U.S. patent application number 11/124421 was filed with the patent office on 2005-12-01 for multi-current elements for magnetic resonance radio frequency coils.
Invention is credited to Adriany, Gregor, Akgun, Can E., Moeller, Steen, Moortele, Pierre-Francois Van de, Snyder, Carl, Tian, Jinfeng, Ugurbil, Kamil, Vaughan, J. Thomas.
Application Number | 20050264291 11/124421 |
Document ID | / |
Family ID | 35394779 |
Filed Date | 2005-12-01 |
United States Patent
Application |
20050264291 |
Kind Code |
A1 |
Vaughan, J. Thomas ; et
al. |
December 1, 2005 |
Multi-current elements for magnetic resonance radio frequency
coils
Abstract
A current unit having two or more current paths allows control
of magnitude, phase, time, frequency and position of each of
element in a radio frequency coil. For each current element, the
current can be adjusted as to a phase angle, frequency and
magnitude. Multiple current paths of a current unit can be used for
targeting multiple spatial domains or strategic combinations of the
fields generated/detected by combination of elements for targeting
a single domain in magnitude, phase, time, space and frequency.
Inventors: |
Vaughan, J. Thomas;
(Stillwater, MN) ; Adriany, Gregor; (Minneapolis,
MN) ; Snyder, Carl; (Minneapolis, MN) ; Akgun,
Can E.; (Woodbury, MN) ; Tian, Jinfeng;
(Minneapolis, MN) ; Ugurbil, Kamil; (Minneapolis,
MN) ; Moortele, Pierre-Francois Van de; (Minneapolis,
MN) ; Moeller, Steen; (St. Louis Park, MN) |
Correspondence
Address: |
Schwegman, Lundberg, Woessner & Kluth, P.A.
P.O. Box 2938
Minneapolis
MN
55402
US
|
Family ID: |
35394779 |
Appl. No.: |
11/124421 |
Filed: |
May 6, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60568889 |
May 7, 2004 |
|
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60569810 |
May 11, 2004 |
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Current U.S.
Class: |
324/318 ;
324/322 |
Current CPC
Class: |
G01R 33/3415 20130101;
G01R 33/34046 20130101 |
Class at
Publication: |
324/318 ;
324/322 |
International
Class: |
G01V 003/00 |
Goverment Interests
[0006] The present subject matter was partially supported by the
National Institutes of Health (NIH) under Agency Grant Numbers
NIH-P41 RR08079, NIH-R01 CA94200, NIH-R01 EB00085 and NIH R33
CA94318, MIND Institute and Keck Foundation. The United States
government may have certain rights in the invention.
Claims
What is claimed:
1. A current unit for a radio frequency coil comprising: a first
radio frequency current path proximate a dielectric; and a second
radio frequency current path independent of the first current path
and proximate the dielectric; and wherein the first radio frequency
current path and the second radio frequency current path are
configured for magnetic resonance imaging.
2. The current unit of claim 1 further including a third radio
frequency current path proximate the dielectric and wherein the
third radio frequency current path is independent of the first
current path and second current path and the third radio frequency
current path is configured for magnetic resonance imaging.
3. The current unit of claim 2 wherein the first radio frequency
current path, the second radio frequency current path and the radio
frequency current path are substantially planar and in parallel
alignment.
4. The current unit of claim 1 wherein at least one of the first
current path and the second current path is embedded in a
dielectric.
5. The current unit of claim 1 wherein at least one of the first
current path and the second current path includes a plurality of
perforations, slots, holes or apertures.
6. The current unit of claim 1 wherein at least one of the first
current path and the second current path is configured to encode
along a z-axis.
7. The current unit of claim 1 wherein at least one of the first
current path and the second current path is configured to encode
along at least one of an x-axis and a y-axis.
8. The current unit of claim 1 wherein at least one of the first
current path and the second current path includes a conductive
loop.
9. The current unit of claim 1 wherein at least one of the first
current path and the second current path is tapered along a
surface.
