U.S. patent application number 09/754054 was filed with the patent office on 2001-10-25 for multiple tunable double ring surface coil with high b1 homogeneity.
This patent application is currently assigned to National Research Council of Canada. Invention is credited to Tomanek, Boguslaw, Volotovskky, Vyacheslav.
Application Number | 20010033165 09/754054 |
Document ID | / |
Family ID | 26870373 |
Filed Date | 2001-10-25 |
United States Patent
Application |
20010033165 |
Kind Code |
A1 |
Tomanek, Boguslaw ; et
al. |
October 25, 2001 |
Multiple tunable double ring surface coil with high B1
homogeneity
Abstract
To perform multinuclear magnetic resonance spectroscopy and
imaging, a coil with a high signal-to-noise ratio and spatially
uniform B.sub.1 field that covers the same volume of interest at
several frequencies is preferred. A radio frequency surface coil
probe that produces a homogeneous field within the same volume for
both .sup.31P and .sup.1H is described. The probe provides signal
to noise characteristics close to those of a single frequency
double ring assembly. It acquired excellent proton images and
.sup.31P spectra from an image selected voxel in rat liver at 7 T.
The probe uses a double ring construction where each ring is tuned
by a parallel LC trap to two or more different frequencies. A
calculation of the values of the inductance and capacitance allows
the double coil arrangement to be tuned to the required frequencies
with a peak of the homogeneity at each frequency occurring at the
same distance from the coil and with good Q. The probe is powered
by a driver coil adjacent to the probe by mutual inductance.
Inventors: |
Tomanek, Boguslaw; (Colpany,
CA) ; Volotovskky, Vyacheslav; (Winnipeg,
CA) |
Correspondence
Address: |
Mr. Adrian D. Battison
Ade & Company
1700-360 Main Street
Winnipeg
MB
R3C3Z3
CA
|
Assignee: |
National Research Council of
Canada
Montreal Road
Ottawa
ON
K2AOR6
|
Family ID: |
26870373 |
Appl. No.: |
09/754054 |
Filed: |
January 5, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60174592 |
Jan 5, 2000 |
|
|
|
Current U.S.
Class: |
324/318 |
Current CPC
Class: |
G01R 33/3635 20130101;
G01R 33/341 20130101 |
Class at
Publication: |
324/318 |
International
Class: |
G01V 003/00 |
Claims
1. A method for use in simultaneous nuclear magnetic resonance
experiments at two different predetermined frequencies matched to
the resonant frequencies of two different nuclei comprising:
providing a sample defining a volume of interest; locating the
sample in a magnetic field; providing a surface probe arranged to
lie adjacent a surface of the sample and offset to one side of the
volume of interest; arranging the probe to lie generally in a
common plane for generating an RF field within the volume of
interest adjacent to the plane; providing a driver connected to a
power source for generating within the surface probe an alternating
current for generating said RF field; the surface probe including
two separate probe coils each forming a loop lying generally within
the common plane with the loop of one of the coils inside the loop
of the other of the coils, the separate probe coils being arranged
to co-operate by mutual inductance to generate said RF field;
providing in each coil inductive and capacitive components arranged
to tune the respective coil to at least two resonant frequencies
different from said predetermined frequencies; and selecting the
values of the inductive and capacitive components relative to the
construction and arrangement of the coils such that: a) the
resonant frequencies of the surface probe obtained by the
co-operation of the two coils are matched to said predetermined
frequencies; b) the field strength of the RF field generated by the
surface probe obtained by the co-operation of the two coils at each
of the different resonant frequencies provides a peak of maximum
homogeneity within the volume of interest at a predetermined
distance spaced from the plane, thus reducing signals from a
portion of the sample immediately adjacent the plane; and c) the
predetermined distance for each of the frequencies is substantially
equal.
2. The method according to claim 1 wherein the values of the
inductive and capacitive components relative to the construction
and arrangement of the coils are selected so as to provide for the
peak at each resonant frequency a high Q.
3. The method according to claim 2 wherein Q is higher than
100.
4. The method according to claim 2 wherein Q is approximately the
same for both peaks.
5. The method according to claim 1 wherein the step of providing a
driver includes providing a matched coil tuned to the predetermined
frequencies and locating the matched coil adjacent the probe to
generate the current therein by mutual inductance.
