can flow into the conduit at a fluid
input 408, and flow out of the conduit at a fluid output 410. In
example detector 400, each of conduit branches 404a-404c and
microcoils 406a-406c may be independent. For example, each of
microcoils 406a-406c may be energized at a different frequency or
range of frequencies, which may permit detection of different
entities in each of conduit branches 406a-406c. However, microcoils
406a-406c may each be used in identical detection experiments,
which may further reduce the susceptibility of the detector to
false positive or false negative detections, by allowing for the
flow rate through the sensitive volumes of the conduits to be
reduced.
[0272] FIG. 5 depicts a portion of example detector 500 comprising
additional fluidic components fluidically coupled to a conduit 502.
An affinity column 504 may be used to trap and hold pathogens or
other entities until a detection experiment can be conducted.
During a detection experiment, a fluid containing a removal reagent
506 may flow past the affinity column 504, and cause pathogens or
other entities to enter the fluid. The fluid can then flow through
the section of the conduit 502 that is wrapped by a microcoil 508.
If an entity is detected by microcoil 508, a valve 510 can be
activated to divert the fluid into a sequestration chamber 512 for
storage or further analysis. If no entities are detected, the valve
510 may allow the fluid to exit the conduit through fluid output
514.
[0273] FIG. 6 depicts a portion of example detector 600 comprising
multiple affinity columns 602a-602c. At the output end of each of
the affinity columns, solenoidal microcoils 604a-604c are wrapped
around conduits 606a-606c. A valve 608 controls the flow of a
removal reagent 610 past the affinity columns 602a-602c. The use of
multiple affinity columns 602a-602c may permit additional
differentiation and increase the throughput of the detector. For
example, a detection method may provide ten levels of discernable
signal differentiation and use ten specific beads in each of the
affinity columns 602a-602c. By processing each affinity column
602a-602c separately, the flow through the microcoils 604a-604c is
multiplexed, permitting thirty distinct potential identifications
using only ten bead characteristic levels. Further, since three
affinity columns 602a-602c are used, a sample can be divided into
three portions, permitting the sample to be analyzed three times
more quickly than with a single column.
[0274] FIGS. 7a-7c depict three example schematic arrangements of
the tuning circuit 110 and the microcoil 102. In FIG. 7a, the
tuning coil 120 and the microcoil 102 are connected in series, and
a resonant circuit is formed with a tuning capacitor 122. In FIG.
7b, the tuning coil 120 and the microcoil 102 are connected in
parallel. In FIG. 7c, the tuning coil 120 and the microcoil 102 are
again connected in parallel, but the microcoil 102 is remotely
located from the tuning circuit 110 via a transmission line 124. In
an example detector, the transmission line possesses a length that
is an odd multiple (i.e. 1.times., 3.times., 5.times.,) of
one-fourth of the wavelength of the alternating current induced in
the microcoil during resonance.
[0275] FIG. 8 depicts an example module 1100 wherein a fluid is
contained in a sample chamber 1112. The sample chamber 1112 is
fluidically coupled to a bead chamber 1114 that contains beads that
can be affixed to a target entity in the fluid. A conduit 1116 is
fluidically coupled to the bead chamber 1114, passes through a
microcoil 1118, and is fluidically coupled to a valve 1120 after
passing through the microcoil 1118. The valve 1120 is fluidically
coupled to a sequestration chamber 1122 and a exit reservoir 1124.
If during a detection experiment the microcoil 1118 detects a
magnetic resonance in the fluid that indicates the presence of a
target entity, the valve 1120 can be activated to divert the fluid
into the sequestration chamber 1122. If no target entity is
detected, the valve 1120 can be activated to direct the fluid into
the exit reservoir 1124. All of components of module 1100 are
mechanically coupled to a substrate 1126, which provides structure
support and maintains the relative position of the components on
the module 1100.
[0276] In an exemplary embodiment, a microcoil with an inner
diameter of 170 microns and a length of approximately 1.1 mm is
wound around a conduit. The conduit and microcoil are disposed in a
magnetic field with a strength of about 1 Tesla, generated by a
permanent magnet, and a magnetic gradient of 0.07 G/mm is applied
along the long axis of the microcoil. As fluid is passed through
the conduit, the data acquisition techniques disclosed herein are
applied to detect the presence of a labeled entity in the
fluid.
[0277] The example device was used in a successful implementation
of the method. A fluid consisting of Magnevist doped water
(T1.about.430 ms) and dilute magnetic beads (5 micron Bangs beads),
was disposed in the conduit, and by following a method of the
present invention, a bead was positively identified. FIG. 9
contains a time series of images, offset for clarity, in which the
bead appears as a dip in the profile as indicated by the arrow.
[0278] FIG. 10 depicts the full time course of a detection
experiment as a contour plot. In FIG. 2, the positively identified
bead appears as a linear feature moving diagonally up and to the
left across the centrally located band. The centrally located band
depicts the location of the sample volume.
[0279] FIG. 11 depicts a data set from a similar detection
experiment using the example device and a method of the present
invention. A positively detected bead is visible as dark bands
trending to the lower right within the light band.
Example 2
[0280] There are many species of bacteria, and within each species
there are also many strains. Of the virulent bacteria some strains
are more virulent than others. Additionally, some strains of a
particular species can be resistant to antibiotics, for example
MRSA (Methicillin-resistant staph. aureus) and GBS. Antibodies are
available or can be produced that provide strain-specific labeling
of bacterial pathogens. Information of this nature may be useful
for the identification of the appropriate antibiotic for clinical
use based on the identification of species or strain of a
bacterium.
[0281] There are also antibodies that are `pan-bacterial` and that
are specific for a particular antigen that is indicative of some
subset of bacteria, for example, all strains in a species, or all
of a particular group that shares a common antigenic
characteristic, or for example, all Gram-positive (G+), or all
Gram-negative (G-) bacteria. In this way a test using the methods
of the invention provide for the screening of blood such that the
presence of any Gram-positive or Gram-negative bacteria is
detected. Further variations include, but are not limited to:
[Gram-positive, Gram-negative, Fungi]; or [Gram-positive,
Gram-negative, Fungi, mycobacteria]; or some set of these plus a
particular protein or multiple proteins, which are indicative of a
particular disease state or other important clinical
information.
[0282] A particular label, with a certain signal characteristic,
can be applied to all bacteria or other pathogens that are
susceptible to a particular antibiotic, and thus a characteristic
signal in a test sample from a patient identifies the patient as
one that would benefit from use of a particular antibiotic. This
labeling approach, to label a set of pathogens with a particular
property or characteristic, may be accomplished by attaching the
several antibodies to all beads, or have a single type of antibody
attached to a bead, and then mixing the several single-target beads
into a mixture bead solution. It is possible to apply more than one
antibody to a label bead, for example pan-[G+] and pan-[G-], or
some specific set of antibodies that make up a desired set, for
example all prevalent G+ bacteria, or all bacteria susceptible to a
particular antibiotic, or an antibody for each of the most
prevalent fungi. This may be desirable in order to reduce the
background bead concentration, as compared to a concentration of
beads for each specific targeted organism.
[0283] In a further embodiment, the G+ label solution contains
beads with multiple antibodies attached, for example 10 different
antibodies on each bead, or 10 types of beads with each subset of
beads having only one of the 10 antibodies attached. Alternatively,
beads are labeled with a pan-bacterial antibody may be mixed with
additional beads labeled with other antibodies, to facilitate the
labeling of particular targets. This embodiment can be used, for
example, where the pan-bacteria label does not adequately attach to
some subset of the pan-bacteria set.
[0284] Exemplary sample fluids that can be used include, but are
not limited to biological specimens such as blood, urine, CSF, or
other fluids; swab samples from skin, wound, or other body sites
may be tested by washing to entrain the sample from the collector
into a fluid; and semi-solid samples, such as fecal samples, can
also be processed by appropriate sample dilution in a liquid.
[0285] Using the methods of the invention, it is possible to
determine if a specific bacterium is present in a sample fluid, by
attaching antibodies specific for a specific bacterial strain to a
bead with a particular signal characteristic. For example, a bead
with a particular amount of iron can be used, which produces a
particular signal level. In this way, the presence of many
different types of bacteria may be determined in a sample, and the
presence of a particular bacterial type, or many types can be done
in a single measurement cycle. The number of different types of
bacteria that may be detected depends on the number of discrete
detection levels that may be determined given the signal
discrimination of the detection process, dictated for example by
the signal-to-noise ratio and the dynamic range of the measurement.
For example 10 individual signal levels may be detected, such that
10 bead types (or ten distinct multiplicities of bead labels) may
be identified, and thereby 10 different types of bacteria may be
identified. Labeling of different entities may also be
differentiated by the number of labels attached to each, rather
than a selection achieved via specific binding.
[0286] In order to increase the number of levels detected, it is
possible to divide the test sample, for example 10 ml, into several
subsamples, for example 2 ml each, and have 5 separate labeling
stages to provide for identification of 50 distinct individual
bacteria, in this particular example. The separate testing of each
individual subset might be done by having a separate detection coil
for each subset, or have valves that sequence the subsamples
through the detection coil.