10. A magnetic resonance radio frequency coil comprising: at least
two current units arranged proximate each other and each current
unit having at least two current paths wherein a first current path
is configured for generating an excitation field and a second
current path is configured for detecting a signal.
11. The coil of claim 10 wherein the at least two current units are
distributed about a volume.
12. The coil of claim 10 wherein the at least two current units are
distributed about a surface.
13. The coil of claim 10 wherein the coil is configured for
operating in a static magnetic field having a field strength of
between 1 and 12 Tesla.
14. A current unit for a magnetic resonance radio frequency coil
comprising: a first current element and a second current element,
each current element having a first current path and a second
current path in parallel relation with the first current path,
wherein the first current element lies in a first plane and the
second current element lies in a second plane and wherein the first
plane is different from the second plane and the first plane and
second plane intersect on a line.
15. The current unit of claim 14 wherein at least one of the first
current element and the second current element include a loop
conductor.
16. The current unit of claim 15 further including a third current
element disposed proximate the first current element and the second
current element.
17. The current unit of claim 15 further including a plurality of
planar current elements, each planar current element in
substantially parallel alignment and disposed proximate the first
current element and the second current element.
18. A method comprising: generating a first field in a radio
frequency magnetic resonance coil using a first current path of a
current unit; generating a second field using a second current path
of the current unit; and detecting a signal using the current
unit.
19. The method of claim 18 wherein detecting includes encoding
along a z-axis of the coil.
20. The method of claim 18 wherein generating includes modulating
at least one of a current, a frequency, a phase angle, space and
timing.
21. The method of claim 18 wherein generating includes modulating
power delivered to the current unit.
22. The method of claim 18 further including tuning the first
current path to a first particular resonant frequency.
23. The method of claim 22 further including tuning the second
current path to a second particular resonant frequency wherein the
first particular resonant frequency differs from the second
particular resonant frequency.
24. The method of claim 18 further including reactively coupling
the first current path and the second current path.
25. The method of claim 18 further including reactively decoupling
the first current path and the second current path.
26. The method of claim 18 further including coupling the first
current path to a first spatial domain and coupling the second
current path to a second spatial domain.
27. A magnetic resonance radio frequency coil comprising: at least
two current units arranged proximate each other and each current
unit having at least two current paths wherein at least one current
path is configured for at least one of generating a field or
receiving a signal.
28. The coil of claim 27 wherein the at least two current units are
distributed about a volume.
29. The coil of claim 27 wherein the at least two current units are
distributed about a surface.
30. The coil of claim 27 wherein the coil is configured for
operating in a static magnetic field having a field strength of
between 1 and 12 Tesla.
Description
RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application Ser. No. 60/568,889, filed May 7, 2004, and entitled
"MULTI-CURRENT ELEMENTS FOR MAGNETIC RESONANCE RADIO FREQUENCY
COILS," which is hereby incorporated by reference.
[0002] This application claims priority to U.S. Provisional Patent
Application Ser. No. 60/569,810, filed May 11, 2004, and entitled
"MULTI-CURRENT ELEMENTS FOR MAGNETIC RESONANCE RADIO FREQUENCY
COILS," which is hereby incorporated by reference.
[0003] This application is related to U.S. patent application Ser.
No. 10/637,261, filed Aug. 8, 2003, and entitled "RADIO FREQUENCY
MAGNETIC FIELD UNIT WITH APERTURE," which is a continuation of U.S.
patent application Ser. No. 09/919,479, filed Jul. 31, 2001, and
entitled "RADIO FREQUENCY MAGNETIC FIELD UNIT WITH APERTURE," each
of which are hereby incorporated by reference.
[0004] This application is related to U.S. patent application Ser.
No. 10/420,541, filed Apr. 21, 2003, and entitled "RADIO FREQUENCY
GRADIENT AND SHIM COIL," which is hereby incorporated by
reference.
[0005] This application is related to U.S. patent application Ser.
No. 10/957,870, filed Oct. 4, 2004, and entitled "PARALLEL
TRANSCEIVER FOR NUCLEAR MAGNETIC RESONANCE SYSTEM," which is hereby
incorporated by reference.
