U.S. patent application number 12/102688 was filed with the patent office on 2009-10-15 for tuning low-inductance coils at low frequencies.
Invention is credited to Andrew F. McDowell.
Application Number | 20090256572 12/102688 |
Document ID | / |
Family ID | 41163450 |
Filed Date | 2009-10-15 |
United States Patent
Application |
20090256572 |
Kind Code |
A1 |
McDowell; Andrew F. |
October 15, 2009 |
Tuning Low-Inductance Coils at Low Frequencies
Abstract
A method and apparatus for tuning and matching extremely small
sample coils with very low inductance for use in magnetic resonance
experiments conducted at low frequencies. A circuit is disclosed
that is appropriate for performing measurements in fields where
magnetic resonance is beneficially utilized. The circuit has a
microcoil, an adjustable tuning capacitance, and added inductance
in the form of a tuning inductor. The microcoil is an electrical
coil having an inductance of about 25 nanohenries (nH) or less.
Because additional inductance is purposefully added, the
capacitance required for resonance and apparatus function is
proportionally and helpfully reduced. The apparatus and method
permit the resonant circuit and the magnet to be made extremely
small, which is crucial for new applications in portable magnetic
resonance imaging, for example.
Inventors: |
McDowell; Andrew F.;
(Albuquerque, NM) |
Correspondence
Address: |
DAVIS WRIGHT TREMAINE LLP - San Francisco
505 MONTGOMERY STREET, SUITE 800
SAN FRANCISCO
CA
94111
US
|
Family ID: |
41163450 |
Appl. No.: |
12/102688 |
Filed: |
April 14, 2008 |
Current U.S.
Class: |
324/318 |
Current CPC
Class: |
G01R 33/3628 20130101;
G01R 33/34 20130101 |
Class at
Publication: |
324/318 |
International
Class: |
G01R 33/32 20060101
G01R033/32 |
Claims
1. A method for obtaining magnetic resonance signals from a
microcoil at low frequency, comprising the steps of: connecting the
microcoil in series with a second coil, whereby the microcoil is an
effective magnetic resonance transmitter or receiver coil; forming
a resonant circuit of the coils with a capacitor whose capacitance
is determined primarily by the inductance of the second coil and;
permitting the resonant circuit to resonate at low frequency, with
alternating current at a resonance wavelength.
2. A method according to claim 1 wherein the step of forming a
resonant circuit comprises forming a parallel resonant circuit.
3. A method according to claim 1 wherein forming a resonant circuit
comprises forming a series resonant circuit.
4. A method according to claim 1 further comprising the steps of:
locating the microcoil remotely from the second coil and capacitor;
and connecting electrically the micro coil in parallel with the
second coil by means of a transmission line having a length that is
an odd multiple of a length corresponding substantially to
one-fourth of the resonance wavelength; whereby the remotely
located microcoil presents, in the resonant circuit, as a large
impedance in parallel to the second coil.
5. A method according to claim 4, wherein the step of connecting
the microcoil in parallel with the second coil comprises connecting
the transmission line to a partial segment of the second coil.
6. A method according to claim 4, wherein the step of connecting
the microcoil in parallel with the second coil comprises connecting
an impedance transformer to the transmission line intermediate to
the microcoil and the second coil.
7. A method according to claim 2 wherein the step of connecting the
microcoil in series with a second coil comprises connecting
electrically a plurality of microcoils together, and connecting
said plurality of microcoils in series with the second coil.
8. A method according to claim 3 wherein the step of connecting the
microcoil in series with a second coil comprises connecting
electrically a plurality of microcoils together, and connecting
said plurality of microcoils in series with the second coil.
9. A method according to claim 4 wherein the step of connecting the
microcoil in series with a second coil comprises connecting
electrically a plurality of microcoils in series, and connecting
said plurality of microcoils in series with the second coil.
10. A method according to claim 4 wherein the step of connecting
the microcoil in series with a second coil comprises connecting
electrically a plurality of microcoils in parallel, and connecting
said plurality of microcoils in series with the second coil.