It is also possible to use a multiplicity of differently-sized NMR
coils, or different flow rates, or different magnetic fields
optimized to detect different magnetic labels. The fluid under test
can be made to pass through each detector sequentially, so that a
multiplicity of distinct entities can be detected.
[0287] One example of a testing scenario on human blood is:
1) Run Screen Module for G+,G-, Fungi
[0288] 2) Example test result: G+, 4 CFU/ml; Fungi, 8 CFU/ml
3) Run G+ Identification Module
4) Example Result; MRSA, 4 CFU/ml
5) Run Fungi Identification Module
[0289] 6) Example Test result: Candida albicans, 8 CFU/ml In
another example, all tests are run from a single module, wherein
the subtests run are dependent on the preceding results:
[0290] 1) Load sample, 10 ml
[0291] 2) Run 2 ml through G+, G-, Fungi subsection
[0292] 3) Example Test result: G+, 4 CFU/ml; Fungi, 8 CFU/ml
[0293] 4) Run 2 ml through G+ subsection
[0294] 5) Example Result; MRSA, 4 CFU/ml
[0295] 6) Run 2 ml through Fung subsection
[0296] 7) Example Test result: Candida albicans, 8 CFU/ml
[0297] In this way the number of possible identified species or
pathogens depends on the number of subsection tests, and the number
of discrete detection levels that may be produced. The module may
also be designed to perform more than one detection protocol in
parallel. For example, the device might rapidly test the sample for
the presence of high concentrations of targets, perhaps
sequentially, while at the same time initiating a longer search for
very rare targets of the same or of a different nature. This
parallel processing may proceed through the use of multiple coils
or a single detector coil, operated in a multiplexed way.
[0298] An additional feature of the detection process is the
measurement of the concentration of the pathogen, for example
bacteria, such that the nature, status, and progress of the
infection may be determined and monitored using test samples
obtained from a given patient at different time points.
[0299] In addition to detecting pathogens, molecular targets such
as proteins may be detected in the device. Individual molecules can
be detected if they have a bead attached, since the detection
amplification process for molecules is identical to that of cells
or organisms. Protein concentrations can also be measured in a
concentration mode of detection. Any other entity to which an
antibody can be raised may also be targeted, for example viruses
(HIV, HepA,B,C) or Prion diseases (vCJD, TSE). The device may be
used to detect mammalian cells, for example cancer cells, in either
rare entity or concentration mode. Thus the technology may be used
to detect, for example, bladder cancer cells in urine, or
circulating cancer cells in blood, or fetal cells in maternal blood
or amniotic fluid.
[0300] While operating in rare entity detection mode, the device is
able to sequester the detected targets into a very small volume
(few to 100 mL, or 1 uL, etc.). This collection of targets will
thereby be concentrated. The collection volume can be small enough
to enable the rapid location of the targets on a microscope slide,
or the rapid processing of only the small volume in subsequent
processes or devices. The collected, concentrated targets may be
further analyzed in the device, either in the coil used to detect
them in the first place, or in a separate detection circuit or
method. Alternatively, the collection volume may be located on the
module in such a way as to facilitate the introduction of that
volume into a second magnet and detector system optimized to study
the volume as a stationary NMR sample. These further NMR studies
may be precise measurements of relaxation times, spectroscopic
identification of objects or molecules, including the
identification of properties or features of the labels that were
not discernable during the rare entity detection phase of the
device's operation.
[0301] The clinical application of the methods and devices of the
invention may be to take the place of blood culture, which is
presently the method used to determine if there are pathogens in
blood. Blood culture takes a significant amount of time to produce
a result, which nevertheless exhibits a high degree of variability,
has a significant rate of false negative, and only determines the
presence of a pathogen, not the identity of the pathogen. Further
identification steps must be employed to determine the actual
pathogen type, including steps performed by a person. For example,
a technician will take a sample from a positive blood culture
bottle, do preparation and staining of the sample, and do a
preliminary qualitative analysis of the pathogen (ie: "Gram-rod",
or "Gram+cocci"). Further identification and testing for antibiotic
susceptibility must be performed in additional processing steps and
instruments. It is common for the process to take 1 to 2 days, and
it is possible for the process to take more than 5 days in some
instances. A device based on the disclosed technology can provide
for the detection of pathogens and also the identification of the
pathogen. Because there are antibodies specific to strains that are
indicative of antibiotic resistance, the disclosed technology is
capable of doing detection, identification, and susceptibility
determination (through strain identification) of pathogens, e.g.
bacteria. Thus the technology can provide a clinician with a result
that allows the timely identification of the optimal antibiotic for
the pathogen that is present in the sample.
[0302] Such a device would be very useful clinically, as the
progression of infectious diseases is frequently much faster than
the time periods associated with blood culture, or culturing of
other samples. For example, bacterial meningitis is identified
through the examination of a sample from the CSF. Although these
samples may be cultured, bacterial meningitis has such a short
mortality period that culture is unable to be used for detection
and identification. Staining and microscope examination of the
sample must be performed, and this is not an optimal way to detect
and identify bacteria. GBS testing of women about to experience
child birth is another example where the swab sample is not able to
be cultured before the result is required for clinical use.
[0303] In these cases, and many others, the disclosed device is
able to produce detection and identification in a time that is
appropriate to the clinical use.
[0304] The disclosed technology is also able to detect proteins
that are associated with pathogens, or with the response of the
host to pathogen or inflammatory responses that are associated with
a disease, or an infection, or some other indication of the status
of a disease or health in general. There are for example many host
response proteins that may be indicative of the development and
progress of sepsis. Although the relevance of these proteins is
being debated at this time, the disclosed technology can detect
these proteins, through magnetic labeling and detection, and thus
identify and quantify the protein in the tested fluid. There may
therefore be a correlation determined such that a particular
combination of proteins, or their concentrations are indicative of
a particular disease state or progress. Or the concentrations of
the proteins taken also with the detected pathogen may indicate a
particular disease state or progress. Or some set of parameters,
such as other clinical information and diagnostic assays or
measurements, taken with the concentration of proteins, and/or with
the identified pathogen, may indicate a disease state or progress.
This may include the time dynamics of said measurements. The device
is be capable of detecting proteins at significantly lower
concentrations than other techniques. In principle, one molecule in
10 mL is possible. If more than one bead can be attached to the
protein molecule, then a homogeneous assay approach for protein
detection is possible.
[0305] One embodiment of the technology involves the use of
affinity chromatography to concentrate the target entity, be it
bacteria, cells, viruses, proteins, protozoa, prions, nucleic
acids, or other biological material of interest, on a column
modified to contain a capture agent which recognizes the target.
For example, a protein (bovine serum albumin; BSA) is modified with
a hexa-histidine peptide tag. This protein can then be bound to
either a nickel-agarose, or a cobalt-agarose column where the
histidine residues recognized the nickel or cobalt ions and bind
the protein to the column. The column is then probed with magnetic
particles to which an anti-BSA antibody is attached. The particles
bind to the immobilized BSA. To release the bound particles, the
column is eluted with imidazole, which competitively interferes
with histidine binding to the metal ions. The result is that the
Bead-Ab-BSA complex is released from the column in a very small
volume. This released material then enters the NMR microcoil and
can be detected by means of the magnetic bead's effect on the
transverse (T.sub.2*) relaxation time of the background water in
the sample. Similar performance can be achieved if the column
contains a bound target protein that recognizes, for example, sugar
groups, such as a lectin, of which Concanavalin A is a prototype.
The column-bound material can then be released by eluting the
column with the sugar to which the lectin bound, in the case of
Concanavalin A, this release agent is glucose. There are many
lectin/sugar combinations which can be used for this purpose. Other
affinity media include antibodies from one species which are
directed against antibodies from a different species, for example,
antibodies raised in goats can be recognized by rabbit, anti-goat
antibodies. Other affinity media include, but are not limited to,
streptavidin/biotin, protein A/antibodies, and dextran/glucose.
[0306] The column may be a matrix of material that has the antibody
for the target of interest attached to the matrix, for example a
specific target such as a protein, a fungus or bacterium or a
broader class of targets, such as all G+ or G- bacteria. As an
example, once the target has been attached to the column, the flow
of the sample may be stopped, and the column washed of excess
materials, and then a fluid containing magnetic label beads may be
flowed through the column. These beads may have the same antibody
as the column, or may have some other antibody. For example, if the
column has G+ antibody, the labels may have an antibody for MRSA,
and thus only attach to MRSA bacteria, if present. Or there may be
several different types of beads, for example each specific type
with a different characteristic signal and an antibody to a
different target G+ bacteria. In this way all G+ bacteria are
collected, and then the determination of the type of G+ bacteria is
made through the particular label antibody that causes specific
attachment of a particular bead type that has a detectable
characteristic signal.
[0307] The sample fluid may also be caused to flow through several
different affinity columns, in parallel, or more preferably in
series, in order to attach a particular pathogen type in each
separate column. For example three affinity columns might be used
in series, one for G+, G-, and fungi. The sample, for example
blood, is caused to flow through each in succession, and in a loop
several times, until the probability of attachment is acceptable.