TECHNICAL FIELD
[0007] This subject matter relates to radio frequency coils for use
with magnetic resonance imaging and spectroscopy and more
particularly, to a method and system for excitation and detection
of magnetic resonance signals using a current element having
multiple current paths.
BACKGROUND
[0008] Traditional radio frequency coils are inadequate for
exciting and detecting signals using magnetic resonance.
SUMMARY
[0009] A radio frequency coil includes a number of current units.
Each current unit includes multiple current elements and thus,
provides a plurality of current paths within a discrete insulated
module. The current flowing in each current path is independent of
current in other current paths of the same current element or of
current in a path of another current unit. A current path can
provide field excitation or detection; A current unit is sometimes
referred to as a current element.
[0010] In one example, each current unit of a coil is separately
addressed and independently controlled.
[0011] Current units (including, for example, transmission line
elements) are configured in a manner to change the RF field in the
coil.
[0012] Other aspects will be apparent on reading the following
detailed description and viewing the drawings that form a part
thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] In the drawings, like numerals describe substantially
similar components throughout the several views. Like numerals
having different letter suffixes represent different instances of
substantially similar components.
[0014] FIG. 1 includes a view of a radio frequency coil having a
number of current units.
[0015] FIG. 2A includes a current unit having a transmission line
current element and a loop current element.
[0016] FIG. 2B includes a conductor of a current element with
apertures.
[0017] FIG. 3 includes a current unit having a pair of orthogonal
loop current elements.
[0018] FIG. 4 illustrates fields corresponding to the current unit
of FIG. 3.
[0019] FIG. 5A includes a current unit having a three conductor
transmission line current element and two loop current
elements.
[0020] FIG. 5B includes a portion of a current unit having a three
conductor transmission line current element.
[0021] FIG. 6 includes a surface coil having four current units
having orthogonal loop current elements.
[0022] FIG. 7A illustrates a coil having a plurality of current
units with z-axis gradation.
[0023] FIG. 7B illustrates a current unit having a tapered
profile.
DETAILED DESCRIPTION
[0024] In the following detailed description, reference is made to
the accompanying drawings that form a part hereof, and in which is
shown, by way of illustration, specific embodiments in which the
present subject matter may be practiced. These embodiments are
described in sufficient detail to enable those skilled in the art
to practice the subject matter, and it is to be understood that the
embodiments may be combined, or that other embodiments may be
utilized and that structural, mechanical, logical and electrical
changes may be made without departing from the scope of the present
subject matter. The following detailed description is, therefore,
not to be taken in a limiting sense, and the scope of the present
subject matter is defined by the appended claims and their
equivalents.
[0025] FIG. 1 includes a view of radio frequency coil 100 having
current units 200. Each current unit 200 includes multiple current
paths. Coil 100 is configured as a volume coil with conductive
outer surface 90, sometimes referred to as a conductive shield
surface. The term shield is sometimes used to describe a barrier
between conductors or to describe an outer conductor of an RF coil.
In various examples, coil 100 includes an outer conductor 90, a
separate conductive shield or both outer conductor 90 and a
separate conductive shield.
[0026] Interior 80 of coil 100 includes the region of interest and
is configured to receive the subject to be examined. In the example
illustrated, each current unit 200 is positioned near an adjacent
current unit by a mechanical structure or an adhesive bond. In one
example, a coil structure includes a plurality of conductive
elements disposed about a contiguous dielectric member.
[0027] In one example, each current unit generates or receives a
quadrature or circularly polarized field. The current units can be
positioned in regular or irregular patterns in a coil circuit in
order to generate a desired field in the coil. The current units
can be arrayed in volume or surface coil configurations. Coils
having current units may be used for magnetic resonance anatomic
imaging, parallel imaging, molecular imaging, multi-nuclear
imaging, and functional imaging. In addition, a coil having one or
more current units can be used for electron paramagnetic resonance
(EPR), electron spin resonance (ESR) and nuclear magnetic resonance
(NMR) spectroscopy.
