U.S. patent application number 11/085800 was filed with the patent office on 2005-12-15 for transceive surface coil array for magnetic resonance imaging and spectroscopy.
Invention is credited to Menon, Ravi S., Pinkerton, Robert G..
Application Number | 20050275403 11/085800 |
Document ID | / |
Family ID | 35459874 |
Filed Date | 2005-12-15 |
United States Patent
Application |
20050275403 |
Kind Code |
A1 |
Pinkerton, Robert G. ; et
al. |
December 15, 2005 |
Transceive surface coil array for magnetic resonance imaging and
spectroscopy
Abstract
A surface coil array comprises a surface coil support and an
arrangement of non-overlapping magnetically decoupled surface coils
mounted on the support. The surface coils encompass a volume into
which a target to be imaged is placed. Magnetic decoupling circuits
act between adjacent surface coils. Impedance matching circuitry
couples the surface coils to conventional transmit and receive
components.
Inventors: |
Pinkerton, Robert G.;
(Harrowsmith, CA) ; Menon, Ravi S.; (London,
CA) |
Correspondence
Address: |
BAKER & DANIELS LLP
111 E. WAYNE STREET
SUITE 800
FORT WAYNE
IN
46802
US
|
Family ID: |
35459874 |
Appl. No.: |
11/085800 |
Filed: |
March 21, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60554350 |
Mar 19, 2004 |
|
|
|
Current U.S.
Class: |
324/318 ;
324/322 |
Current CPC
Class: |
G01R 33/365 20130101;
G01R 33/34046 20130101; G01R 33/3415 20130101 |
Class at
Publication: |
324/318 ;
324/322 |
International
Class: |
G01V 003/00 |
Claims
What is claimed is:
1. A surface coil array comprising: a plurality of spaced surface
coils arranged about a volume; and magnetic decoupling circuits
acting between said surface coils.
2. A surface coil array according to claim 1 wherein magnetic
decoupling circuits act between adjacent surface coils.
3. A surface coil array according to claim 2 wherein said magnetic
decoupling circuits are purely capacitive.
4. A surface coil array according to claim 3 wherein the capacitive
reactance of each magnetic decoupling circuit is equal to the
mutual inductive reactance between adjacent surface coils.
5. A surface coil array according to claim 4 wherein each magnetic
decoupling circuit comprises a single capacitor.
6. A surface coil array according to claim 4 wherein said surface
coils are generally equally spaced about said volume.
7. A surface coil array according to claim 5 further comprising a
dielectric support on which said surface coils are mounted.
8. A surface coil array according to claim 7 further comprising
dielectric loss reducing elements distributed about said surface
coils.
9. A surface coil array according to claim 8 wherein said
dielectric loss reducing elements are low-loss lumped
capacitors.
10. A surface coil array according to claim 4 further comprising
baluns coupled to said surface coils.
11. A surface coil array according to claim 10 wherein said baluns
are impedance matched to feed network circuitry coupled
thereto.
12. A surface coil array according to claim 11 wherein said baluns
are impedance matched to 50.OMEGA. component-based feed network
circuitry.
13. A surface coil array according to claim 10 further comprising a
feed network coupled to said baluns and communicating with said
surface coils.
14. A surface coil array according to claim 13 wherein said baluns
are balanced and impedance matched to said feed network.
15. A surface coil array according to claim 14 wherein said feed
network comprises conventional transmit and receive components.
16. A surface coil array according to claim 15 wherein said
conventional transmit and receive components comprise 50.OMEGA.
preamplifiers and low cost radio frequency (RF) power
amplifiers.
17. A surface coil array according to claim 16 wherein said feed
network further comprises a 50.OMEGA. coaxial cable connecting said
baluns to said feed network.
18. A surface coil array according to claim 13 wherein said baluns
are configured to provide transmit signals generated by said feed
network to said surface coils for transmit operation and to provide
receive signals received by said surface coils to said feed network
for receive operation.
19. A surface coil array according to claim 18 wherein said feed
network comprises conventional transmit and receive components.
20. A surface coil array according to claim 19 wherein said
conventional transmit and receive components comprise 50.OMEGA.
preamplifiers and low cost RF power amplifiers.
21. A surface coil array according to claim 13 wherein said baluns
are configured to condition said surface coil array to a
receive-only mode, in said receive-only mode, said baluns isolating
said surface coils from said feed network when said feed network
generates transmit signals.
22. A surface coil array according to claim 21 wherein said feed
network comprises conventional transmit and receive components.
23. A surface coil array according to claim 22 wherein said
conventional transmit and receive components comprise 50.OMEGA.
preamplifiers and low cost power amplifiers.
24. A surface coil array according to claim 4 wherein a magnetic
decoupling circuit acts between each adjacent pair of surface
coils.
25. A surface coil array according to claim 4 wherein said surface
coils are arranged in sets, magnetic decoupling circuits acting
between adjacent pairs of surface coils in each set.