11. A method for obtaining magnetic resonance signals from a
microcoil at low frequency, comprising the steps of: connecting the
microcoil in series with a tuning coil having an inductance at
least ten times larger than the inductance of the microcoil,
whereby the microcoil is an effective magnetic resonance
transmitter or receiver coil and contributes no substantial
inductance needed for resonance; forming a resonant circuit of the
coils with a capacitor whose capacitance is determined primarily by
the inductance of the tuning coil and; permitting the resonant
circuit to resonate at low frequency, with alternating current at a
resonance wavelength.
12. A method according to claim 11 wherein the step of forming a
resonant circuit comprises forming a parallel resonant circuit
wherein the microcoil contributes no substantial inductance needed
for parallel resonance.
13. A method according to claim 12 wherein the step of connecting
the microcoil in series with a tuning coil comprises connecting
electrically a plurality of microcoils together, and connecting
said plurality of interconnected microcoils in series with the
tuning coil.
14. A method according to claim 12 further comprising the steps of:
locating the microcoil remotely from tuning coil and capacitor; and
connecting electrically the microcoil in parallel with the tuning
coil by means of a transmission line having a length that is an odd
multiple of a length corresponding substantially to one-fourth of
the resonance wavelength; whereby the remotely located microcoil
presents, in the resonant circuit, as a large impedance in parallel
to the tuning coil.
15. A method according to claim 14, wherein the step of connecting
the microcoil in parallel with the tuning coil comprises connecting
the transmission line to a partial segment of the tuning coil.
16. A method according to claim 11 wherein the step of forming a
resonant circuit comprises forming a series resonant circuit
wherein the microcoil contributes no substantial inductance needed
for series resonance.
17. A method according to claim 16 wherein the step of connecting
the microcoil in series with a tuning coil comprises connecting
electrically a plurality of microcoils together, and connecting
said plurality of interconnected microcoils in series with the
tuning coil.
18. A method according to claim 16 further comprising the steps of:
locating the microcoil remotely from tuning coil and capacitor; and
connecting electrically the microcoil in parallel with the tuning
coil by means of a transmission line having a length that is an odd
multiple of a length corresponding substantially to one-fourth of
the resonance wavelength; whereby the remotely located microcoil
presents, in the resonant circuit, as a large impedance in parallel
to the tuning coil.
19. A method according to claim 18, wherein the step of connecting
the microcoil in parallel with the tuning coil comprises connecting
the transmission line to a partial segment of the tuning coil.
20. A method according to claim 18, wherein the step of connecting
the microcoil in parallel with the tuning coil comprises connecting
an impedance transformer to the transmission line intermediate to
the microcoil and the tuning coil.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention (Technical Field)
[0002] The present invention relates to resonant electrical
circuits containing inductors and capacitors, particularly when the
inductance is very low and the circuit must resonate at a low
frequency. More specifically, this disclosure is directed to a
method for tuning and matching extremely small sample coils with
very low inductance for use in magnetic resonance experiments
conducted at low frequencies.
[0003] 2. Background Art
[0004] Magnetic resonance experiments and procedures typically are
performed using an electrically resonant circuit, with a coil
(inductor L) wrapped around a sample to be evaluated, and a
capacitor (capacitance C) connected to the coil to form a series or
parallel resonant LC circuit. The frequency at which the electrical
resonance must occur is determined by the strength of the magnetic
field used, as well as the properties of the nucleus (or electron)
being studied.
[0005] The arts of Nuclear Magnetic Resonance (NMR) and Magnetic
Resonance Imaging (MRI) recently have advanced in the field of
sample miniaturization. Miniature sample magnetic resonance systems
employ very small sample coils. Such small coils allow NMR
experiments on, for example, mass-limited samples or highly
localized MRI information. The use of small coils, which have low
inductance, in electrically resonant circuits becomes increasingly
difficult as the size of the coil is reduced and/or the required
resonance frequency is reduced. The resonance frequency is
typically low for miniaturized experiments, when a small, weak
permanent magnet is used. To date, NMR and MRI experiments at low
frequencies (below 100 MHz) have been performed with "millicoils"
or "mini-coils" of relatively large size. Experiments using very
small coils have always been carried out at relatively much higher
frequencies. In contrast, the invention disclosed hereinafter
allows the use of the smallest possible coils in NMR and MRI
procedures carried out at the lowest possible frequencies.