[0028] FIG. 2A includes current unit 200A having a transmission
line current element and a loop current element. The transmission
line current element includes first conductor 210A and second
conductor 215A. First conductor 210A and second conductor 215A are
in parallel alignment and separated by dielectric 220. First
conductor 210A and second conductor 215A, in various examples,
includes copper, aluminum or other alloy conductors. Dielectric 220
includes a dielectric or insulative material, examples of which
include synthetic or polymer materials. In addition, current unit
200A includes loop current element 225. Loop current element is
shown to be disposed on lower surface 240A of current unit 200A and
separated from second conductor 215A by insulative layer 205.
Insulative layer 205 may be the same or different from the material
of dielectric 220. In one example, sides 235A are flat surfaces
configured for bonding or otherwise affixing to an adjacent current
unit and are angled with respect to each other in a manner to
define a cylindrical or other shaped volume. In one example, outer
surface 230A denotes the exterior of the coil. Either one or both
of first conductor 210A and second conductor 215A may include
perforations, slots or openings or, as shown in the figure with
respect to second conductor 215A, a solid surface.
[0029] A current carrying current unit 200A can be used to generate
a radio frequency (RF) field for an RF coil. In one example,
current unit 200A includes multiple conductor elements configured
for supporting currents and fields of two or more phase angles,
magnitudes and frequencies.
[0030] FIG. 2B illustrates current unit 200A having second
conductor 215B with apertures. Second conductor 215B includes three
openings 217, however it will be understood that one or more
openings 217 are contemplated.
[0031] FIG. 3 includes current unit 200B having a pair of loop
current elements in orthogonal relation. First loop current element
245A and second loop current element 250A each lie in planes that
intersect at a line forming angle .theta. of approximately 90
degrees, however, other angles are also contemplated. Angle .theta.
is illustrated by way of axis 246A and axis 251A. Outer surface
230B, sides 235B and inner surface 240B define some of the exterior
dimensions of current unit 200B.
[0032] In the example illustrated, first loop current element 245A
generates one magnetic dipole, and second loop current element 250A
generates a second magnetic dipole. The fields of these two dipoles
are driven in relative quadrature phase, as shown in FIG. 4, thus
producing a circularly polarized field. In one example, first loop
current element 245A and second loop current element 250A each
receive a radio frequency signal and each are tuned to resonate at
a particular frequency which may be different for each current
element.
[0033] In one example, the first loop current element 245A and
second loop current element 250A are tuned to different
frequencies. In one example, the first loop current element 245A
and second loop current element 250A are driven at different
magnitudes. In one example, the first loop current element 245A and
second loop current element 250A are adjusted electrically or
mechanically to different phase angles. In one example, the first
loop current element 245A and second loop current element 250A are
switched on or off according to an excitation regimen. Switching on
or off can include modulating the power delivered to a current
element or unit. Either one or both of the first loop current
element 245A and second loop current element 250A can be used for
transmitting, receiving or both. In one example, the first loop
current element 245A and second loop current element 250A are
activated concurrently or sequentially. In various examples, the
first loop current element 245A and second loop current element
250A are of similar or different sizes and are arranged with
different spatial position and orientation relative to one another.
More than two current elements can be used.
[0034] In FIG. 3, each of the two illustrated current elements lies
in a plane and the two planes intersect on a line.
[0035] FIG. 4 illustrates the fields corresponding to current unit
200B of FIG. 3. In particular, current element 245A is configured
to excite, or detect, field 252 shown in alignment with axis 251B.
In addition, current element 250A is configured to excite, or
detect, field 247 shown in alignment with axis 246B.