26. A surface coil array according to claim 11 wherein said feed
network circuitry includes a separate low power transmitter for
each surface coil.
27. A surface coil array according to claim 18 wherein said feed
network includes a separate low power transmitter for each surface
coil.
28. A surface coil array according to claim 11 wherein receive
signals received by said surface coils bypass said feed network
circuitry.
29. A surface coil array comprising: a surface coil support; a
plurality of generally evenly spaced surface coils mounted on said
support and surrounding a volume into which a target to be imaged
is placed; and capacitive decoupling circuitry acting between said
surface coils to reduce magnetic coil-to-coil coupling.
30. A surface coil array according to claim 29 further comprising a
feed network and impedance matching circuitry acting between said
feed network and each of said surface coils.
31. A surface coil array according to claim 30 wherein said feed
network comprises conventional transmit and receive components.
32. A surface coil array according to claim 31 wherein said
conventional transmit and receive components comprise 50.OMEGA.
preamplifiers and low cost RF power amplifiers.
33. A surface coil array according to claim 31 wherein said
impedance matching circuitry is configured to provide transmit
signals generated by said feed network to said surface coils for
transmit operation and to provide receive signals received by said
surface coils to said feed network for receive operation.
34. A surface coil array according to claim 33 wherein said
conventional transmit and receive components comprise 50.OMEGA.
preamplifiers and low cost RF power amplifiers.
35. A surface coil array according to claim 30 wherein said
impedance matching circuitry comprising a .lambda./4 lattice balun
associated with each surface coil.
36. An RF resonator comprising: a surface coil array including an
arrangement of non-overlapping magnetically decoupled surface coils
encompassing a volume; and a feed network coupled to said surface
coil array, said feed network at least receiving signals received
by said surface coils during imaging of a target within said
volume.
37. An RF resonator according to claim 36 wherein said feed network
comprises conventional 50.OMEGA. transmit and receive
components.
38. An RF resonator according to claim 37 wherein said surface coil
array comprising impedance matching circuitry acting between each
surface coil and said feed network.
39. An RF resonator according to claim 38 wherein said feed network
provides transmit signals to said surface coils via said impedance
matching circuitry.
40. A surface coil array comprising: a surface coil support; and an
arrangement of non-overlapping magnetically decoupled surface coils
mounted on said support and encompassing a volume into which a
target to be imaged is placed.
41. A surface coil array according to claim 40 wherein each of said
surface coils is generally identical.
42. A surface coil array according to claim 41 wherein said surface
coils are of varying shape and/or size.
43. A surface coil array according to claim 42 wherein said surface
coils are grouped into sets, the surface coils in each set being
magnetically decoupled independent of other sets.
44. A surface coil array according to claim 43 wherein said surface
coils receive transmit signals from at least one radio frequency
transmitter.
45. A surface coil array according to claim 44 wherein said surface
coils receive transmit signals from a single transmitter.
46. A surface coil array according to claim 45 wherein each surface
coil receives transmit signals from an associated transmitter.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 60/554,350 filed on Mar. 19, 2004 for an
invention entitled "Transceive Surface Coil Array For Magnetic
Resonance Imaging and Spectroscopy".
FIELD OF THE INVENTION
[0002] The present invention relates generally to magnetic
resonance imaging (MRI) and more specifically, to a transceive
surface coil array for magnetic resonance imaging and
spectroscopy.
BACKGROUND OF THE INVENTION
[0003] Nuclear Magnetic Resonance (NMR) Imaging, or Magnetic
Resonance Imaging (MRI) as it is commonly known, is a non-invasive
imaging modality that can produce high resolution, high contrast
images of the interior of the human body. MRI involves the
interrogation of the nuclear magnetic moments of a subject placed
in a strong magnetic field with radio frequency (RF) magnetic
fields. A MRI system typically comprises a fixed magnet to create
the main strong magnetic field, a gradient coil assembly to permit
spatial encoding of signal information, a variety of RF resonators
or RF coils as they are commonly known, to transmit RF energy to,
and receive signals emanating back from, the subject being imaged,
and a computer to control overall MRI system operation and create
images from the signal information obtained.
[0004] The design of RF resonators operating in the near-field
regime plays a major role in the quality of magnetic resonance
imaging. Over the past few decades, the designs of RF resonators
have significantly evolved from the simple solenoid coils of wire
that were typically used. The volume coil, such as for example the
saddle coil, hybrid birdcage or TEM, has been a popular choice of
MRI transmitter and/or receiver for large fields of view (FOV) due
to the ability to match it to 50.OMEGA. impedance transmit and
receive components and its homogeneous sensitivity profile.