Thereby if there is G+, G-, or fungus separately or together in the
sample, the pathogen is attached to the associated column. After
the attachment to the column, the sample flow may be turned off, a
wash step may be used to remove unbound material, and then a
solution with label may be flowed through the column. The bead
solution flowing through the column allows the beads to attach to
the pathogen for which the bead has antibody. Once the bead
solution has flowed through the column sufficiently to have an
acceptable level of labeling probability, for example in a loop,
the bead solution flow is stopped. Then the column is washed, and
subsequently eluted with a reagent that causes the attached
pathogen, with the label, to release from the column and pass
through the detection circuit.
[0308] The label solution, containing magnetic beads, can be flowed
through the columns in series or in parallel. The magnetic beads
may have a specific antibody attached in order to target particular
bacteria, as described herein. For example, there may be 30
different levels, 10 each of types of G+, G-, and Fungi. In this
way the solution containing the beads may be flowed through the
affinity filters in series, and the labels attaching to the
specific pathogen for which the bead has antibody.
[0309] In another embodiment, there may be 10 levels only of
discernable signal differentiation (for example based on signal to
noise of the detection method), and in this case 10 specific beads
may be used in each of the 3 affinity filters. The solution for
each column is processed separately, and the flow through the
detection circuit is multiplexed, such that the determination of
the type of pathogen includes the signal level from the bead, for
example level 7, and the column from which the solution originated,
for example, the fungi column. This provides 30 distinct
identifications, using only 10 bead characteristic levels.
[0310] It is also possible to have distinct layers in the affinity
column such that the target pathogens are attached to a specific
region. For example there may be 10 layers in a column with each
layer having a specific antibody associated with it (FIG. 12). The
bacteria would then attach to the associated region, and be
labeled. The release wash would then produce a flow of liquid
through the detection coil that has a certain profile, in time,
associated with the position on the column. A detection event from
the fluid associated with a particular level would indicate a
certain bacteria. Thus the time from wash flow initiation to the
detection event would associate the fluid to a particular region on
the column.
[0311] The layers could also be for a general type of pathogen, and
the bead labels provide further specificity. For example, there
might be three layers, one each for G+, G-, and fungi, and there
may be 10 magnet labeled beads, each with a specific signal
characteristics and antibody. One level is associated with 3
antibodies, one particular species or strain of G+, G-, and fungi.
With the pathogen attached to the layer, the bead solution is
caused to flow through the column, labeling the specific bead to
the target pathogen. When released, identification is achieved for
a particular pathogen by the signal level, and the time from the
release, which is related to the layer from which the bead
originated.
[0312] Additionally, the layered column might be held within the
detection coil, and thus the identification can be made by the
layer associated with the detected signal (FIG. 13). Using MRI
based techniques, the location of the signal may be determined, and
thereby the associated layer. As previously stated, a multi layer
affinity column may be constructed. In this case the label beads
may have a specific antibody, or general antibody, as long as the
pathogens of each layer are labeled sufficiently for detection. The
specificity is attained by the specific antibody associated with a
layer, or a position on the column, and a signal strength
associated with a particular label that has antibody for a specific
pathogen. For example, a affinity column layer may have a pan G+
antibody to localize any G+ bacteria, and then specific label beads
with characteristic signal strengths for the various G+ bacteria
would label the individual targets. Identification is determined by
the location on the column and the signal strength associated with
the labels for a particular G+ target bacteria.
[0313] If an affinity column step is not used, then it is preferred
that the magnetic beads are multivalent so that they cross-link
targets, or multiple beads are attached to a single target entity,
and the NMR signal arises from the differential effect of the
aggregated vs. dispersed magnetic beads on the surrounding
water.
Blood Safety
[0314] The screening of donated blood in a blood bank requires the
detection of small amounts of a number of potential pathogens.
Bacterial contamination is acknowledged as the most frequent
transfusion-transmitted infection of all blood components. However,
parasitic, viral, host-leukocyte, and prion contamination are also
important players in the transmission of disease via transfusion.
The disclosed methods and devices of the invention can provide
ultrahigh-sensitivity detection of each and every one of the
following blood-supply threats.
[0315] Bacteria. Currently, the human blood supply is subject to
several safeguards, however, despite a battery of tests for
pathogens, only a single bacterial strain, Treponema pallidum, the
bacterium that causes syphilis, has historically been selected for
routine screening. Due to the absence of bacterial-monitoring,
transfusion-associated bacterial sepsis is estimated to occur in 1
case per 6 contaminated units transfused, with fatality occurring
in up to 25% of these cases. In the UK, data from the 1996-2001
Serious Hazards of Transfusion (SHOT) study indicated that 60% of
all reported cases of transfusion-transmitted infection were
attributed to bacterial contamination. In France, data from the
Haemovigilance Network reported 15% of transfusion-related deaths
between 1996-1997 were due to bacterial contamination. Although
automated culture methods are available for detecting the presence
of different classes of bacteria, these systems have limitations,
including sampling errors, false-positive and false-negative
results, and long lead times of more than 48 hours for
reporting.
[0316] Bacteria are a particular problem in platelet concentrates
because of their storage at room temperature, which favors rapid
bacterial growth. In addition, bacteria are an under-reported
problem in platelet transfusions. The estimated incidence of
bacterial contamination is 1 in 2000 random donor or apheresis
platelet units.
[0317] Parasites. In addition to bacteria, other blood supply
threats include parasites. Detection tests are not yet developed
for the presence of any recognized parasites in donated blood. With
the increase in global travel and migration over the last 20 years,
blood-borne parasitic diseases such as malaria, Chagas' disease and
Leishmaniasis have become more common in blood transfusions.
Parasite screening tests are not routinely used, and
notoriously-unreliable donor interviews remain the primary defense
against such infections.
[0318] Globally, malaria is estimated to infect more than 400
million people every year in tropical regions. This disease,
transmitted through blood transfusion, accounted for 91 cases in
the US between 1963 and 1999. Currently, blood donors who have
visited areas in which malaria is endemic may be deferred from
donating blood for six months.
[0319] Trypanosoma cruzi, the causative agent of Chagas' disease,
is endemic in much of South America. Although the disease can be
transmitted through blood transfusion and has been documented in
the US and Canada, routine screening of blood for T. cruzi is not
performed. Studies in Los Angeles, Calif. have indicated that in
some regions as many as 1 in 7,500 blood donors show evidence of
infection.
[0320] Leishmaniasis, caused by the Leishmania sp., is a serious
illness endemic to subtropical regions. Recent studies of
Leishmania/HIV co-infection have revealed that the true prevalence
of Leishmania in Spain and Southern Europe may be under-reported
and the parasite can pose a serious health risk to
immunocompromised patients.
[0321] Viruses. Not only bacteria and parasites, but also viruses
and prions contaminate the human blood supply. Viral safety is also
an important concern for manufacturers of biopharmaceutical
products. Although as many as 25 known viral pathogens may be
present in blood, only a handful of viruses, including HIV, HCV,
and HBV, are required to be screened after collection. The need for
scrutiny has not kept up with the infection incidence. For example,
over the past two decades, Table 1 shows the new viral threats to
the blood supply that have been identified.
TABLE-US-00001 TABLE 1 Year of detection of Viruses. Year Virus
1980 HTLV-1 1982 HTLV-2 1983 HIV-1 1986 HIV-2 1986 HDV 1989 HCV
1990-91 HEV 1994-96 HHV-8 1995-96 HGV 1997 TTV 1999 SEN-V 2002 BDV
2002 WNV 2003 SARS*
[0322] One of these, West Nile Virus (WNV), provides a notable
example of the ability of pathogens to migrate across continents
and emerge as threats in new regions. Over the last 40 years, WNV
has caused sporadic outbreaks of human disease in Europe. The most
recent European outbreak in humans occurred in 1996-7 in Bucharest,
Romania, and involved more than 500 recorded cases, with a fatality
rate of .about.10%. Moreover, since 2002, a new strain of this
virus has spread rapidly through the United States, with 12,375
cases of human WNV infection reported by Feb. 18, 2003, and 466
associated deaths. Furthermore, as of April 2003, it had been
confirmed that 23 cases of WNV illness had developed in patients as
a result of infection from a blood transfusion or organ
transplant.
[0323] Although most of the routine screening tests in common use
are directed against viral pathogens, the blood supply still
remains vulnerable to many known viral diseases. Cytomegalovirus
(CMV) is such a common infection that it is not routinely screened
due to its high prevalence (from 40-100% in Western adult
populations). However, CMV can pose a serious threat to
immunocompromised individuals, who receive CMV-seronegative blood
units from a limited supply. Although leukoreduction has been
proposed as an alternative to the use of CMV-seronegative units,
some studies have indicated that leuko-filtration is not completely
effective at preventing transmission of CMV. Parvovirus B19 is
another virus which is widespread (up to 70-90% estimated
seroprevalence in adults), and is not included in the panel of
blood bank screening tests. B19 infection can be hazardous for
certain populations such as those who are immunocompromised, sickle
cell/thalassemia patients, and pregnant women.