[0006] Very small coils for use in NMR spectroscopy at relatively
high frequencies have been described previously.
[0007] T. L. Peck, et al., were among the first to extend the
analysis of coil circuit performance in magnetic resonance to very
small coils. They identified the signal-to-noise ratio advantage of
small diameter coils for samples with limited volume. A Peck, et
al. study, conducted at 4.7 Tesla (200 MHz), was of small coils
with diameters ranging from 0.050 mm to 0.860 mm and larger.
However, even with their smallest coil, which had an inductance of
only 4 nH, Peck, et al. did not face the challenge of resonating a
very small coil at a very low frequency. Their tuning circuit
layout discussion focused on the lengths of the leads of their
small coil, although they discuss only the resistance of these
leads, and not their stray inductance. Peck, et al., did not use a
tuning inductor. T. L. Peck, R. L. Magin, and P. C. Lauterbur,
"Design and Analysis of Microcoils for NMR Microscopy," J. Magn.
Reson. 108, 114-124 (1995).
[0008] U.S. Pat. No. 6,788,061 to Sweedler, et al., appears to
describe an NMR apparatus with a sample holder having a containment
region that holds a volume of less than about 1 microliters of the
analyte sample, and a coil which encloses the containment region of
the analyte sample holder and the contained sample. The coil is
operatively associated with the analyte sample in the containment
region of the sample holder, such that the coil can transmit and/or
receive energy from the analyte sample in the containment region.
This '061 patent contemplates operation of the system at very high
frequencies, and mentions a sample of volume <10 microliters and
magnets less than 50 kg. However, even though small coil usage in
micro-NMR applications at low magnetic field are discussed in the
'061 patent, no reference is made to resonance tuning. The '061
patent describes experiments carried out in a magnetic field of 7
Tesla, corresponding to a resonance frequency of 300 MHz. Hence,
Sweedler, et al. did not confront the problem of resonating a small
coil at low frequency.
[0009] U.S. Pat. No. 5,684,401 to Peck, et al., appears to teach
compensation of magnetic susceptibility variation in NMR
microspectroscopy detection coils. The disclosed apparatus does not
employ a tuning inductor. When discussing a tuning circuit, the
'401 patent concedes that their preference to move the tuning
capacitance physically away from the small coil will result in a
degradation of electrical performance. This concession follows
convention, which teaches that extra "lead," or "stray" inductance
is to be avoided.
[0010] Seeber, et al., explored microcoil performance
experimentally, using a field of 9 Tesla (383 MHz). While they used
coils as small at 0.020 mm in diameter, their tuning circuits
evidently never contained a tuning inductor, as their high
operating frequency made this unnecessary. Seeber, et al.,
attempted to reduce stray inductance by placing very small
capacitors as close as possible to the coils. They also made an
extensive, systematic study of the deleterious effects of stray
inductance. D. A. Seeber, R. L. Cooper, L. Ciobanu, and C. H.
Pennington, "Design and testing of high sensitivity microreceiver
coil apparatus for nuclear magnetic resonance and imaging," Rev.
Sci. Inst., 72, 2171 (2001).
[0011] U.S. Pat. No. 6,242,915 to Hurd seems to teach a
field-frequency lock system for an MRI system that includes a small
coil and resonant sample located to sense changes in the polarizing
magnetic field. The apparatus is operated at a high frequency of
205 MHz. Changes are detected as a shift in frequency of the NMR
signal produced by the resonant sample, and the frequency shift is
used to compensate the MRI system. This patent deals with
moderately large coils for use at moderately high frequency and
does not seem to require any special tuning schemes beyond the
traditional.
[0012] The prior art examples listed above suggest that the
challenge of resonating a very small coil (<20 nH) at low
frequencies (<100 MHz) has not been faced because the work has
all been carried out in strong magnetic fields, and has involved 1H
nuclei.
[0013] Moresi and Magin describe a low field NMR system using a
permanent magnet operating at 0.6 T (25.5 MHz), and discuss
motivations for assembling a small system using a permanent magnet.