Although these volume coils operate well at low magnetic field
strengths, they become less effective in the high field regime i.e.
at magnetic field strengths greater than 3 T. As the static
magnetic field strength used in MRI increases, the wavelength of
the associated Larmor RF approaches the dimensions of the volume
coil and volume of interest (VOI). Several imaging problems arise
in this full wavelength regime namely, increased radiation losses,
increased local and global specific absorption rate (SAR), and
dielectric resonance effects that create both inhomogeneous images
and signal loss.
[0005] By using surface coils to receive, one can reduce the
detrimental effects of dielectric resonance on signal homogeneity
commonly observed with volume coils at higher field strengths
(>3 T). Surface coils can also be designed to target specific
VOIs thereby to reduce unnecessary power deposition within the
patient. Furthermore, the increased sensitivity of surface coils
for reception, in comparison to volume coils, presents an
opportunity for increased image signal-to-noise ratio (SNR) in both
receive-only and transmit and/or receive ("transceive") modes.
[0006] With the advent of fast parallel imaging techniques such as
SMASH, SENSE and transmit SENSE, there exists a greater need for
flexible placement and combination of multiple receiver and/or
transmitter RF coils. The sensitivity profiles of these multiple RF
coils are required for and influence the efficiency of these fast
parallel imaging methods. Fast parallel imaging techniques provide
the ability to remove or unfold aliasing artefacts in under-sampled
images. This ability to un-alias images provides a means to
increase temporal resolution. Images that may have been impossible
to acquire within the time constraints of breath hold techniques
may be realizable through the reduction of motion artifacts.
[0007] There are several design approaches for both volume and
surface coils that have been implemented to acquire multiple
sensitivity profiles with considerable success. For example, the
degenerate mode birdcage has been shown to be useful in sensitivity
encoding although receive-only surface coil arrays provide higher
SNR and are more suitable for high field MRI. The predominant
impediment to surface coil array design is however, the strong
magnetic coil-to-coil coupling.
[0008] There are several design approaches available to reduce this
magnetic coil-to-coil coupling, including preamplifier decoupling,
strip transmission line arrays, overlap geometries, and capacitive
decoupling networks
[0009] Magnetic coil-to-coil coupling can be substantially
eliminated between two (2) neighboring surface coils using a unique
overlap of surface coils. Unfortunately, the resultant overlapping
sensitivity profiles are less than ideal for fast parallel imaging
techniques, which are more effective when sensitivity profiles of
individual surface coils do not overlap.
[0010] Alternatively, the effect of magnetic coil-to-coil coupling
on both nearest neighbor and next nearest neighbor surface coils
can be reduced using a decoupling method employing low impedance
preamplifiers. Unfortunately, low (or high) impedance preamplifiers
are not generally available off-the-shelf and are therefore, more
complex, expensive and time consuming to implement. Furthermore,
low input impedance RF power amplifiers are not widely commercially
available off-the-shelf, thereby practically limiting this
coil-to-coil magnetic coupling reduction technique to receive-only
applications. More complicated capacitive ladder networks have been
employed to reduce magnetic coil-to-coil coupling at lower field
strengths. However, considerable electric field loss and strong
coupling between lattice networks limits their application at
higher field strengths.
[0011] Other coil-to-coil coupling reduction techniques have also
been considered. For example, U.S. Pat. No. 5,973,495 to Mansfield
discloses a method and apparatus for eliminating mutual inductance
effects in resonant coil assemblies in which a plurality of coils
is situated in sufficiently close proximity to create small mutual
inductances between the coils. Mutual inductances are evaluated
using a T star or other transformation of the relevant parts of the
circuit thereby to isolate the inductances in such a way that
series capacitances may be introduced to tune out the mutual
inductances at a common frequency, reducing the coil array to a
synchronously tuned circuit. Unfortunately, this design requires a
common ground resulting in electric field losses and requires a
common connection between all of the coils, which is geometrically
restrictive.
[0012] U.S. Pat. No. 6,788,059 to Lee et al. discloses an RF
detector array based on a microstrip array decoupling scheme. The
detector array comprises a plurality of conductive array elements
that is substantially parallel to a conductive ground plane and a
plurality of capacitors. At least one capacitor is shunted from
each conductive array element to the ground plane to adjust a
corresponding electrical length of each conductive array element. A
combination of each respective conductive array element, at least
one corresponding capacitor and the ground plane forms a resonator
that resonates at a selected frequency. A decoupling interface and
a plurality of matching boxes match each decoupled strip to a
selected impedance.
[0013] U.S. patent application Publication No. 2002/0169374 to
Jevtic discloses a capacitive ladder network to achieve next
nearest neighbor (NNN) coil-to-coil decoupling. Unfortunately, this
ladder network is complex and appears to be limited to low field
MRI applications, as considerable electric field losses, and strong
coupling between lattice networks would limit its application at
high field strengths.
[0014] U.S. patent application Publication No. 2003/0184293 to
Boskamp et al. discloses a multiple channel array coil for magnetic
resonance imaging, that similar to Lee et al., is based on a
microstrip array decoupling scheme. The array coil includes a
plurality of conductive strips formed within a dielectric medium.