[0324] Prions. Prion diseases, such as Transmissable Spongiform
Encephalopathy (TSE) and variant Creutzfeldt-Jacob disease (vCJD)
are caused by the misfolded prion protein (PrPSc). Prion diseases,
can spread through the food supply, organ transplants, contaminated
medical instruments, the blood supply, or pharmaceuticals made from
animal products. Recently, two patients died of variant
Creutzfeldt-Jacob disease from blood transmission from an
asymptomatic infected individual. A rapid blood screening test for
prion diseases is needed. Prion peptide reagents, that bind the
pathogenic prion protein isoform, PrPSc, have been coated on
magnetic beads to effectively capture and concentrate human PrPSc
from brain homogenates. Rapid blood screening tests of PrPSc from
serum can use the prion-specific antibody 15B3. This magnetic prion
assay can achieve the sensitivity and throughput required for
screening human blood samples.
[0325] Donor Leukocytes. The presence of viable donor leukocytes in
donated blood has long been recognized as a source of risk for
recipients. Transfusion-associated graft-versus-host disease
(TA-GVHD) caused by proliferation of donor T-cells in the recipient
may carry a mortality rate as high as 84%. In addition, cytokines
released by leukocytes during unit storage can cause febrile
non-hemolytic transfusion reactions. Leukocytes can also harbor
latent viruses such as CMV and HTLV. If one detects leukocytes, one
can utilize current safeguards against leukocytes. These often take
the form of gamma irradiation, which is used to inactivate donor
leukocytes. Leukoreduction is another standard tool for reducing
risk to recipients, and leukofiltered units contain a much-reduced
number of white cells.
Example 3
Construction Methods
[0326] Winding coils--As our goal is to construct small portable
NMR systems at reasonable cost, we seek a method for manufacturing
microcoils that is significantly simpler than the focused ion beam
techniques used to construct the coil for our first experiments.
The classic coil construction method is "hand-winding" using
standard (usually enameled) wires. A simple gear-synchronized
device for accurately winding very small coils on pulled pipette
tips has been previously described [17]. The main requirements of
any such device are methods for holding the sample tube and
rotating it and for controlling the position of the very fine wire
as it is taken up on the tube. We find that these requirements are
readily met through the use of a miniature lathe [Taig Tools,
Chandler, Ariz.] in conjunction with optical fiber chucks [Newport]
appropriately sized to hold micro-capillary tubes [Vitrocom]. The
tube is mounted to the headstock of the lathe using the fiber
chuck. One end of the wire is taped to the fiber chuck while the
other is taped to a support whose position is controlled by the
saddle and cross-feed of the lathe. A 7.times.-30.times. dissecting
microscope is used to visually monitor the coils as they are wound
by gradually turning the headstock and repositioning the cross-feed
of the lathe by hand. The capillary tubes are very flexible, and
their deflection while winding aids in maintaining the proper
tension on the wires [typically 50 gauge enameled copper,
California Fine Wire]. When finished, the coil is secured to the
tube using standard five-minute epoxy. With minimal practice, coils
can be wound in about 15 minutes.
[0327] One key to this rapid coil construction lies in our choice
to close-wind our coils. Generally, NMR practitioners assume that
coils should be wound with some space between the turns [25,27] in
order to avoid strong proximity effects that can increase the
high-frequency resistance of the coil windings, leading to excess
electrical noise. However, we are operating our coils at rather low
frequencies, low enough that the diameter of our wire is only about
2.5 times larger than the radio frequency (RF) skin depth.
Furthermore, the enamel layer on our wire is about 6 .mu.m thick,
yielding a measured 37 .mu.m turn-to-turn spacing. (The diameter of
50 gauge bare copper wire is 25 .mu.m.) Under these conditions, the
RF resistance is enhanced over the DC resistance by 4% due to the
skin effect and only an additional 4-11% due to the proximity
effect [1]. On the other hand, a close-wound coil has the highest
possible pitch and hence the highest signal detection sensitivity.
The high performance of close-wound micro-coils has been
demonstrated experimentally [17]; the SNR was found to be maximal
for minimal turn spacing. However, as discussed herein, this
practice has been discouraged by NMR practitioners as leading to
strong proximity effects that ultimately lead to excess noise [25,
27]. One practical benefit of close winding is that it greatly
reduces the need to accurately control the wire during coil
construction.
[0328] Designing the probe circuit--In our earlier work with
microcoils, we introduced the counterintuitive idea of using a
large, fixed-value auxiliary inductor in the resonant circuit of
the probe. While not strictly necessary for the "large" (550 .mu.m
diameter) coil in our earlier work, such an auxiliary inductor is a
practical necessity for smaller coils operating at low frequency.
There are at least two benefits of the auxiliary inductor. Very
small sample coils cannot be resonated at the low operating
frequencies typical of permanent magnet based NMR devices without
the use of very high capacitances. Either a physically large
variable capacitor (contrary to the goal of miniaturization) or a
large amount of fixed capacitance in parallel with a variable
capacitor (which reduces the tuning range, awkward given the field
drifts of permanent magnets) would be required. Furthermore, it is
difficult to construct a resonant circuit for a microcoil without
introducing paths for circulating currents whose inductances are
larger than the branch of the circuit that contains the microcoil.
This is especially true for circuits that contain more than one
tuning capacitor in parallel. Under such conditions, the circuit as
a whole may resonate at the desired frequency, but the current
paths in this resonant mode will largely avoid the branch of the
circuit containing the sample coil. An auxiliary inductor, placed
in series with the microcoil, both forces the resonant current to
flow through the sample coil and reduces the need to include
numerous capacitors and their multiple, competing current
paths.
[0329] The auxiliary inductor raises the Q of the probe circuit.
For high resistance microcoils, the resulting Q is still modest
(.about.10-20), rendering the electrical resonance easy to detect
without presenting difficulties with respect to probe stability or
excessive ring-down.
[0330] Proper design of the auxiliary inductor insures that it does
not degrade the SNR performance of the probe over what
theoretically can be achieved without the auxiliary inductor. One
key idea is that the auxiliary inductor should not contribute to
the resistance of the resonant circuit. It is stray resistance that
degrades the performance of NMR circuits, not stray inductance.
[27] Once the microcoil has been wound, its RF resistance can be
measured or calculated, and this value serves as the starting point
for auxiliary inductor design. Ideally, the RF resistance of the
auxiliary inductor should be much less than that of the microcoil.
If the inductor adds 10% to the circuit's RF resistance, the SNR is
reduced by 5%. To minimize its resistance, the auxiliary inductor
is wound using large diameter wire. For our probes, we typically
choose 14 gauge copper wire (diameter=1.63 mm). Given that the
current is carried only at the outer surface of the wire, we can
calculate the longest piece of such wire consistent with our goal
of low resistance. It can be shown that the maximal inductance that
can be constructed from a wire of length l.sub.wire is achieved
with a coil of radius:
r coil = 25 l wire kd wire 46 .pi. ( 1 ) ##EQU00002##
where d.sub.wire is the wire diameter, and kd.sub.wire is the
turn-to-turn spacing (k=1.3, typically). Our microcoils have RF
resistances that are typically 0.2-1.0.OMEGA. at our operating
frequency (44 MHz), and the calculated auxiliary inductors are
conveniently sized (radius 0.3-0.6 cm, 2-4 turns).
[0331] With the auxiliary inductor properly constructed, we build
the remainder of our NMR probe circuit. For example, a small
rectangle of acrylic sheet (roughly 4 cm.times.3 cm.times.0.7 cm)
can serve as a mechanical support. Small pieces of adhesive-backed
copper foil are cut and placed on the acrylic block so that the
sample coil, auxiliary inductor, and tuning and matching capacitors
can all be mounted on the block. The capillary tube on which the
sample coil is wound is glued into two lengths of flexible tubing
(we have used PEEK and Radel) using standard five-minute epoxy.
This structure is in turn glued to the acrylic block so that the
fragile capillary tube is isolated from mechanical shocks
transmitted along the flexible tubing. The auxiliary inductor and
capacitors are soldered to the copper foil, and the
acrylic-block-mounted module is then installed in a shielded probe
body (we have used cast aluminum boxes, or plastic boxes covered
with copper foil). As we show herein, these very simple and crude
probe construction techniques yield NMR detectors that achieve
optimal performance.
Results
[0332] Using the methods described herein, we have wound a series
of microcoils of different sizes, as detailed in Table 1.
TABLE-US-00002 TABLE 1 Microcoil probe parameters. Sample Sample
Capillary Length Calculated diameter volume OD/ID (.mu.m) Turns
(.mu.m) inductance (nH) (.mu.m) (nL) 170/100 4 148 3 100 1.2
250/150 5 185 7 150 3.3 330/200 6 222 13 200 7.0 550/400 17.5 648
120 400 81
[0333] We designate the capillary size with two numbers, the outer
diameter and inner diameter of the tube, in micrometers. The length
of the coil is calculated from the number of turns and measured
total wire diameter of 37 .mu.m. The inductance is calculated using
the standard formula. [25] The three smallest coils were built into
probe circuits that contained an auxiliary inductor: 3 turns of 14
gauge bare copper wire with a 14.5 mm diameter (calculated
inductance 150 nH). The probe circuit for the largest coil did not
employ an auxiliary inductor.