Moresi, et al., purport to that show a system can perform NMR by
providing a sample coil with diameter 4 mm and inductance 168 nH.
With such a relatively large coil, they faced no particular
challenge in building a resonance circuit, thus employed a
conventional circuit design. G. Moresi and R. L. Magin, "Miniature
permanent magnet for table-top NMR," Concept. Magn. Reson. 19B,
35-43 (2003).
[0014] Goloshefsky, et al., discuss small-coil-based NMR and MRI
systems for industrial applications, and the advantages of using
very small coils. Goloshefsky et al.'s system operates at a low
magnetic field of 0.6 Tesla, corresponding to an operating
frequency of around 25.5 MHz. However, their coils are relatively
very large (two flat spirals, each with outer diameter 3.5 mm),
with a large enough inductance (81 nH) that they can resonate in
the traditional manner without significant difficulty. A. G.
Goloshevsky, J. H. Walton, M. V. Shutov, J. S. de Ropp, S. D.
Collins, M. J. McCarthy, "Development of low field nuclear magnetic
resonance microcoils," Rev. Sci. Inst. 76, 024101 (2005).
[0015] Sorli, et al., describe the design and construction of a
planar small coil system for operation at 2 Tesla (85 MHz). They
point out that very small coils have the potential for integration
with microfluidic "lab-on-a-chip" devices. The coil of the Sorli,
et al. system is rather large (0.5 mm on a side), and they describe
their resonance circuit as "conventional." B. Sorli, J. F.
Chateaux, M. Pitival, H. Chaliboune, B. Favre, A. Briguet, P.
Morin, "Micro-spectrometer for NMR: Analysis of small quantities in
vitro," Meas. Sci. Technol. 15, 877-880 (2004).
[0016] The three immediately previous publications, concerning the
use of so-called "microcoils" to perform NMR at low frequency,
utilized relatively large (.about.1 mm diameter, L.gtoreq.100 nH)
coils. Another particularly active area of application of small
coils at relatively low frequency is in MRI, where the typical
magnetic field strength is 1.5 Tesla, yielding a resonance
frequency of about 64 MHz. The following publications are
representative of efforts in this general area.
[0017] U.S. Patent App. Publication No 2005/0245814 to Anderson, et
al., appears to disclose a method for determining the position
and/or orientation of a catheter or other interventional access
device or surgical probe using phase patterns in a magnetic
resonance (MR) signal. The process employs a large coil (4 mm
outside diameter), said to operate at about 1.5 Tesla (.about.64
MHz). There is no apparent reference to any resonance tuning
procedures for the coils.
[0018] Published International Patent Publication No. WO2005026762,
to Weiss, appears to show an MR process for locating a medical
instrument with a very small coil attached thereto in the
examination volume of an MR device. The coil is part of a resonant
circuit matched to the resonant frequency of the MR device and
having no external controls. According to the disclosure, the small
coil is part of a resonant circuit tuned to the resonant frequency
of the MR device, which circuit is unconnected to any of the other
components of the MR device. However, the does not appear to be any
detailed teaching of a tuning method or of the magnetic filed
used.
[0019] European Patent EP1304581 to Gleich shows a method for
localizing an object, preferably a medical instrument, introduced
into a body. The object is in the examination volume of an MR
device, which evaluates the interaction between an electromagnetic
resonant circuit, mounted on the object, and an RF field applied in
the MR device for nuclear magnetization of the body. While an
extremely small coil is used in the resonance circuit, no
description is provided of a method or means for tuning.
[0020] International Patent Publication No. WO0173460 to Fuderer,
et al., offers an interventional magnetic resonance method
utilizing a very small coil. The method purports to enable
localization of an interventional instrument by detection of
magnetic resonance signals from the surroundings of the small coil
under the influence of magnetic field gradients. The disclosure
focuses on localization method through micro-NMR, but does not
include any tuning references or frequency details.