6. The method according to claim 5 wherein fine tuning of the
driver to either frequency is achieved by moving a strip of a foil
between the matched coil and the probe.
7. The method according to claim 5 wherein matching of the
inductance of the matched coil to that of the power source is
effected by moving the matched coil in a plane parallel to that of
the probe.
8. The method according to claim 1 wherein the inductive and
capacitive components include a parallel LC trap.
9. The method according to claim 1 wherein the coils are
coplanar.
10. The method according to claim 1 wherein the coils are
annular.
11. The method according to claim 1 wherein the coils are
coaxial.
12. A probe for use in simultaneous nuclear magnetic resonance
experiments in a sample at two different predetermined frequencies
matched to the resonant frequencies of two different nuclei in the
sample comprising: a surface probe body arranged to lie
substantially in a common plane for locating adjacent a surface of
the sample and offset to one side of a volume of interest in the
sample for generating an RF field within the sample; a driver for
connection to a power source for generating within the surface
probe body an alternating current for generating said RF field; the
surface probe body including two separate probe coils each forming
a loop lying generally within the common plane with the loop of one
of the coils inside the loop of the other of the coils, the
separate probe coils being arranged to co-operate by mutual
inductance to generate said RF field; each coil having inductive
and capacitive components arranged therein to tune the respective
coil to at least two resonant frequencies different from said
predetermined frequencies; the values of the inductive and
capacitive components being selected relative to the construction
and arrangement of the coils such that: a) the resonant frequencies
of the surface probe obtained by the co-operation of the two coils
are matched to said predetermined frequencies; b) the field
strength of the RF field generated by the surface probe obtained by
the co-operation of the two coils at each of the different resonant
frequencies provides a peak of maximum homogeneity within the
volume of interest at a predetermined distance spaced from the
plane, thus reducing signals from a portion of the sample
immediately adjacent the plane; and c) the predetermined distance
for each of the frequencies is substantially equal.
13. The probe according to claim 12 wherein the values of the
inductive and capacitive components relative to the construction
and arrangement of the coils are selected so as to provide for the
peak at each resonant frequency a high Q.
14. The probe according to claim 13 wherein Q is higher than
100.
15. The probe according to claim 13 wherein Q is approximately the
same for both peaks.
16. The probe according to claim 12 wherein the driver includes a
matched coil tuned to the predetermined frequencies which is
located adjacent the probe to generate the current therein by
mutual inductance.
17. The probe according to claim 16 including a strip of a foil
between the matched coil and the probe.
18. The probe according to claim 16 wherein the matched coil is
movable in a plane parallel to that of the probe.
19. The probe according to claim 12 wherein the inductive and
capacitive components include a parallel LC trap.
Description
[0001] This invention relates to a probe for use in nuclear
magnetic resonance experiments and to a method for use in such
experiments.
BACKGROUND OF THE INVENTION
[0002] In magnetic resonance experiments on a sample within a
magnetic field, the RF field, generally known as the B1 field, is
generated by an NMR probe located at or adjacent the sample. The
probe can be provided by a surface coil arrangement which is
generally planar and located immediately adjacent the surface of
the sample for experiments on a volume of interest within the
sample and spaced from the plane of the probe. Surface coils are
widely used in magnetic resonance imaging (MRI) and spectroscopy
(MRS) due to their high sensitivity. The sensitivity, however,
decreases rapidly with increasing distance between the coil and the
volume of interest due to the non-homogeneous excitation as well as
signal reception. This results in variations in the intensity of MR
images and adversely affects MRS experiments.
[0003] Some attempts to overcome this have used composite or
adiabatic pulses applied to the probe, although these require
relatively high RF power.
[0004] In the alternative, volume coils, which are no longer
planar, can provide a homogeneous B.sub.1 but with a reduced signal
to noise ratio (SNR) compared to surface coils. Separate transmit
and receive coils are often used. In this case a volume coil or a
large surface coil provides a highly homogeneous B.sub.1 field and
a smaller receiving surface coil is used to obtain high SNR.
However, such a configuration still suffers from inhomogeneous
signal reception and the small coil must be actively de-coupled
requiring cumbersome electronics that tends to introduce additional
noise and reduce the SNR. In addition, high RF power is required
for excitation, often limiting the pulse sequences not to exceed
specific absorption rate (SAR) limits.