The conductive strips are arranged into a generally cylindrical
configuration with each of the strips having a length selected to
cause each of the conductive strips to serve as a resonator at a
frequency corresponding to a proton MRI frequency. The cylindrical
configuration of the conductive strips forms a multiple channel,
volume resonator in which each of the conductive strips is isolated
from the remaining strips.
[0015] As will be appreciated, there exists a need for a surface
coil array that is capable of transmit and/or receive operation for
use in fast parallel imaging techniques such as SENSE imaging.
There is a further need for a surface coil array that is capable of
operation at both low and high magnetic field strengths without
succumbing to SAR limitations. There is a further need for a
surface coil array that can operate with conventional 50.OMEGA.
transmit and receive components including preamplifiers and less
expensive low power amplifiers, while maintaining the SNR benefits
of receive-only surface coils.
[0016] It is therefore an object of the present invention to
provide a novel surface coil array for magnetic resonance imaging
and spectroscopy.
SUMMARY OF THE INVENTION
[0017] Accordingly, in one aspect there is provided a surface coil
array comprising:
[0018] a plurality of spaced surface coils arranged about a volume;
and
[0019] magnetic decoupling circuits acting between said surface
coils.
[0020] In one embodiment, the magnetic decoupling circuits act
between adjacent pairs of surface coils. The magnetic decoupling
circuits are purely capacitive with each magnetic decoupling
circuit including a single capacitor. The capacitive reactance of
each magnetic decoupling circuit is equal to the mutual inductive
reactance between adjacent surface coils.
[0021] The surface coils are generally evenly spaced about the
volume and are mounted on a dielectric support. Dielectric loss
reducing elements in the form of low-loss lumped capacitors are
distributed about the surface coils.
[0022] Baluns are coupled to the surface coils and are impedance
matched to feed network circuitry coupled thereto. The feed
circuitry is 50.OMEGA. component-based. The baluns in one
embodiment are configured to provide transmit signals generated by
the feed network circuitry to the surface coils for transmit
operation and to provide receive signals received by the surface
coils to the feed network for receive operation. Alternatively, in
another embodiment the baluns are configured to condition the
surface coil array to a receive-only mode. In the receive-only
mode, the baluns isolate the surface coils from the feed network
circuitry when the feed network circuitry generates transmit
signals.
[0023] According to another aspect there is provided a surface coil
array comprising:
[0024] a surface coil support;
[0025] a plurality of generally evenly spaced surface coils mounted
on said support and surrounding a volume into which a target to be
imaged is placed; and
[0026] capacitive decoupling circuitry acting between said surface
coils to reduce magnetic coil-to-coil coupling.
[0027] According to yet another aspect there is provided an RF
resonator comprising:
[0028] a surface coil array including an arrangement of
non-overlapping magnetically decoupled surface coils encompassing a
volume; and
[0029] a feed network coupled to said surface coil array, said feed
network at least receiving signals received by said surface coils
during imaging of a target within said volume.
[0030] According to still yet another aspect there is provided a
surface coil array comprising:
[0031] a surface coil support; and
[0032] an arrangement of non-overlapping magnetically decoupled
surface coils mounted on said support and encompassing a volume
into which a target to be imaged is placed.
[0033] The surface coil array is advantageous in that it provides
the flexibility of using a single array for transmitting and/or
receiving RF signals, which has benefits in fast parallel imaging
techniques and limits image artefacts associated with using
separate transmit and receive coil arrays. The surface coil array
is also less SAR limited at high field strengths due to the
proximity of the surface coil array to the imaging volume, thereby
limiting electric field losses. Further, the surface coil array can
be tuned for a variety of paramagnetic nuclei (e.g. .sup.13C,
.sup.1H, .sup.23Na, .sup.31P, etc. . .) for use in many MR imaging
and spectroscopy applications.
[0034] The surface coil array provides for the ability to vary
surface coil size and geometry thereby offering great flexibility
in custom imaging applications (e.g. whole body imaging (TIM)). As
the surface coil array can be used with multiple, more economical
lower power amplifiers for transmit applications, more control in
imaging sequences is available. Also, the surface coil array
provides for use with conventional 50.OMEGA. transmit and receive
components while maintaining isolation between surface coils.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] Embodiments will now be described more fully with reference
to the accompanying drawings in which:
[0036] FIG. 1 is a perspective view of a transceive surface coil
array for magnetic resonance imaging and spectroscopy and an
associated feed network;
[0037] FIG. 2 is an equivalent circuit diagram for the transceive
surface coil array in a transceive mode;
[0038] FIG. 3 is an equivalent circuit diagram for the transceive
surface coil array in a receive-only mode.