[0021] Canadian Patent CA2342047 to Raghavan, et al., shows a
device, such as a medical device, having a distribution of
"microcoils" (pairs) that may be used within an organism under MRI
visualization. At least one or each microcoil of the opposed pair
of microcoils has at least a region where a diameter circumscribed
by a first winding is greater than the diameter circumscribed by at
least one complete second winding, especially an adjacent winding
displaced from the first winding along an axis or core of the
medical device or an axis of the microcoil. The second winding is
nearer to or farther from an intermediate region between the
microcoils that define the pair of microcoils. The device
description does not include references or details regarding the
frequency or the tuning procedure. The preferred coils have
relatively large diameters between 1 mm and 4 mm.
[0022] U.S. Pat. No. 6,512,941 to Weiss, et al., discloses a device
and method for exciting the nuclear magnetization in a limited
volume of an object to be examined, utilizing a very small coil
which is present in the volume and is attached, for example, to an
interventional instrument during the formation of a magnetic
resonance image of the object to be examined. However, no tuning of
the coil is described.
[0023] U.S. Pat. No. 6,397,094 to Luedeke, et al., teaches an MR
method which utilizes a very small coil without connection leads
which causes a change in phase of an external RF magnetic field in
its direct vicinity within an object to be examined. This increase
apparently can be used to localize the coil, to image the direct
vicinity, or to track the propagation of a liquid flow passing
through the direct vicinity. However, no tuning of the microcoil is
described.
[0024] Accordingly, the prior art apparently makes no reference to
the difficulty of resonating very small coils at low frequencies.
All coils with inductances comparable to 25 nH have been utilized,
it is believed, at frequencies of 200 MHz and above. Coils used at
64 MHz or less have all been large, with diameters on the scale of
millimeters, with inductances exceeding 80 nH. The particular
challenge of resonating the smallest coils at low frequencies has
not been faced, and therefore it previously has not been
solved.
[0025] The challenge has also not been anticipated because those
skilled in the art likely would apply the conventional solution to
resonating a small coil at low frequency, that is, by simply
increasing the capacitance. The problem with such an approach is
that resonating a 10 nH inductor at 40 MHz, for example, would
require a capacitor of value of 1580 pF. Since the electrical
resonance frequency must match the magnetic resonance frequency,
the capacitance must be adjustable. An adjustable capacitor of
value above 1000 pF would be physically very large. An alternative
is to use both a fixed-value capacitor of small physical size but
large capacitance together with a physically small adjustable
capacitance. The drawback of this perceived solution is a dramatic
reduction in the range of capacitance adjustability.
[0026] The historical development of the art relating to the
optimization of detector coil performance in magnetic resonance
experiments is helpfully revealed in the seminal publication by D.
I. Hoult and R. E. Richards, "The SNR of the NMR Experiment," J.
Magn. Reson. 24, 71 (1976). Hoult and Richards highlight the
then-state-of-the-art understanding of coil performance
(exemplified by their first reference: A. Abragam, Principles of
Nuclear Magnetism, Oxford University Press, pp. 71-83 (1961)), by
identifying shortcomings in the known art and proposing a more
fundamental, general, and accurate approach to understanding the
performance of detector coils.
[0027] The state-of-the-art prior to the 1976 Hoult and Richards
publication expressed coil performance in terms of coil volume,
sample filling factor (the fraction of the coil volume occupied by
the sample in the usual situation where there is a single coil used
as the NMR sample coil), coil inductance, and resonant circuit
quality factor ("Q factor"), among other concepts. The conventional
wisdom with regard to sample filling factor was to maximize the
sample's "exposure" to the radiofrequency magnetic field generated
by all parts of the resonance circuit. This meant, for example,
minimizing the lengths of wires connecting the coil to the rest of
the resonant circuit in order not to generate "wasted" fields.
[0028] Hoult and Richards developed an approach based on concepts
more closely tied to the physics of the detection process. They
utilized the Principle of Reciprocity to rigorously calculate the
strength of the signal that would be detected, and ascribing the
electrical noise that serves to obscure the desired signal to
various details of the experiment. The end result of Hoult and
Richards' efforts was an equation for calculating detector coil
performance that is better grounded in fundamental principles than
previous approaches. Specifically, their result is expressed in
terms of the coil's efficiency in producing magnetic field at the
sample location (defined as the field produced per unit current in
the coil) and the resistance in the resonant circuit. Taking this
result as a guide for designing detector coils and circuits, a
researcher in the art is inexorably led to the conclusion that the
best design maximizes the magnetic field intensity generated by a
unit current in the coil, divided by the square root of the circuit
resistance. A practical realization of this criterion is to
minimize the time required to tip the spins by, say, 90 degrees,
the so-called "90-degree pulse."