[0036] FIG. 5A includes a partial view of current unit 200C having
a three conductor transmission line current element and two loop
current elements. Outer surface 230C includes first conductor 210B
and inner surface 240C includes second conductor 215C, each
separated by dielectric 236 of current unit 200C. First conductor
210B and second conductor 215C are in parallel alignment and each
is also parallel with intermediate conductor 280A. The combination
of the three parallel conductors, namely first conductor 210B,
second conductor 215C and intermediate conductor 280A, provides
three separate transmission line elements and thus three discrete
current paths. For example, discrete transmission lines are
provided between first conductor 210B and second conductor 215C,
first conductor 210B and intermediate conductor 280A and between
second conductor 215C and intermediate conductor 280A. Any or all
of the conductive elements of current unit 200C can include slots,
perforations or apertures along the z-axis.
[0037] In addition, current unit 200C includes first loop current
element 260 and second loop current element 270 disposed, or
embedded, within dielectric 236 and proximate intermediate
conductor 280A. Angle .alpha. denotes the relative angle between
the plane of first loop current element 260 and the plane of second
loop current element 270. Angle .beta. denotes the relative angle
between the plane of first loop current element 260 and
intermediate conductor 280A. Angle .alpha. and angle .beta. are
selected to provide the desired excitation field or detection
sensitivity. In various embodiments, angle .alpha. is approximately
90 degrees and angle .beta. is 45 degrees, however other angles are
also contemplated.
[0038] FIG. 5B includes a partial view of current unit 200E having
conductors 210C, 280B and 215D arranged as a transmission line. For
clarity, the dielectric is omitted from the figure and the outline
of current unit is shown in dashed lines. The smaller separation
distance between conductor 210C and 280B allows targeting of
spatial domain 370. The larger separation distance between
conductor 210C and 215D allows targeting of spatial domain 380.
Targeting a particular spatial domain can include driving the
spatial domain with an excitation signal or receiving a signal from
the spatial domain.
[0039] FIG. 6 includes surface coil 300 having four current units
200D, each having orthogonal loop current elements. For example,
current unit 200D includes loop current element 245B and loop
current element 250B, aligned at an angle of substantially 90
degrees. Voxels 350A, 350B, 350C and 350D represent spatial domains
in the region of interest. In addition, the fields associated with
each current element are represented as line segments. For example,
and with respect to voxel 350A, current element 245B generates
excitation field 360A and detects a signal produced from field 360B
using current element 250B. Each of current units 200D also
generate fields 365A, 370A and 380A and detects signals produced by
fields 365B, 370B and 380B, corresponding to voxels 350B, 350C and
350D, respectively.
[0040] In the foregoing example, one current element of each
current unit provides excitation and the other current element of
the same current unit provides signal reception. In addition, one
current element of the coil can provide excitation and each other
current element can provide reception or the same current element
can provide both excitation and reception.
[0041] Other combinations are also contemplated. For example, a
first current unit 200D can be used to transmit an excitation
signal and a second current unit 200D can be used to receive the
generated signal.
[0042] An example of an RF coil according to the present subject
matter includes coil 700 illustrated in FIG. 7A. The figure
illustrates a multi-unit coil where current units 200F are
configured to generate a z-axis field gradient. The walls of each
current unit are tapered along the z-axis of the coil. Adjacent
current units 200F are aligned in alternate directions in the
example illustrated, however, other configurations are also
contemplated. Segment 710 serves as one conductor of a transmission
line element not parallel to the others. Each current unit 200F is
coupled to a signal cable and each signal cable is connected to a
transmitter, a receiver or both a transmitter and receiver through
a transmit/receive switch. Each current unit 200F can serve as
either a driving unit, a receiving unit or both a driving and a
receiving unit.
[0043] FIG. 7B illustrates exemplary current unit 200F having a
tapered profile. In the example, dimension 730A is greater than
dimension 730B and dimension 740A is greater than dimension 740B,
however, other configurations for providing a z-axis gradient are
also contemplated.
[0044] In one example, the z-axis is encoded by means of holes or
slots along the z-axis of the current unit, as illustrated in FIG.
2B. The spacing, diameter, shape position and other factors can be
varied to achieve z-axis encoding.