[0039] FIG. 4 shows the spectral profile for each surface coil of
the transceive surface coil array;
[0040] FIG. 5 shows the spectral profile for an isolated and
unisolated surface coil;
[0041] FIG. 6 is a graph showing isolation between surface coils as
function of surface coil separation;
[0042] FIG. 7 shows field profiles of a cylindrical oil phantom
generated using the transceive surface coil array;
[0043] FIG. 8 are SNR maps within a human generated using the
transceive surface coil array and using a hybrid birdcage;
[0044] FIG. 9 shows transmit fields and contour plots for the
transceive surface coil array in the transceive mode and for a
hybrid birdcage; and
[0045] FIG. 10 shows SNR images and smoothed contour plots of a
human head captured using the transceive surface coil array in the
transceive mode, the transceive surface coil array in the
receive-only mode, a hybrid birdcage and an oversized head
coil.
DETAILED DESCRIPTION OF THE EMBODIMENT
[0046] Turning now to FIGS. 1 and 2, a transceive surface coil
(TSC) array for magnetic resonance imaging and spectroscopy is
shown and is generally identified by reference numeral 20. As can
be seen, TSC array 20 comprises a cylindrical supporting shell 22
on which are mounted a plurality of generally equally spaced,
rectangular surface coils 24. The shell 22 is formed of acrylic and
has inside and outside diameters of approximately 24.1 cm and 25.4
cm respectively. In this particular example, the TSC array includes
eight (8) surface coils 24. Each of the surface coils 24 is
substantially identical and is constructed of conductive strips, in
this case copper tape, having a thickness equal to approximately 50
.mu.m. Each surface coil 24 includes two legs 26 having a length of
approximately 21 cm and two legs 28 having a length of
approximately 8.1 cm. The width of each leg is approximately 63.5
mm and the inter-surface coil spacing is approximately 2 cm.
[0047] A magnetic decoupling circuit (MDC) 30 extends between each
adjacent pair of surface coils 24. The magnetic decoupling circuits
30 are purely capacitive with each MDC including a single capacitor
having a capacitance C.sub.m. The capacitive reactance
(1/(.omega.C.sub.m)) is equal to the mutual inductive reactance
(.vertline..omega.M.vertline.) between each surface coil at 170.3
MHz. The value of C.sub.m is function of the inter-surface coil
spacing as will be described.
[0048] Each surface coil 24 is coupled to a feed network 38 via a
quarter wavelength (.lambda./4) lattice balun 40 and a 50.OMEGA.
coaxial cable 42. The inductor and capacitor arrangement of the
.lambda./4 lattice baluns 40 is shown in FIG. 2. Low-loss lumped
capacitors 44 are also distributed about the surface coils 24 to
reduce dielectric losses (known as irrotational or coulomb electric
field losses at high magnet fields) within the volume of interest
of the TSC array 20.
[0049] The feed network 38 in the present example includes
conventional 50.OMEGA. transmit and receive components including
50.OMEGA. preamplifiers and low cost, low power RF amplifiers. The
feed network 38 splits a single transmit signal into eight (8)
equally power divided signals (-9 dB) with a phase delay of 2.pi./8
between each signal and applies the signals to the surface coils 24
via the coaxial cables 42 and .lambda./4 lattice baluns 40 to
approximate current distributions of a birdcage coil operating in a
homogenous mode. The feed network 38 also recombines signals
received by the surface coils 24 into four (4) channels via four
(4) 90.degree. branch line couplers and provides the received
signals on coaxial cables 46.
[0050] The capacitors 30 act as magnetic decoupling circuits
between nearest neighbor surface coils 24 that substantially
eliminate the effects of nearest neighbor mutual inductance on
resonance splitting and signal and noise correlation. The
capacitance C.sub.m has a unique relationship with the magnitude of
the mutual inductance (M) given by: 1 C m = 1 2 M [ 1 ]
[0051] It should be noted that even though km can be both positive
and negative, the decoupling reactance remains capacitive and may
remain in the same circuit layout. Strong and weak magnetic
coupling between surface coils 24 can be substantially eliminated
irrespective of the inter-surface coil spacing by adjusting the
capacitance C.sub.m of the capacitors 30.
[0052] Although non-next nearest neighbor coupling may introduce
peak splitting into the spectrum, loading due to both patient
conductivity and impedance matching to the 50.OMEGA. coaxial cable
has the effect of de-Qing the surface coils 24 thereby to provide
spectral smoothing. This spectral smoothing effectively reunifies
numerous peaks caused by non-next nearest neighbor interactions.
For cylindrical TSC array geometries commonly used in head imaging
and in this example, the non-next nearest neighbor coupling is
sufficiently weak that the act of impedance matching to the
50.OMEGA. coaxial cables 42 unifies any resonance splitting.