[0029] These design goals are equivalent to maximizing the
Q-factor, inductance, filling factor, etc., only for a simple
solenoid that is full of the sample and is a part of a compact
resonant circuit--the usual case for NMR/MRI. In the general case,
one must use the Hoult and Richards criterion, and not the older
approach. This fact seems to have remained unrecognized prior to
the present disclosure, even in the extension of the Hoult and
Richards methodology to the regime of very small detector coils
presented by Peck, et al. T. L. Peck, et al., "Design and Analysis
of Microcoils for NMR Microscopy," J. Magn. Reson. B 108, 114
(1995).
[0030] An adverse aspect of the Hoult and Richards approach is that
the concepts used to describe coil performance are abstract and
somewhat difficult to measure for many practitioners. The
conceptual units of the previous approach of Abragam, for example,
are more concrete and readily measured. Ideas like inductance,
volume, filling factor, Q-factor, etc. are understood from
introductory-level physics or radio technology, and there are
inexpensive instruments to measure these quantities and simple
methods for calculating them. Hoult and Richards recognized the
conceptual value of the other approach, and showed in an appendix
how their result could be used to derive the Abragam-style result,
under the assumption that the filling factor was maximized.
Significantly, while the Hoult and Richards result can be used to
derive the Abragam-style result, the reverse is not true. The Hoult
and Richards result is more general.
[0031] Thus, it is believed that the early formulation of coil
performance in terms of inductance, volume, filling factor, and
Q-factor have seen continued use in the art of magnetic resonance,
and Abragam's book remains a popular reference in the field. The
fact that the Abragam-style approach dominates current thinking is
readily seen in the literature, including references mentioned
hereinabove. Evidently, the Hoult and Richards result did not
succeed in altering the standard methodology for designing optimal
detector coils for magnetic resonance experiments, because the
early formulation works for the vast majority of NMR/MRI
applications. Indeed, when the Hoult and Richards article is cited
in the literature, it is often used to support the notion that
filling factor should be maximized, which is a misinterpretation of
the result given in their appendix. From this it is evident that
concepts such as the filling factor, Q-factor, etc. are highly
misleading when it comes to building optimized detector coils.
[0032] The present invention was developed against the foregoing
background. The present approach is contrary to prior art teaching,
because the prior art has not recognized the full implications of
the Hoult and Richards result. This disclosure provides a solution
for tuning small inductances at low frequencies that is much more
convenient than the conventional approaches. Beyond convenience,
the disclosed apparatus and method permits the resonant circuit and
the magnet to be made very small, which is crucial for new
applications in portable MRI, for example.
SUMMARY OF THE INVENTION
Disclosure of the Invention
[0033] There is provided an apparatus and method for tuning
low-inductance coils at low frequencies. An innovative circuit is
disclosed that would be appropriate for performing measurements in
fields where magnetic resonance is beneficially utilized. The
circuit has a microcoil, an adjustable tuning capacitance, and a
tuning inductor. In this description and claims, "microcoil" refers
to an electrical coil having an inductance of about 25 nanohenries
(nH) or less. (It should be noted that prior art publications
occasionally use "microcoil" to refer to coils of comparatively and
significantly larger inductance.) A microcoil having such a very
low inductance is often much smaller than a common tuning inductor.
The circuit elements are electrically connected, as by wires which
may be attached to the elements as "leads," or added separately
during construction using other connection techniques known to
practitioners of the art. The innovation features the inclusion of
the tuning inductor in addition to the microcoil. As additional
inductance is purposefully added, the capacitance required for
resonance and apparatus function is proportionally and helpfully
reduced.