[0334] To determine the SNR, we performed a single-pulse experiment
and acquired a single two-channel free induction decay (FID)
signal. For all of our experiments, we used a 1.04 Tesla permanent
magnet with a 5 cm clear gap equipped with linear gradient/shim
coils. This is hardly a portable magnet; however it has the same
field as our 0.6 kg 1 Tesla magnet, and the much larger gap
facilitates rapid detector prototyping. The NMR console was a
compact imaging system from MRTechnology (Japan). The sample fluid
was Magnevist(Gd)-doped water (T.sub.1.about.430 ms) delivered by
syringe though the tubing attached to the capillary. To determine
the SNR, the detected signal was corrected for baseline offset and
then mathematically adjusted to be on resonance with all of the
signal power in one of the channels.
TABLE-US-00003 TABLE 2 Measured performance and resistance of
microcoils. Observed Detector Line width Resistance Power SNR
bandwidth (Hertz) (.OMEGA.) (.mu.W) Probe (.+-.10%) (Hertz)
(.+-.10%) (.+-.0.01 .OMEGA.) (.+-.15%) 170/100 8.6 .+-.250 1.5 0.43
7.6 250/150 38 .+-.250 1.5 0.44 14 330/200 63.5 .+-.250 2.8 0.57 20
550/400 485 .+-.250 5 1.61 25
[0335] The noise value was taken to be the standard deviation of
the data in the baseline region, the last 50% or so of the data
set. The signal was determined by extrapolating the FID back to
zero time. Extrapolation was necessary because our narrow bandwidth
filters ring for a significant time after the RF pulse. (Our
console, designed for imaging, is not optimized for operation at
our narrow bandwidths and is not equipped with a adjustable
receiver muting circuit.) The measured SNR values are given in
Table 2 and have an estimated uncertainty of 10%, which comes from
both the extrapolation and from the uncertainty in the standard
deviation of the baseline data.
[0336] The minimum achieved line width (FWHM of a Lorentzian fit to
the Fourier Transform of FID data) is also given in Table 2. The
probe was carefully positioned in the most homogeneous location in
the magnet, and the linear shims were adjusted to maximize the
lifetime of the FID (and optimize its shape). The shims typically
provided a 2-5 fold reduction in the line width. The line widths
reported in Table 2 are the narrowest we achieved. Typically,
careful placement of each probe into the same position in the
magnet allowed the line width to be reproduced to within 20%. Note
that the FID data used to determine minimum line widths were not
the same as those used to determine maximum SNR.
TABLE-US-00004 TABLE 3 Calculated resistance and SNR performance of
microcoil probes. Skin Auxiliary Calcu- R.sub.DC depth Proximity
R.sub.RF inductor Total lated Capillary (.OMEGA.) factor factor
(.OMEGA.) (.OMEGA.) (.OMEGA.) SNR 170/100 0.43 0.040 0.039 0.45
0.047 0.50 13 250/150 0.44 0.040 0.044 0.47 0.047 0.52 36 330/200
0.57 0.040 0.050 0.61 0.047 0.66 67 550/400 1.61 0.040 0.114 1.82
none 1.82 474
[0337] The DC resistance of the microcoils, given in Table 2, was
measured with a benchtop DMM (Fluke 8840A/AF) operating in two-wire
mode. The resistance of the microcoil together with the leads
connecting it to the rest of the probe circuit was measured. The
DMM lead resistance of 0.040.+-.0.005.OMEGA. was subtracted from
the measured values, which have an overall uncertainty of about
0.01.OMEGA.. The right-most column in Table 2 gives the RF power
required to produce the 100 .mu.s .pi./2 pulse in our experiments.
This power was determined by measuring the peak-to-peak voltage of
the pulse seen by the probe circuit by routing this pulse to a
50.OMEGA.-terminated oscilloscope. The uncertainty in the
measurement comes from the roughly .+-.10% uncertainty of reading
the oscilloscope screen.
Discussion
[0338] Comparison to original microcoil results--We have achieved
an SNR of 485.+-.50 with the 50 gauge wire-wound 550/400 microcoil
using a detection bandwidth of .+-.250 Hz. To facilitate comparison
to our previous focused ion beam (FIB) coil result, we re-analyzed
the previous data using the method described herein. (Henceforth,
we refer to this coil as the "FIB-coil.") The re-analyzed SNR value
for the FIB-coil was 38.+-.2 in a detection bandwidth of .+-.5000
Hz. At this bandwidth, our wire-wound coil should give an SNR of
108.+-.12. To account for the different lengths of the sample
volumes, we note that extending our wire-wound coil to the 2.1 mm
length of the FIB-coil would enclose 3.24 time more volume while
increasing the resistance by the same factor. We therefore scale
the SNR by {square root over (3.24)} and calculate that a wire
wound coil of the same size as the FIB-coil should yield an SNR of
194.+-.20. Hence, the wire-wound coil achieved a 5-fold
(5.1.+-.0.8) improvement in SNR performance over the FIB-coil. The
DC resistances of a wire-wound coil that is the same length as the
FIB-coil is 5.22.OMEGA., compared with the 5.42.OMEGA. of the FIB
coil. The pitch of the wire-wound coil is two times finer than the
FIB-coil. These two differences account for a factor of two
improvement in the SNR. The FIB-coil's probe circuit contained
other sources of resistive losses, including the 45 cm quarterwave
cable and the long (10 cm) leads between components in the two
shielded boxes. These resistances are eliminated in the present
design, which helps account for the observed 5-fold improvement in
SNR.
[0339] The line width achieved in the wire-wound 550/400 probe was
115 ppb, a factor of 2 worse than our FIB-coil result. In our
previous paper [26], we speculated that the narrow line had been
achieved due to the high cylindrical symmetry of the FIB-coil. Due
to the following experiments, we now believe that the main reason
was the relative lack of metal present in that coil.
[0340] Before winding the 550/400 coil described in Table 1, we
wound a 20-turn coil with 40-gauge enameled copper wire on a
550/400 tube. This coil was 1.5 mm long and contained a sample
volume of about 190 nL. The turn-turn spacing was .about.80
microns, nearly the same as the FIB-coil, while the volume was
about 3/4 of the FIB-coil's sample volume. However, the narrowest
line achieved in this 40-gauge probe was 1 ppm, nearly 20 times
worse than the FIB-coil. Since a narrower line had been achieved
over a larger sample volume in the FIB-coil, homogeneity of the
background magnetic field can be ruled out as a factor limiting the
line width. A series of modifications were made to this 40-gauge
probe in order to ascertain the source of the excess line
broadening. Guided by the original FIB-coil probe, in which no
piece of the probe circuitry or support structure came within 3 mm
of the sample volume, new versions of the 40-gauge probe were
constructed in which the support structures, electrical circuitry,
and fluid handling tubing were all moved well away from the sample
volume; no improvement in the line width was observed. The final
modification was to replace the 40-gauge with 50-gauge wire. This
50-gauge coil achieved the much smaller line width given in Table
2.
[0341] The 550/400 coil wound with 50-gauge wire yielded a line
width a factor of 2 worse than our original FIB-coil result, while
the 40-gauge coil was a factor of 10 worse than the 50-gauge coil.
These line width differences correlate very well with the
cross-sectional areas of the wires in each of the these coils, 4900
.mu.m.sup.2, 490 .mu.m.sup.2, and 325 .mu.m.sup.2 for the 40-gauge,
50-gauge, and FIB-coils, respectively. (The wire-wound coils were
copper. The FIB coil was gold, 3 times more diamagnetic than
copper, over a very thin chromium layer, which would compensate for
the gold to some extent.) Due to the skin-depth effect, only the
outer surface of the 40-gauge wire (an effective area of 250
.mu.m.sup.2) carries current, so most of the copper in that wire is
"wasted." This excess copper, although it would not affect the SNR
performance of the coil [Peck1995][1], affects the homogeneity of
the field in the sample volume. It appears that the presence of
copper near the sample volume should be minimized in order to
achieve the highest resolution, indicating that wires much larger
than the skin depth should be avoided. Of course, susceptibility
matching techniques [3,6] may be employed to overcome the
distortions of the field due to the wires, but these would
complicate our device.
[0342] Performance of smaller coils--As shown in Table 2, the
resolution achieved in the three smallest coils is extremely good,
approaching 30 ppb using only linear shims. Furthermore, the SNR is
high enough that the FID is visible in a single shot, even in the
smallest coil's .about.1 nL volume. It is clear that our probe
circuit design and simple, fast, inexpensive construction methods
produce useable NMR detectors. However, we may ask if these
detectors are performing optimally.