[0053] The capacitive reactance (X.sub.c) required to eliminate
resonance peak splitting at each separation is equal and opposite
to the reactance of the mutual inductance (X.sub.M), (L.sub.ind=260
nH) [cross correlation between X.sub.Cm& X.sub.M=-96.5%,
p=1.6E-6, confidence interval a=0.95, N=11]. This magnetic
decoupling reactance (X.sub.Cm) effectively shorts the positive
feedback coupling between resonance modes into degeneracy. As a
result, the magnetic decoupling circuits 30 are able to decouple
both strong and weak magnetic coupling. In the extreme case of
k.sub.m=1, the required decoupling capacitance (3.3 pF) is
physically realizable. As the mutual inductance approaches zero,
the capacitance required for decoupling approaches infinity.
Fortunately, the magnetic decoupling circuits 30 become redundant
when there is no mutual inductance between the surface coils
24.
[0054] Closed loop current paths in close proximity to surface
coils 24 at high fields couple and degrade spectral responses,
analogous to the effects of neighbouring surface coils within the
TSC array 20. The benefits of using .lambda./4 lattice baluns 40 to
reduce conservative electric field generated noise outweigh the
detriments due to coupling. To avoid the additional closed loop
current paths introduced by matching networks, the .lambda./4
lattice baluns 40 are used to both balance and match the surface
coils 24 to the 50.OMEGA. coaxial cables 42. Furthermore, the
parallel to series capacitance ratio may be used to reduce the
effects of the varying patient to patient to loading on the overall
matching. For the transceive mode, an open parallel circuit matched
and tuned design has a spectral profile less influenced by other
surface coils although it is more susceptible to impedance mismatch
in patient-to-patient variance. Without proper balancing, the
transmission line shield voltage potential rises above ground
potential due to direct conductive coupling, and the shield acts as
an antenna and increases the amount of conservative electric field
coupling to the sample and noise correlation detected in the
system.
[0055] The TSC array 20 an also be conditioned to a receive-only
mode by altering the .lambda./4 lattice baluns 40. As shown in FIG.
3, in the receive-only mode, the .lambda./4 lattice baluns 40
include PIN diodes 60. The PIN diodes 60 electrically open each of
the surface coils 24 during transmit operation of feed network
38.
[0056] To test the TSC array 20, a number of images were captured.
All imaging data were obtained using 4 T Varian Unity INOVA
whole-body MRI/MRS system (Palo Alto, Calif., USA) interfaced to
Siemens Sonata Gradients and amplifiers (Erlangen, Germany).
[0057] In a first series of tests, the MR images were obtained
using a spoiled steady state free precession imaging sequence,
TR=22 ms, TE=15 ms, NRO=256 NPE=256, sweep width 60 kHz, FOV
20.times.20 cm, .DELTA.z =1 cm. The SNR from the both a hybrid
birdcage and the TSC array 20 was calculated according to modified
sum of squares method proposed by Constantinides et al. in the
article entitled "Signal to noise measurements in magnitudes from
NMR Phase Array" published in Magnetic Resonance Medicine Vol. 38,
pages 852 to 857 (1997) to account for the noise amplification of
magnitude images when combining data from multiple receivers. The
four (4) receiver configuration of the TSC array 20 restricted use
of all eight (8) channels independently. The feed network 38
equally transmitted power to all eight surface coils 45.degree. out
of phase and recombined coils in quadrature pairs to conform to the
four (4) receiver configuration.
[0058] The gradient coupling between the non-shielded TSC array 20
and system gradients was not compensated for. A resonance frequency
of 171.2 MHz was measured post acquisition and would suggest that
better SNR is attainable.
[0059] Two key parameters to assess the TSC array 20 are the
spectrum profile, and the isolation between neighboring surface
coils 24. Provided the non-next nearest neighbor interactions are
weak, or there is sufficient loading, the array of surface coils 24
act as they would independently. A shielded field probe was
centered on the inside of the transmit coil with all other coils
loaded with 50.OMEGA., and a loader shell to mimic the human head.
As shown in FIG. 4, the spectral profile of each individual surface
coil 24 is identical with the spectral response of that of an
isolated surface coil as shown in FIG. 5, which validates the
effectiveness of the magnetic decoupling scheme. The average
isolation between surface coils 24 as a function of separation,
measured with a Hewlett-Packard 4395A network/spectrum/impedance
analyzer, is shown in FIG. 6 with respect to a through calibration.
As expected, there is cylindrical symmetry of isolation with
respect to surface coil separation. Since arbitrary orientation was
chosen to measure separation, symmetry about 180.degree. is
expected. The maximum coupling between surface coil pairs is
approximately -25 dB.
[0060] A centered uniform cylindrical oil phantom having a diameter
equal to 12.7 cm was used to test the recombination of orthogonal
surface coils 24 through the feed network 38. An annulus loader
shell (13 g NaCl, 1.9 g CuSO.sub.4, 5.9235 L, inner diameter 16.5
cm, outer diameter 22.9 cm ) was used to load the TSC array 20 to
50.OMEGA.. The field profiles of the four recombined receive
channels are shown in FIG. 7. Each of the four receivers can be
seen to be a superposition of orthogonal surface coils exemplifying
the aforementioned high isolation between the receivers.