[0034] Several objects and advantages of the disclosed apparatus
and method are evident. The disclosure permits the resonating of
low inductance coils at low frequencies without:
[0035] (1) the use of physically large adjustable capacitors;
[0036] (2) the use of high-fixed-value capacitors together with a
variable capacitor, which reduces the range of adjustability;
and
[0037] (3) the need to mount capacitors very near the coil, which
can be a significant challenge and may distort the magnetic field
at the sample.
[0038] Other objects and advantages include:
[0039] (1) enabling the miniaturization of coils used in resonant
circuits known to the prior art, when the resonant frequency must
be kept low;
[0040] (2) the operation of small coils in resonant circuits at
arbitrarily low frequencies, generally less than 100 MHz.;
[0041] (3) the invention allows the miniaturization of small coils
used in NMR and MRI;
[0042] (4) the provision of an apparatus that works with a
multiplicity of small coils; and
[0043] (5) the location of the miniaturized sample coil remotely
from the tuning capacitance and inductance.
[0044] Other objects, advantages, and novel features, and further
scope of applicability of the present invention will be set forth
in part in the detailed description to follow, taken in conjunction
with the accompanying drawings, and in part will become apparent to
those skilled in the art upon examination of the following, or may
be learned by practice of the invention. The objects and advantages
of the invention may be realized and attained by means of the
instrumentalities and combinations particularly pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0045] The accompanying drawings, which are incorporated into and
form a part of the specification, illustrate several embodiments of
the presently disclosed apparatus and, together with the
description, serve to explain the principles thereof. The drawings
are only for the purpose of illustrating preferred embodiments, and
are not to be construed as limiting the invention. In the
drawings:
[0046] FIG. 1 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;
[0047] FIG. 1A is a schematic depicting the circuit shown in FIG.
1, except with the single sample coil replaced by a plurality of
coils connected in series;
[0048] FIG. 1B is a schematic depicting the circuit shown in FIG.
1, except with the single sample coil replaced by a plurality of
coils connected in parallel;
[0049] FIG. 2 is a schematic diagram indicating how an electrical
circuit according to the present disclosure could be implemented in
a magnetic resonance experiment;
[0050] FIG. 3 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;
[0051] FIG. 4 is a schematic diagram, similar to FIG. 3, 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;
[0052] FIG. 5 is a schematic diagram, similar to FIG. 3, 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;
[0053] FIG. 6 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;
[0054] FIG. 7 is a schematic diagram elaborating somewhat on FIG.
1; and
[0055] FIG. 8 is a schematic diagram elaborating somewhat on FIG.
3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Best Modes for Carrying Out the Invention
[0056] There is disclosed herein an apparatus and method for tuning
low-inductance coils at low frequencies. A purpose of the disclosed
apparatus and method is to allow a very small coil of negligible
inductance to be resonated electrically at low frequency, while
readily maintaining frequency adjustability over a wide range of
values using standard variable capacitors. "Low frequency," in this
application, means less than about 100 MHz. The main application of
the disclosed circuitry is in the field of Nuclear Magnetic
Resonance (NMR), although it is useful for any situation requiring
a tunable resonant circuit containing a very small coil. Properly
implemented, the disclosed circuit introduces few or no electrical
losses into the resonant circuit, and the circuit maintains maximal
efficiency for the detection of NMR signals.
[0057] The disclosure includes a LC resonance circuit that would be
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.
[0058] This present disclosure is contrary to conventions in the
art. In addition to suggesting the standard solution of adding more
capacitance to achieve resonance at lower frequency, the prior art
appears to teach against adding inductance that is not a part of
the NMR coil. This additional inductance often is referred to as
"stray" inductance. Prior efforts in the related art, such as U.S.
Pat. No. 5,684,401 to Peck, et al., and articles by D. A. Seeber et
al. (Rev. Sci. Iist., 72, 2171 (2001)) and G. Moresi, et al. (G.
Moresi Concept. Magn. Reson. 19B, 35-43 (2003)), take steps to
reduce this unwanted stray inductance by placing the capacitors as
close as possible to the coil.