[0343] The SNR value we expect from these coils and probe circuits
can be calculated using [1]
SNR = k 0 ( B 1 / i ) .upsilon. s N .gamma. 2 I ( I + 1 ) .omega. 0
2 / 3 2 k B T 4 k B TR .DELTA. f ( 2 ) ##EQU00003##
where k.sub.0 is a constant that accounts for geometric effects
associated with irregularity of the B.sub.1 field produced by the
finite solenoid as well as non-uniform excitation of the spins in
the sample volume. (B.sub.1/i) is the efficiency of the coil in
generating B.sub.1, and for a solenoid it has the value
.mu..sub.0n, where .mu..sub.0=4.pi..times.10.sup.-7 Tm/A and n is
the turns per unit length. v.sub.s is the sample volume,
N=6.7.times.10.sup.28/m.sup.3 the number density of hydrogen nuclei
in our water samples, .gamma.=2.675.times.10.sup.8 radians/sec/T is
the gyromagnetic ratio for hydrogen nuclei, I=1/2 for hydrogen,
.omega..sub.0=2.78.times.10.sup.8 radians/s for hydrogen nuclei in
our field, T=297K, R the high frequency resistance of the LC
resonant circuit, and .DELTA.f the detection bandwidth. k.sub.0 is
the only parameter that cannot be measured or calculated easily. It
should have a value close to but slightly less than 1.0, and should
not differ substantially for our probes because they all have
similar coil and sample geometries. We set k.sub.0=1.0 in our
calculations. Our detectors differ mainly in their sample volumes
and resistances.
[0344] To calculate the expected SNR, we need to know the RF
resistances of each element of the LC resonant circuit. The other
parts of the probe, including the matching capacitor, the
connections to ground, etc., play a much smaller role in
contributing to losses in the circuit because the currents flowing
in these sections of the probe are much smaller than in the
resonant loop itself. We can calculate the resistances for each
element of the resonant circuit by measuring their physical
dimensions and considering that current will only flow in the
region within a skin depth (10 .mu.m in copper at our frequency) of
the surface. For the 50-gauge wire, we measure the DC resistance of
the microcoil and its leads; this value is given in Table 2 and
repeated in the 2.sup.nd column of Table 3 for convenience. We
convert this DC resistance to a calculated RF resistance by using
Tables 2 and 4 in Peck, et al., [1]. The RF resistance of the leads
is raised by the skin depth effect, the strength of which may be
found using Table 2 of Peck, et al. The factional enhancement to
the DC resistance is about 4% for our coils, as shown in the third
column of our Table 3. For the wires in a coil, the proximity
effect also plays a role. The strength of the proximity effect
depends on the number of turns and the coil aspect ratio, and can
be calculated using FIGS. 2 and 4 in Peck, et al. The 4.sup.th
column in our Table 3 gives the fractional enhancement to the DC
resistance due to the proximity effect in each of our coils. The
5.sup.th column in our table is the total RF resistance, calculated
by applying the enhancement factors to the measured DC resistance,
with the proximity effect applied only to the fraction of the wire
length in the microcoils. The calculated RF resistance of the
auxiliary inductor is given in the 6.sup.th column, and the total
RF resistance of the LC circuit is given in the 7.sup.th. We find
that the copper foil and the tuning capacitors contribute
negligibly to the resistive losses.
[0345] One is faced with a number of definitional questions in
linking measurements of absolute SNR to the theory [28,29].
Johnson's measurements were single-channel incoherent measurements
of sub-megahertz voltage signals. Our NMR spectrometer processes
the signal from the NMR coil to yield two channels of voltage data,
each derived from a coherent combining of the NMR voltage signal
with a local oscillator. In order to make the most direct
connection with the older measurements, we process our two-channel
signals by frequency- and phase-shifting them so that the entire
signal appears in one of the channels. From this point forward, we
treat this single channel as our entire signal, measuring the
signal level from its amplitude at zero time and calculating the
noise as the standard deviation of the values in the baseline, as
described earlier.
[0346] A second consideration applies to the definition of the
bandwidth .DELTA.f. In Nyquist's treatment, the rms voltage noise
is white (that is, it has equal powers in every frequency interval
.DELTA.f). The number 4 in the denominator of Eq. 2 is appropriate
for an ideal filter that passes (without any weighting) only
frequencies within the bandwidth .DELTA.f and excludes all others.
A single pole filter, with its Lorentzian transfer function, will
admit more noise, and the factor 4 should be multiplied by .pi./2,
assuming that .DELTA.f represents the FWHM of the Lorentzian. Our
filters are eighth order, so they closely approximate the ideal
filter.
[0347] The calculated SNR values in Table 3 agree very well with
the measured values in Table 2, indicating that we accurately
accounted for the losses in the probe circuit.
[0348] It is also possible to relate the SNR performance of the
system to its characteristics in transmit mode. [27] If the current
i that gives rise to B.sub.1 flows through the resistance R
responsible for the loss, then the power dissipated is P=iR.sup.2
and Eq. (2) may be recast in the form:
P = ( A .upsilon. s B 1 SNR ) 2 ( 3 ) ##EQU00004##
with A=2.31.times.10.sup.14 watt.sup.1/2 m.sup.-3 T.sup.-1 (for
k.sub.0=1, .DELTA.f=500 Hz). A 100 .mu.s .pi./2 pulse corresponds
to B.sub.1=0.59 G=5.9.times.10.sup.-5 T. Using Eq. (3), the sample
volumes v.sub.s from Table 1, and the measured SNR values from
Table 2, we calculate expected RF power requirements of 9.1 .mu.W,
14 .mu.W, 23 .mu.W, and 52 .mu.W for our four probes. The agreement
with the measured values given in Table 2 is within uncertainties
for the three smallest coils. Hence, the SNR measurements are
consistent with the transmit performance of the probe, indicating
that the SNR values are not corrupted by excess noise in our
receiver. For the largest coil, the agreement is not quite as good,
even when the uncertainties in the measured SNR and power are taken
into account. We do not currently have a detailed explanation for
this, except to note that the largest coil is substantially larger
than the others and has a correspondingly much higher SNR.
[0349] Table 3 shows that our auxiliary inductor does not decrease
the SNR performance unacceptably. We see that in the probes tested,
the auxiliary inductor accounts for about 10% of the resistive
losses, leading to a 5% decrease in the SNR that might be achieved
without it. Removing the inductor therefore will not make a very
large improvement in the SNR performance of these probes. However,
without the auxiliary inductor, the probe circuit is much more
difficult to construct and operate.
[0350] Building and testing a better probe--The agreement between
the observed and calculated SNR in our first set of probes
indicates that we have a good quantitative understanding of the
factors contributing to the SNR performance. We now ask if we can
improve the SNR.
[0351] This is equivalent to asking if there are stray resistances
that can be removed. The first set of coils had rather long
connections between the microcoils themselves and the remainder of
the probe circuit, in order to avoid the homogeneity-degrading
effects of large conductors too near the sample. Since these
connections are made with the narrow gauge wire, they contribute
substantially to the resistance (and hence noise) in our detector,
without contributing signal. We have measured the lead lengths of
our coils and then used the results of Peck, et al. [1] to
calculate the separate contributions of the microcoils and the
leads to the resistance of the LC circuit. The values are given in
the 2.sup.nd and 3.sup.rd columns of Table 4. The 4.sup.th column
gives the total calculated RF resistance of the coil and its leads,
while the 5.sup.th column gives the value of this same quantity
that we calculated based on the measured DC resistance (see Table
3). The last column gives the calculated RF resistance of the
auxiliary inductor, also from Table 3.
TABLE-US-00005 TABLE 4 Contributions to the resistive losses in the
microcoil probe circuits. Microcoil Leads Total "Measured" Total
Auxiliary Capillary (.OMEGA.) (.OMEGA.) (.OMEGA.) (.OMEGA.)
inductor (.OMEGA.) 170/100 0.097 0.35 0.44 0.45 0.047 250/150 0.17
0.22 0.39 0.47 0.047 330/200 0.26 0.29 0.55 0.61 0.047 550/400 1.29
0.31 1.60 1.82 none
[0352] Table 4 indicates that probes with shorter leads can achieve
higher SNR performance. Shortening the leads requires bringing
larger conductors closer to the sample volume. Given our experience
with the 40 gauge wire and the 550/400 coil, we expect that the
resolution will eventually be degraded as larger amounts of copper
are placed closer to the sample volume. A compromise between
resolution and SNR, informed by the requirements of the
measurements to be performed, will govern the optimization of the
probe circuit design.
Conclusions
[0353] We have shown that very small microcoils can readily be
operated as NMR detectors in the low magnetic fields of permanent
magnets, a new size and frequency regime for miniaturized NMR
detectors. The close agreement in the measured SNR and the values
calculated based on a detailed analysis of the sources of resistive
losses in our NMR probes indicate that the simple construction
methods we employ yield outstanding probes. In particular, the SNR
performance confirms that the auxiliary "tuning" inductor does not
degrade SNR performance. The detailed analysis of the resistive
losses also points the way toward available improvements in SNR
performance: reduction of the leads connecting the microcoil to the
remainder of the probe circuit. However, reduction of these lead
lengths must respect the fact that bringing more metal closer to
the sample volume can readily degrade the field homogeneity.
Example 4
Tuning Coils
[0354] The example includes a LC resonance circuit that is
appropriate for performing magnetic resonance experiments, among
other beneficial purposes. The circuit has a microcoil, an
adjustable tuning capacitance, and a tuning inductor. A microcoil
having an inductance of 25 nH or less is often much smaller than a
tuning inductor. The circuit elements are connected by wires, which
may be attached to the elements as "leads," or added separately
during construction using connection techniques known to
practitioners of the art. The innovation features the inclusion of
the tuning inductor in addition to the microcoil.