[0061] SNR maps as shown in FIG. 8 were used to compare the TSC
array 20 to a hybrid birdcage. The TSC array showed a 38% increase
in average SNR measured throughout the entire head volume. A 9 fold
increase in SNR was observed when the patient was in close
proximity to the TSC array 20. In the center of the brain there is
an 8% decrease in SNR (centered 11.times.11 voxel ROI).
[0062] In a second series of tests, the MR images were obtained
using a spoiled steady state free precession (FLASH) sequence. The
.vertline..beta..sup.+.vertline. profile measurements were made by
implementing the method proposed by Wang et al. in the article
entitled "Measurement and correction of transmitter and receiver
induced non-uniformaties in vivo" published in Magnetic Resonance
Medicine, Vol. 53, pages 408 to 417 (2005) that uses the signal
intensities of 2 spin echo sequences (TR/TE 5000/13 ms,
FOVx=FOVy=25 cm, NRO=NPE=64) with exciting/refocusing flip angles
of 60.degree./120.degree. and 120.degree./240.degree. respectively.
The imaging parameters used for all head SNR images were:
FOVx=FOVy=24 cm, TR=20 ms, TE=5 ms, a slice thickness of 1 cm, tip
angle of 11.degree., NRO=NPE=256, with 2 averages per image. SNR
maps were calculated for all surface coils 24 using the magnitude
NMR phased array method of Constantinides et al. referenced above
to account for additional magnitude noise accumulating from
combination of multiple receiver channels. All isolation and
spectral profiles (with the use of shielded RF field probes)
measurements were made with a Hewlett-Packard 4395A
network/spectrum/impedance analyzer.
[0063] The capacitive decoupling circuits 30 achieve approximately
-25 dB of isolation (or better) between surface coils 24. In
comparison, a typical isolation between a surface coil pair of
similar geometry decoupled through natural decoupling (10% overlap)
at 170.3 MHz is approximately -20 dB. This high degree of isolation
between all surface coils 24 within the TSC array 20 allows each
surface coil to act as it would in isolation. Hence, each surface
coil 24 transmits and receives independently of each other. The RF
spectral profiles as measured with field probes of each surface
coil 24 within the TSC array 20 (loaded with an annular loader
shell) do not differ from the spectral profile of a single surface
coil in complete isolation, confirming the independence of each
surface coil within the TSC array 20. Upon biasing in receive-only
mode, the active decoupling of the TSC array provides greater than
-20 dB of isolation relative to its unbiased field probe
measurement. As a result, the transmit power for the oversized head
(OH) coil required to achieve a 90.degree. in both the transceive
mode and the receive-only mode (using the TSC array modified for
receive-only operation and the OH coil to transmit) are
identical.
[0064] A limitation of a surface coil for volume imaging is the
penetration depth that it can achieve. To assess SNR performance,
measurements were made within the human head of a volunteer. The
transmit field .vertline..beta..sup.+.vertline. profiles for the
TSC array 20 in the transceive mode and a hybrid birdcage are shown
in FIG. 9. The maximum (mean) transmit field inhomogeneity
(relative to each surface coil's maximum field intensity) for the
TSC array and hybrid birdcage are 4.2% (97.0) and 1.2% (99.3%)
respectively. SNR images and smoothed contour plots of a human head
captured using the TSC array 20 in the transceive mode (TR), the
TSC array 20 in the receive-only mode (RO), a hybrid birdcage (BC)
and an oversized head coil (OH) are shown in FIG. 10. These SNR
images were reconstructed using the phased array reconstruction
method proposed by Constantinides et al. referred to above. The
mean SNR values within the brain volume and at the center of the
brain (50.times.50 pixel region of interest (ROI) as outlined by
the white box in part (d) of FIG. 10 are shown in Table 1
below:
1 TABLE 1 Coil Type TR RO BC OH Mean SNR 57.3 48.1 41.6 23.8 Center
SNR 48.1 41.9 53.9 28.6
[0065] Compared to the standard hybrid birdcage a decrease of 10.7%
(22.3%) in SNR is measured in the center brain region and an
increase of 37.7% (15.6%) in SNR over the whole brain volume for
the TSC array 20 in the transceive (receive-only) mode. A maximum
9-fold increase in SNR is measured in the brain region using the
TSC array 20 as compared to the hybrid birdcage. There exists a
10.7% decrease of SNR in the center brain region of the TSC array
20 in the transcieve mode compared to the hybrid birdcage. Both
hybrid birdcage values demonstrate a pronounced dielectric
resonance in the centre of the array, with the centre SNR
considerably enhanced relative to the mean.
[0066] The SNR of the TSC array 20 in the transceive mode is
slightly higher than in the receive-only mode (see Table 1 and FIG.