[0005] A dual-ring surface coil with the sensitivity and power
requirements of a traditional surface coil and the B.sub.1
homogeneity of a volume coil over a targeted volume of interest has
been described in a paper published by Tomanek B. Ryner L, Hoult D
I, Kozlowski P, Saunders J K entitled Dual Surface Coil with
High-B.sub.1 Homogeneity for Deep Organ MR Imaging in Magn Reson
Imag 1997; 10:1199-1204. Further details are published in a paper
published by King S B, Ryner L N, Tomanek B, Sharp J C, Smith I C P
entitled MR Spectroscopy using Multi-Ring Surface Coils in Magn
Reson Med 1999; 42:655-664.
[0006] This arrangement is used for .sup.1H MR imaging and .sup.1H
spectroscopy. In this design an additional inductively coupled
coaxial ring produces a B.sub.1 field that compensates for the
roll-off of the B.sub.1 field produced by a larger surface coil
resulting in high B.sub.1 homogeneity over a specified volume.
[0007] The combination of MR imaging with spectroscopy provides a
powerful tool for clinical management of problems such as stroke,
tumor monitoring and the assessment of other diseases. Especially
the integrated acquisition of proton images and localized spectra
is essential for the practical application of spectroscopic
techniques to human and animal research and it is highly desirable
to use a single RF probe in order to minimize operational problems
such as changing coils from one experiment to another. Advantages
of a such a probe in the form of a double-tuned coil also include
more accurate localization of spectra and easier shimming for
.sup.31P spectroscopy since the shimming can be done at the .sup.1H
frequency with the same coil over the same volume. Absolute
quantitation methods using proton signal are as well easier to
perform when correction for substantially different B.sub.1 profile
is not required.
[0008] Early publications on double resonant RF devices dealt
mostly with surface coils. One example is shown in U.S. Pat. No.
4,742,304 of Schnall et al issued May 3, 1988 which discloses a
frequency splitting circuit used on a simple coil which allows the
coil to resonate at two separate frequencies which can be tuned to
the required frequencies for the nuclei under examination. However
this arrangement has the disadvantages of the simple surface coil
of the inhomogeneous B1 field leading to high levels of surface
effects which interfere with the analysis of the signals from the
volume of interest, often requiring surgical removal of skin and
overlying tissue to allow effective investigation of an underlying
tissue of interest.
[0009] While the above patent thus provides an arrangement for
simultaneous investigation at the two or more required different
frequencies for a simple coil, this technique could not be applied
to more complex coil arrangements since the formulae disclosed for
calculating the values of the inductive and capacitive components
for the frequency splitting circuit could not be applied to more
complex coil arrangements. Experimental analysis of the values
required for more complex coil constructions is simply not
practical because of the complex mutual induction between the coil
components.
[0010] Recent assemblies for double resonant RF devices have
therefore abandoned the above patent and are instead mostly based
on volume coils where coaxial inserted birdcage resonators are
often employed. Such a set-up implies for volume coils usually a
high power requirement and lower SNR; very often VOI for two
frequencies are not the same because of different size or position
of coaxial resonators.
SUMMARY OF THE INVENTION
[0011] It is one object of the present invention to provide a
method for use in simultaneous nuclear magnetic resonance
experiments at two different predetermined frequencies matched to
the resonant frequencies of two different nuclei using a probe of
the surface coil type.
[0012] The present invention provides a combination of a multi-ring
surface coil in which each coil includes a multi frequency trap
circuit. The present invention provides a technique for calculating
the inductance and capacitance values necessary to tune the probe
defined by the combined surface coils to the required frequencies,
while at the same time locating the peak of maximum homogeneity at
the same distance from the plane of the coil and at the same time
providing acceptable Q values for effective experiments.
[0013] In the example presented herein, imaging is performed on the
.sup.1H nucleus while spectroscopy is performed on the .sup.31P
nucleus. However other frequencies can be selected for analysis of
different nuclei.
[0014] Thus the B.sub.1 field for the nuclear magnetic resonance
experiments is transmitted to the sample most efficiently from a
resonant RF surface coil placed in proximity to the sample and
connected to the RF driving apparatus. Either the same probe or a
second probe can be connected to the RF receiving apparatus to
receive the MR signals which are induced by the precessing
magnetism of the nuclei in conventional manner.
[0015] The probe of the present invention provides the following
advantages
[0016] 1. Operation of the probe at two or more frequencies for
both sequential and simultaneous acquisition of MR data from two or
more nuclei.