[0355] One embodiment of the circuit of the present invention is
depicted in FIG. 14. This circuit is appropriate for magnetic
resonance experiments, among others. The circuit of FIG. 1 may be
construed as parallel or series resonant, depending upon how other
MR/NMR system components are coupled to it. For the discussion
immediately following, FIG. 14 may be assumed to be a parallel
resonant circuit. The circuit has a microcoil 10, an adjustable
tuning capacitor 12, and a second coil functioning as a tuning
inductor 14. A microcoil 10, preferably having an inductance of
approximately 25 nH or less, is generally much smaller than a
tuning inductor. The tuning capacitance 12 and the tuning
inductance 14 together constitute the principal elements of the
resonant circuit. The circuit elements are connected by wires,
which may be attached to the elements as "leads," or added
separately during construction using connection techniques known to
practitioners of the art. The presence in the circuit of both the
tuning inductor 14 and the microcoil 10 is a basic facet of the
present disclosure.
[0356] The microcoil 10 may be any electronically conducting
structure intended to create a magnetic field at the location of a
material under study. The microcoil 10 may be designed and built in
any manner, using techniques known to practitioners of the art. For
example, the microcoil 10 can be built by winding copper, silver,
gold, or other wire to form a helical coil. The wire may be round,
rectangular, or elliptical in cross section. Microcoil 10 may also
be constructed by patterning a metallic layer on a non-conducting
support material, so that the metallic layer has the configuration
of a helix. In an experiment that uses such a coil, the material
being studied is placed inside the helix. Alternatively, the
microcoil 10 may be flat (or planar) in shape, and the material
being studied is placed very close to a planar face of the coil.
Microcoil 10 may also have the form of a flat planar coil that has
been bent to define a curve, so that the concavo-convex coil fits
around or inside the material under study. The microcoil 10 may
also be a structure compounded of the shapes described above. The
microcoil 10 may be designed and built in any manner, using
proprietary or public domain techniques, as required for any
particular experiment. In a first embodiment of the disclosed
apparatus, the microcoil 10 preferably has an inductance
sufficiently low such that, without the use of the auxiliary tuning
inductor, an inconveniently large tuning capacitor would have been
required.
[0357] The microcoil 10 need not necessarily be a single coil. As
particular applications may suggest, alternative circuit
embodiments may substitute a plurality of microcoils 10' connected
electrically in series (FIG. 14A) or in parallel (FIG. 14B) for a
single coil 10. The assembled plurality of microcoils 10' is then
connected in series or parallel with the tuning inductance 14 in
the resonating circuit.
[0358] Similarly, the adjustable tuning capacitor 12 may be
constructed from one or more capacitors, which may each be either
fixed or adjustable. Any capacitor technology and any arrangement
of multiple capacitors may be used.
[0359] The tuning inductor 14 is constructed using any of the
techniques known in the art. For example, it may be formed from
wire wound in a helix or in a flat spiral, or it may take any other
form as long as it remains an inductor. The wire may be copper,
silver, gold, or any other electrically conducting material. The
wire may be solid, stranded, woven, a specialty wire such at "Litz"
wire, or evaporated onto a capillary. The wire may be operated at a
reduced temperature, and may be operated in a superconducting
state. The tuning inductor 14 may be compounded of separate
inductors, which each separate inductor having the same or a
different construction or form.
[0360] The tuning inductor 14 very preferably has a sufficiently
large inductance that it can be resonated with a tuning capacitor
that is of a pragmatically convenient size. In many cases, the
inductance of the tuning inductor 14 preferably is at least 2, 3,
4, 5, 6, 7, 8, 9, or 10 times larger than the inductance of the
microcoil 10. Thus, the tuning coil 14 has an inductance
substantially larger than the inductance of the microcoil 10, so
that the microcoil is an effective magnetic resonance transmitter
or receiver coil, and yet contributes no substantial inductance
needed for resonance.
[0361] When the closed series resonance LC circuit of FIG. 14 is in
use, its electrical signals are communicated to the remainder of a
NMR or MRI apparatus. This can be done in a large variety of ways.
One example, shown in FIG. 15, employs a matching or "coupling"
capacitor 16 connecting the junction between the tuning capacitance
12 and the tuning inductor 14. An additional connection typically
is provided to the other terminal of the tuning capacitance 12, so
that the overall resonance circuit presents two terminals to the
remainder of the NMR/MRI electronics. A nearly limitless variety of
coupling circuits, utilizing either capacitive or inductive
coupling, are known to practitioners skilled in the art, and may be
adapted for use in the method. All of such coupling/communication
schemes apply to the microcoil tuning circuit shown in FIG. 14.
[0362] This apparatus and method is most beneficial in
circumstances where the use of a very small sample coil (the
"microcoil") is desired, yet while the sample coil must be brought
into electrical resonance at a frequency so low that a capacitor of
inconveniently large physical size otherwise would be required
(i.e. if a conventional resonance circuit was employed). In
describing the operation of the invention, it is assumed that the
operator already possesses a microcoil, and seeks to resonate that
microcoil at an inconveniently low frequency.
[0363] Known circuit theory teaches that in an LC resonant circuit,
the total inductance L and total capacitance C serve to store
electrical energy. The stored energy alternates between being
stored in the capacitance and being stored in the inductance. The
rate at which the energy moves back and forth is the electrical
resonance frequency, which may be calculated using:
f = 1 2 .pi. 1 LC ##EQU00005##
Hence, if the total inductance L has a very low value, a very large
value of capacitance C is required to achieve a low value of the
frequency f. For example, if the only inductance in the circuit is
a microcoil of inductance 10 nH and the experiment is carried out
in a field of only 1 Tesla (which sets the proton nuclear magnetic
resonance frequency to be 42.6 MHz), the required capacitance is
1400 pF.
[0364] In the circuit of FIG. 14, the total inductance L is the sum
of the inductance of the microcoil 10 and the inductance of the
tuning inductor 14. By introducing the tuning inductor 14, we can
raise the value of total inductance to the point where the
capacitance required to achieve the appropriate electrical
resonance frequency is a convenient value. In fact, the tuning
inductor 14 that achieves this convenience is very similar to a
standard-sized sample coil for operation at the same required
frequency. Persons knowledgeable in the art of NMR/MRI circuit
construction are readily enabled to construct the required tuning
inductor 14.
[0365] It is preferred that the radiofrequency resistance of the
tuning inductor is much less than the radiofrequency resistance of
the microcoil 10. Otherwise, the tuning inductor 14 may add to the
electrical noise of the resonant circuit, and the signal detection
performance of the circuit is degraded. This occurs, for example,
if the tuning inductor 14 is constructed from the same wire as the
microcoil 10. Preferably, the tuning inductor 14 has less than
about one-tenth of the RF resistance of the microcoil 10. This can
be achieved, for example, by fabricating the tuning inductor 14
from very thick wire, by using stranded wire, by using Litz wire,
by cooling the tuning inductor, or by using superconducting
wire.
[0366] Thus, the microcoil 10 is in series with the second coil 14
which may, but need not necessarily, have a substantially larger
(e.g., at least a factor of two, or even a factor of ten or more)
inductance. The connected coils 10 and 14 form a resonant circuit
with the capacitor 12, and the microcoil 10 functions as an
effective magnetic resonance transmitter/receiver coil.
[0367] Using a separate tuning inductor 14 with the microcoil 10
may as well be practiced in a series resonant circuit. Attention is
invited to FIG. 19 in this regard. Accordingly, there is disclosed
a method and apparatus for obtaining magnetic resonance signals
from a microcoil 10 at low frequency, where the microcoil is
electrically connected in series with a tuning coil 14 having an
inductance substantially larger than the inductance of the
microcoil, and yet where the microcoil functions effectively as a
magnetic resonance transmitter or receiver coil while contributing
no substantial inductance needed for resonance, and also where the
resonant circuit of the coils 10, 14 with a capacitor may be either
series resonant circuit or a parallel resonant circuit.
[0368] Attention is invited to FIG. 16, which shows an alternative
embodiment of the disclosed apparatus, in which the microcoil 10 is
placed some distance from the tuning capacitance 12 and the tuning
inductance 14. Such an embodiment permits the invention to be
practiced under circumstances where the receiving/transmitting
"sensor" microcoil 10 must be remote physically from the remainder
of the system, such as may be required under certain field or
laboratory conditions (e.g., character or shape of the sample being
evaluated, need to isolate microcoil in extreme conditions such as
being placed in a cryogenic bath, and the like). The remotely
located microcoil 10 is connected to the remainder of the resonant
circuit by a cable or transmission line 18 that contains two
separate conductors. The ends of the microcoil 10 are connected to
the distal ends of the two conductors in the transmission line 18.
At the proximate end of the transmission line 18, its conductors
are connected to the tuning circuit elements, so that the microcoil
10 is disposed in electrical parallel with the tuning capacitance
12, the tuning inductance 14, or some portion of the tuning
inductance or capacitance.