[0045] Exemplary Alternatives
[0046] In one example, the coil includes a plurality of current
units with each current unit having multiple current paths. The
current units, in various examples are configured about a volume or
in a surface. A transverse electromagnetic (TEM) coil configured
according to the present subject matter includes a plurality of
current units. The current units can be arranged to provide at
least one aperture in a surface of a volume coil. In addition, the
current units assembled using a backplane that is solid or includes
an end aperture. In one example, the backplane is conductive.
[0047] In addition, the current units can be arranged in a birdcage
structure having at least one end ring for current flow and a
number of rungs. At least one rung can be a current unit having
multiple current paths.
[0048] In one example, a coil includes separate current units, each
having a number of conductors and assembled as a unit for magnetic
resonance excitation or reception. In one example, a coil is
fabricated of a contiguous dielectric and a number of conductors
are assembled on or in the dielectric for magnetic resonance
excitation or reception.
[0049] The current path, in various embodiments, includes a
transmission line or a loop path. The transmission line can include
two or more coaxial conductive elements, a stripline, a microstrip,
a waveguide or a configuration of parallel conductors separated by
an insulative dielectric. The conductive element can include a
solid strip, a perforated or slotted strip, a wire or a tube.
[0050] The current unit can generate two or more fields. Other
circuitry or structures are coupled to the coil and configured to
direct or control the generated fields into a spatial domain within
a region of interest. The region of interest is excited by the
fields and a stimulated signal is received from the region of
interest. In one example, the current unit detects a field in a
spatial domain within a region of interest.
[0051] In one example, the current paths of a current unit generate
a desired field by adjusting the current in the conductive path.
The magnitude, phase, frequency, timing and spatial position of the
current in a conductive path can be selected independent of the
current in any other current element or current path.
[0052] In one example, each current element can be independently
energized to generate an excited field in the region of interest.
Each excited field is coupled to one or more spatial domains within
the region of interest.
[0053] In one example, the individual conductor paths enable
circular polarization of a multi-current unit coil or circular
polarization in the neighborhood of each current element.
[0054] In one example, a single current unit can provide quadrature
drive with each current element contributing to one or more
fields.
[0055] In one example, the current elements of a coil can be
operated at a common frequency or one or more current elements can
be operated or tuned to different resonant frequencies. In
addition, each current element can be driven with a current of a
different magnitude, phase, frequency, timing or spatial
position.
[0056] In one example, the loop current element produces a field
corresponding to a magnetic dipole with the field direction
determined as a function of the current flow.
[0057] In one example, the current unit includes two conductive
paths having predetermined configurations or variable
configurations which produce two different excitation fields. The
fields differ in terms of spatial orientation, phase angle,
magnitude, frequency, timing or any combination thereof. In
particular, the spatial orientation and the magnitude define a
different spatial domain.
[0058] In one example, two or more current loops are nested. In
various examples, the current loops of a current unit are
configured to lie in either the same or a different plane and each
is coupled to the same spatial domain using a different phase
angle. In one example, the current elements are coupled to a
spatial domain with independent vectors.
[0059] In one example, a current unit includes two current elements
generating two orthogonal fields. Selection of suitable frequencies
allows detection of different nuclei.
[0060] In one example, the current unit includes two or more
conductive paths wherein the current in each path can be
manipulated in current phase, current magnitude, current frequency,
current switching or current spatial positioning.
[0061] In various examples, the coil is configured for use with a
magnet generating a B.sub.0 static field of 1 Tesla to 12 Tesla as
well as greater or lower field strengths.
[0062] In one example, the current units are discrete modules
having two or more current elements. In one example, the current
elements are arranged in separate modules and operated in a manner
to provide excitation and detection of a spatial domain as
described herein.
[0063] In one example, the current units are coupled to different
portions of a region of interest. For example, a first current unit
is coupled superficially to the region of interest and another
current unit couples at a greater depth of penetration.
[0064] In one example, two or more current elements are disposed in
a current unit at different positions relative to a shield. For
example, a first current element is located approximately 1 cm from
a shield and a second current element is located approximately 2 cm
from the shield. The shield can be a separate conductor or a
conductor of a transmission line of the current unit.