10). Since the receive profiles are identical for both modes of
operation (through the use of active decoupling), it would suggest
that the TSC array 20 has better transmit capabilities. In
practice, it is common to use a larger transmit RF coil than the
receiver (when in receive-only mode) since two coils cannot occupy
the same space within the magnet. The transmit capabilities of the
TSC array 20 (when driven properly) are approximately equivalent to
that of a comparable size hybrid birdcage. The SNR increase of the
TSC array 20 in the transceive mode as compared to the TSC array
operating in the receive-only mode could be attributed to better
phase performance than the larger oversized head coil, resulting in
effectively better quadrature than the larger coil. Although active
decoupling provides a high degree of isolation, it is possible that
the copper tape used to form the surface coils 24 shielded the
sample from approximately 25% of the transmitted RF from the
oversized head coil. The recent trend towards prolific use of
multiple surface coils to increase fast parallel imaging efficiency
would make this detrimental shielding effect more pronounced in the
receive-only mode, making the transceive mode more desirable for
higher SNR.
[0067] The 9-fold increase of SNR at the periphery of the brain
with the TSC array 20 as compared to the hybrid birdcage can be
attributed to the superior SNR sensitivity of the surface coils 24
in close proximity to the VOI (as seen in FIG. 10). This increase
in peripheral SNR is comparable to results previously cited in the
literature for surface coil arrays (SNR increase of six (6) in the
human head for a receive-only surface coil array as proposed by
Bodurka et al. in the article entitled "Scalable Multichannel MRI
Data Acquisition System" published in Magnetic Resonance Medicine,
Vol. 51, pages 165 to 171 (2004)). A notable decrease in SNR is
shown in the deep brain compared to the twofold increase shown by
Bodurka et al. Dielectric resonance of the human head at 4 T has a
"bulls-eye" profile, which effectively decreases the sensitivity of
the birdcage resonator in the peripheral regions of the brain and
increases its sensitivity in the center of the brain. This apparent
increase of signal for birdcage or TEM coils in the middle of the
brain may explain the decrease (11%) in deep brain SNR measured for
the TSC array 20, which inherently displays signal fall off deep
into the VOI and which therefore may not have such striking
dielectric effects. These results suggest that a conformal surface
coil array may find application at very high fields, e.g. 7 T,
where dielectric resonance effects dominate.
[0068] The variability of subject loading at each surface coil
within the TSC array results in a correspondingly variable transmit
power required for a 90.degree. tip. A solution to this problem
would be to use multiple low power RF amplifiers to transmit to
each surface coil independently, or to implement the design on an
elliptically conformal coil former to provide equal loading of all
surface coils. This first solution would provides the ability to
individually tailor the B1 field (both magnitude and phase) of each
surface coil allowing for a means to achieve transmit SENSE and a
means to accurately calibrate the transmit power delivered to each
surface coil while being less costly than a single high power RF
amplifier.
[0069] Through the use of capacitive decoupling circuits and
surface coil placement, the TSC array 20 is able to transmit and
receive through each surface coil independently while maintaining
the use of conventional 50.OMEGA. amplifiers and preamplifiers. The
high SNR of receive-only surface coils and fast parallel imaging
capability can be achieved with a single multiple surface coil
array, which can easily be incorporated into existing MR systems.
In addition to proton applications, the TSC array could also find
use in X-nucleus applications, where a homogeneous body coil for
transmitting is not available.
[0070] Although the TSC array has been described as including eight
(8) surface coils, those of skill in the art will appreciate that
the TSC array may include more or fewer surface coils. Also, the
surface coils need not be mounted on a cylindrical supporting
shell. Other surface coil support configurations can be used
depending on the particular target to be imaged. Further, the
surface coils need not be rectangular. The shape and sizes of the
surface coils can be tailored to the particular imaging environment
in which the TSC array is being used. In addition, while magnetic
decoupling circuits are shown interconnecting each adjacent pair of
surface coils, those of skill in the art will appreciate that
different magnetic decoupling circuit configurations can be used.
For example, separate sets of surface coils can be magnetically
decoupled independently of one another. In particular, in the case
of a TSC array configured to image a patient's prostate, the TSC
array includes two separate four (4) surface coil sets, each set of
which is magnetically decoupled independently.
[0071] Although a single feed network is shown, those of skill in
the art will appreciate that a separate circuit may be used to
provide each surface coil with a transmit signal. This reduces the
power required on a per surface coil basis enabling the use of
multiple lower power RF transmitters and enabling transmit SENSE
capabilities. In addition, the signal received by each surface coil
can be applied to downstream circuitry independently of the feed
network through TR switched or other technology enabling receive
parallel imaging such as SENSE or SMASH.
[0072] Although preferred embodiments have been described, those of
skill in the art will appreciate that variations and modifications
may be made without departing from the spirit and scope thereof as
defined by the appended claims.
* * * * *