[0369] The transmission line 18 may be any electrically conducting
structure that can carry signals along its length. At the
frequencies of most magnetic resonance experiments, the
transmission line 18 typically is a coaxial cable, although
microwave-style waveguides, twisted pair wires, and other
technologies known to practitioners of the art may be used. The
length of the transmission line 18 is such that it achieves an
impedance transformation between its proximate and distal ends (in
order that the impedance of the microcoil 10 is transformed to
different impedance at the point where the line 18 connects to the
tuning elements 12, 14). This is most simply accomplished by
providing a transmission line 18 having a length that is an odd
multiple (i.e., 1.times., 3.times., 5.times.) of the length of
one-fourth the wavelength of the alternating current at resonance.
In this embodiment utilizing a transmission line 18, the microcoil
10 can be operated at any practically any distance from the
tuning/matching components. However, a remotely operated microcoil
exploits a transmission line 18 having a length of at least
one-fourth the resonant wavelength. Thus, it is explicitly
understood that the operation potentially may be practiced with the
microcoil at practically any straight-line distance of physical
separation from the tuning/matching components, regardless of the
presence of the transmission line 18. But the advantage of remote
operation is realized when the microcoil is functioning at a
significant separation distance. Thus, in this description and in
the claims, "remote" and "remotely" refer to operation of a
microcoil that is located a substantial distance from the remaining
elements of the resonance circuit, but remains in communication
therewith by means of a transmission line. As mentioned, the
transmission line 18 preferably has a length corresponding to at
least one-fourth the resonant wavelength, so the microcoil 18 may
function at least such a physical separation distance from the
other circuit components. A skilled practitioner of the art readily
understands how to select a transmission line length to achieve an
impedance transformation that allows a convenient choice for the
tuning capacitance 12 and tuning inductance 14.
[0370] To use this alternative embodiment, the electrical signals
in the circuit of FIG. 16 are communicated to the remainder of the
NMR/MRI electronics. This task is the same as previously discussed
for the preferred embodiment, and may be achieved with any of the
methods described there, or by other methods known to those skilled
in the art. The microcoil 10, tuning capacitance 12, and tuning
inductance 14 of the alternative embodiment operate substantially
the same as the corresponding elements explained in the preferred
embodiment of FIG. 14. The transmission line 18, however, must be
constructed or selected so that it does not add appreciably to the
resistance of the microcoil circuit.
[0371] FIG. 17 illustrates a variation of the embodiment shown in
FIG. 16. In this alternative embodiment, the microcoil 10 again is
located remotely from the tuning/matching elements 12, 14. The
transmission line 18 preferably has a length corresponding to an
odd multiple of the length of one-fourth the wavelength of the
alternating current at resonance. The microcoil 10 has a parallel
connection with the tuning inductance 14; however, the transmission
line 18 is connected to the tuning inductor, across a partial
section thereof. Thus, only a selected segment of the overall
length of the tuning coil is disposed in electrical parallel with
the microcoil 10.
[0372] Referring to FIG. 18, yet another embodiment of the "remote
microcoil" version of the apparatus is disclosed which disposes an
impedance transformer 20 between the microcoil 10 and the tuning
inductance 14. This embodiment is substantially similar to the
embodiment of FIG. 16, except that the microcoil is connected in
parallel with the tuning inductance by means of the transmission
line 18 and a separate impedance transformer 20. The incorporation
of the impedance transformer 20 allows the microcoil 10 to be
presented to the tuning inductance 14 as a large capacitor, so that
the remotely located microcoil 10 is presented to the resonant
circuit as a large impedance in parallel with the tuning
inductance. As a result, the microcoil 10 serves as an effective
magnetic resonance transmitter/receiver coil without making a
significant contribution to the inductance needed for
resonance.
[0373] In possible applications of the foregoing disclosure, and
referring to FIGS. 20 and 21, a circuit contains a variable tuning
capacitance C.sub.T, in resonance with an inductance, and variable
coupling (or matching) capacitance C.sub.M used to properly couple
the resonant circuit to the 50.OMEGA. transmitter and receiver of a
NMR spectrometer (not shown). The capacitances C.sub.T, C.sub.M can
consist of single or multiple variable capacitors, combined in
series or parallel, which may be additionally mounted in parallel
or series with fixed capacitors. The inductance is the very small
sample microcoil together with a (typically larger) tuning coil
L.
[0374] Two examples for combining the sample microcoil and "tuning"
coil are contemplated. The circuit of FIG. 20, which is an
elaboration on the information provided in FIG. 14, adds the two
coils in series. The circuit of FIG. 21, expanding on the
disclosure of FIG. 16, places the sample coil at the distal end of
a cable of length roughly equal to a quarter-wavelength at the
resonant frequency. The other, proximate, end of the cable is
connected in parallel with the larger coil. The effect is to
transform the reactance of the sample coil to a much larger value
(via the cable) and then place this reactance in parallel with the
larger coil. (The small inductance of the sample coil is
transformed into a larger capacitive reactance by this cable.) The
value of the large inductance, L, can be chosen so that the circuit
achieves electrical resonance at the desired frequency without
requiring awkwardly large values for the capacitances.
[0375] The circuits of this example can be used in obtaining
magnetic resonance signals from a microcoil at low frequency. It
includes in most basic procedure the steps of connecting a
microcoil in series to a second, tuning, coil typically having a
substantially larger inductance, and forming a resonant circuit of
the combined coils with a capacitor, such that the microcoil
functions as an effective magnetic resonance transmitter/receiver
coil, but without the microcoil making a significant contribution
to the inductance needed for parallel resonance so that the
capacitor's capacitance is determined primarily by the inductance
of the tuning coil. It should be appreciated that connecting a
microcoil in series with a second, tuning, coil may be a step of
connecting electrically in mutual series a plurality of microcoils,
so that an assembly of coils may be substituted for a single
microcoil. Alternatively, such a plurality of microcoils may be
mutually connected in parallel to constitute the assembly of coils
that is substituted for a single microcoil, and then connecting the
plural assembly in series with the second or "tuning" coil.
[0376] A method according to this disclosure permits obtaining
magnetic resonance signals, at low frequency, from a microcoil
located remotely from the tuning/matching elements. This benefit is
realized by connecting, in parallel, a microcoil and a second or
tuning coil with a transmission line that is an odd multiple of the
length of one-fourth of the wavelength of the alternating current
at resonance. The second coil is constructed to provide with a
substantially larger inductance and is electrically connected to a
capacitor to form part of a parallel resonant circuit. The remotely
located microcoil presents as large impedance in parallel to the
second coil to act as an effective magnetic resonance
transmitter/receiver coil, yet without making a significant
contribution to the inductance needed for parallel resonance. As
with the basic preferred process, connecting a microcoil in
parallel with a second or "tuning" coil may be the step of
connecting in mutual series or in parallel a plurality of micro
coils, so that an assembly of microcoils may be substituted for a
single microcoil.
[0377] In the method for obtaining magnetic resonance signals from
a microcoil located remotely from the tuning/matching elements and
at low frequency, the process steps may include connecting in
parallel the microcoil with a cable that is an odd multiple of the
length of one-fourth the wavelength of the alternating current at
resonance to only a partial section of the large-inductance second
coil. Connecting these with a capacitor to provide a resonant
circuit, an apparatus thereby is provided whereby the remotely
located microcoil will function as impedance in parallel to the
second coil to act as an effective magnetic resonance
transmitter/receiver, coil without making a significant
contribution to the inductance needed for parallel resonance.
[0378] Alternative method steps include connecting in parallel the
microcoil with a transmission line and an impedance transformer
that will present the microcoil as a large capacitor to a second
coil having substantially larger inductance, and connecting these
to a capacitor to define part of a resonant circuit, whereby the
microcoil at a distance from the resonant circuit presents the
microcoil as a large impedance in parallel to the said second coil
to be an effective magnetic resonance transmitter/receiver coil
without making a significant contribution to the inductance needed
for resonance.
[0379] In sum, "extra" inductance, in the form of a large coil, is
deliberately included in the circuit of this example. Such stray
inductance is generally and previously regarded as flaw in NMR
resonant circuit design, due to an assumption that the stray
inductance will decrease the efficiency of signal detection.
However, we have determined that application of known circuit
design principles avoids degradation in detection efficiency,
provided the extra inductance does not add appreciably to the
electrical resistance of the circuit. Conventional circuit design
approaches for NMR resonant circuits, which do not allow the extra
inductance, are not capable of yielding practical, easy-to-tune
circuits for small sample coils operating at low resonance
frequencies, such as below 100 MHz. Indeed, no resonant circuits
for very small sample coils (<25 nH impedance) are known to be
operable at resonant frequencies below 100 MHz. According to the
present example, the capacitances and tuning inductance can be
mounted in a position remote from the "micro" sample coil. Readily
manipulated knobs or other adjustment aids can be attached to the
capacitances, since they are not subject to space constraints. A
transmission cable can be used to connect the sample coil to the
main resonant circuit, facilitating remote operation and the
miniaturization of a NMR probe.
[0380] Various arrangements and embodiments in accordance with the
present invention have been described herein. All embodiments of
each aspect of the invention can be used with embodiments of other
aspects of the invention. It will be appreciated, however, that
those skilled in the art will understand that changes and
modifications may be made to these arrangements and embodiments, as
well as combinations of the various embodiments without departing
from the true scope and spirit of the present invention, which is
defined by the following claims.
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