[0065] In one example, the current units of a coil are mutually
coupled by a reactive coupling, such as inductively or
capacitively, or hardwired. In one example, one or more current
units of a coil are mutually decoupled by shielding one from
another or by reactively decoupling.
[0066] In various examples, the coil is used in magnetic resonance
imaging, electron paramagnetic resonance and electron spin
resonance and other applications. According to one example, a
current unit having multiple conductors is capable of generating
and detecting currents and fields of two or more phase angles,
magnitudes and frequencies.
[0067] In one example, the current unit is energized with circular
phase polarization for generating orthogonal fields for improved
signal to noise ratio. According to one theory, the signal to noise
ratio improves by a factor of {square root}{square root over (2)}
in nuclear magnetic resonance signal, for example, can be used to
improve the signal intensity, spatial resolution, or speed of image
acquisition. In one example, current units couple to discrete
sample space domains for parallel imaging applications.
[0068] In one example, an orthogonally phased field couples to an
independent magnetization vector field in the sample to improve
parallel imaging performance.
[0069] In one example, multiple current paths of the current unit
are driven at different magnitudes to extend field coupling to
different regions in the sample. Multi-current magnitude and phase
can be combined for targeting regions of interest.
[0070] Multiple current paths of the current unit can be tuned to
multiple frequencies for multinuclear applications such as
metabolic imaging and spectroscopy.
[0071] In one example, multiple current paths and ground paths are
used to shield or to couple mutual elements.
[0072] In one example, a coil includes a plurality of current units
with each current unit having at least one current element. For
example, a first current element (of one current unit) has a first
configuration and all other current elements of the remaining
current units have a second configuration which differs from the
first configuration. Consider a coil having loop current elements.
At least one current element is aligned as illustrated by current
path 245A of FIG. 3 and at least one other current element is
aligned as illustrated by current path 250A. In one example,
adjacent current units have current elements that alternate in
alignment. In one example, each current unit has a current element
that is aligned differently from all other current elements. In
addition, another example includes a coil having loop current
elements located at differing positions relative to a shield
conductor. For example, a first current unit has a current
conductor disposed at a first distance from the shield conductor
and a second, adjacent, current unit has a current conductor
disposed at a second distance from the shield conductor.
[0073] In one example, the frequency, phase, magnitude, position or
timing of the current in each of the different current element is
selected independent of the current in any other current
element.
[0074] In one example, the coil includes current units configured
for generating or receiving a radio frequency signal having a
gradient along the x-axis, the y-axis or the z-axis. The z-axis, in
one example, is aligned with a major dimension of the current unit.
By way of examples, non-parallel sides or varying dielectric
thickness, apertures or other structures can be used to provide a
gradient.
[0075] In one example, a 16-channel stripline transverse
electromagnetic (TEM) coil is configured such that the magnitude of
RF magnetic field is a gradient along the x-axis, the y-axis and
the z-axis. This spatially varying RF profile allows for SENSE
reduction in all three dimensions.
[0076] The exemplary coil has an inside diameter of 25 cm and a
length of 16 cm and is constructed to produce homogeneous head
images at a field magnitude of 7 T. The 16 current elements are
equally spaced on a Teflon dielectric and independently tuned and
matched to a proton's Larmor frequency at 7 T such that the
elements can be driven (transmit and receive) in concert. In the
example illustrated, adjacent current units are reactively
decoupled. For each element, the Teflon dielectric has a tapered
profile in either the superior or inferior direction creating a
spatially varying shunt capacitance. In one example, the conductor
width to dielectric thickness ratio is a constant and thus the
impedance is also a constant.
[0077] Other configurations are also contemplated for creating a
spatially varying RF magnetic field that increases SENSE
performance by admitting k-space sub-encoding in the z-direction
without adversely inhibiting coil performance in the axial plane.
The exemplary coil allows SENSE encoding in the z-direction as well
as the x and y directions, with current elements that do not create
a magnetic field in the z-direction.
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