[0059] The prior art also admonishes that the "filling factor" (the
fraction of the inductance occupied by the sample material) must be
maximized to optimize coil performance. Such teachings are found
in, for example, E. Fulushima and S. B. W. Roeder, "Experimental
Pulse NMR: A Nuts and Bolts Approach," Addison-Wesley, Reading,
Mass. (1981), p 311, p 342, p 374; K. R. Minard, R. A. Wind,
"Solenoidal microcoil design, Part I: Optimizing rf homogeneity,"
Concepts in Magn. Reson. 13, 128-142 (2001); A. Abragam,
"Principles of Nuclear Magnetism," Clarendon Press, Oxford (1961),
p 82). In these instances, the prior art leads circuit builders to
eliminate stray inductances because they are inductances that exist
in regions where there is no sample. The present method, in marked
distinction, purposefully introduces a very large amount of extra
("stray") inductance, and reduces the filling factor of the circuit
design to nearly zero, yet works well.
[0060] The challenge of resonating a low inductance coil at low
frequencies is worth facing and solving. The use of small
(low-field) permanent magnets together with microcoils offers many
advantages in cost, portability, ease of integration with other
techniques, and siting of the equipment in restrictive environments
and non-laboratory locations. These advantages may open up new
applications for NMR in education, industry, point-of-care and
point-of-diagnosis medicine, environmental testing, and science.
Many MRI applications of microcoils may benefit from the reduction
of the coil size, at which time the challenge of resonating the
smaller coils must be addressed. Advances in magnetic resonance
techniques that require both miniaturizing the sample coil and
reducing the operating frequency require a method for resonating
low inductance coils at low frequencies. The present disclosure
addresses these developing needs.
[0061] A preferred embodiment of the circuit of the present
invention is depicted in FIG. 1. This circuit would be 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. 1 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.
[0062] 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.
[0063] 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. 1A) or in parallel (FIG. 1B) 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.
[0064] 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.
[0065] 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.
[0066] 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 ten
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.
[0067] When the closed series resonance LC circuit of FIG. 1 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. 2, 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. 1.
[0068] 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.
[0069] 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 ##EQU00001##
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.
[0070] In the circuit of FIG. 1, the total inductance L will be 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.
[0071] Care must be taken, however, when constructing the tuning
inductor 14 so that the radiofrequency resistance of the tuning
inductor is much less than the radiofrequency resistance of the
microcoil 10. Otherwise, the tuning inductor 14 adds 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.
[0072] 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. The
purposeful addition of inductance--which becomes increasingly
important as the ratio of tuning coil to microcoil inductance
exceeds ten--is contrary to the teachings of the prior art, but
promotes the practical utility of the present invention.
[0073] The foregoing teachings have related to the practice of the
invention in the context of a parallel resonant circuit. However,
the invention 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. 6 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.
[0074] Attention is invited to FIG. 3, 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.
[0075] 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.
[0076] To use this alternative embodiment, the electrical signals
in the circuit of FIG. 3 must be 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. 1. The transmission line 18,
however, must be constructed or selected so that it does not add
appreciably to the resistance of the microcoil circuit.
[0077] FIG. 4 illustrates a variation of the embodiment shown in
FIG. 3. 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.
[0078] Referring to FIG. 5, 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. 3, 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.
[0079] In possible applications of the foregoing disclosure, and
referring to FIGS. 7 and 8, 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.
[0080] Two possibilities for combining the sample microcoil and
"tuning" coil are contemplated. The circuit of FIG. 7, which is an
elaboration on the information provided in FIG. 1, adds the two
coils in series. The circuit of FIG. 8, expanding on the disclosure
of FIG. 3, 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.
[0081] The methodology of the present invention is apparent from
the foregoing, but may be summarized in a procedural format. The
method of the present disclosure is for 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.
[0082] 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.
[0083] 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.
[0084] 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.
[0085] In sum, "extra" inductance, in the form of a large coil, is
deliberately included in the circuit of this disclosure. 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
presently disclosed system, 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.
[0086] Although the invention has been described in detail with
particular reference to these preferred embodiments, other
embodiments can achieve the same results. Variations and
modifications of the present invention will be obvious to those
skilled in the art and it is intended to cover in the appended
claims all such modifications and equivalents. The entire
disclosures of all references, patents, and publications cited
above are hereby incorporated by reference.
* * * * *