[0017] 2. Good or improved homogeneity of the RF field where the
peak of maximum homogeneity is at the same distance from the coil
for both or all of the different frequencies. This allows the
following:
[0018] a) an improved spatial localization since the higher RF
field homogeneity, the more precisely the voxel of interest can be
chosen;
[0019] b) an improved water suppression since the higher RF field
homogeneity, the better efficiency of water peak suppression
procedure.
[0020] c) an improved ability to measure mean metabolite
concentrations quantatively over a tissue volume since in
homogeneous RF field the NMR signal directly corresponds to
metabolite concentration, but for non-homogeneous RF field, its
complicated non-linear spatial dependence should be taken into
account.
[0021] 3. Improved signal to noise ratio relative to volume
coils.
[0022] 4. Reduced transmitter power requirements relative to volume
coils.
[0023] 5. The ability to shim, that is slightly adjust, the
magnetic field using the relatively stronger signal received from
one nucleus (which is in the example described hereinafter the
proton or .sup.1H nucleus), while performing spectroscopy with the
relatively weaker signal received from a different nucleus (which
is in the example the .sup.31P nucleus).
[0024] 6. Decreased contamination from signals generated outside
the volume of interest and particularly by surface material of the
sample immediately adjacent the surface probe such as the skin and
adjacent tissues of an animal or human sample.
[0025] In the example described hereinafter there are two rings
which are circular, coaxial, straight and either parallel or
coplanar. However the number of rings can be different from two.
The rings do not need to be parallel or straight but in general the
rings lie in or adjacent to a surface plane which defines the
surface of the probe for placing immediately adjacent the surface
of the sample. The number of resonant frequencies is preferably two
but can be increased from that number by increasing the number of
inductor-capacitor (LC trap) combinations in each of the rings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] One embodiment of the invention will now be described in
conjunction with the accompanying drawings in which:
[0027] FIG. 1 is a schematic illustration of the probe and method
according to the present invention and particularly the geometry of
the double tuned dual surface coil forming the probe.
[0028] FIG. 2 is a graph of the reactance of the probe as a
function of frequency where .omega..sub.0 is the resonance of the
parallel trap L2 and C2.
[0029] FIG. 3 is a graph plotting the B1 field (arbitrary units)
produced by the two-coil assembly at different frequencies.
[0030] FIG. 4 is a snapshot-flash proton image of a rat liver
obtained with the dual surface coil of FIG. 1.
[0031] FIG. 5 is a phosphorus spectrum from a voxel of the same
liver.
DETAILED DESCRIPTION
[0032] In FIG. 1 is shown schematically the components for magnetic
resonance experiments including a magnet 10 within the field of
which is located a sample 11 having a volume of interest within the
sample indicated at 12 and a surface area of the sample indicated
at 13. A surface probe 14 is located immediately adjacent the
surface area 13 of the sample. The probe includes two coils 14A and
14B which in the example are coplanar, coaxial, circular and
straight. However the coils do not have to have any of these
characteristics except that they need to be arranged so that they
provide mutual inductance with one coil generally inside the other
so that it is smaller in transverse dimension and with both coils
forming a surface coil arrangement thus defining generally a plane
of the probe which is arranged at a surface of the sample generally
in contact therewith.
[0033] The two coils are housed within a suitable container which
locates the coils in position and maintains them fixed relative to
each other. The housing provides insulation to prevent current from
being directly communicated from the coils to the surface of the
sample.
[0034] Each coil includes a capacitor C1 together with an LC trap
circuit T including a capacitor C2 and an inductance I. This
arrangement as explained hereinafter provides tuning of each of the
two coils separately to two different resonant frequencies.
[0035] The coils are powered with the RF oscillating current by
mutual inductance with a driving loop L which itself has capacitor
CL and trap circuit TL. The driver loop L is mounted on a support
schematically indicated at S which is driven longitudinally in a
plane parallel to the plane of the probe by a manual drive or motor
schematically indicated at M. A foil conductor F is positioned
between the plane of the loop L and the plane of the probe and is
again mounted on a support S1 and driven longitudinally in a plane
parallel to the plane of the probe by a manual drive or motor
schematically indicated at M1. The motor M and motor M1 are
controlled to effect matching of the impedance (generally 50 OHMS)
of the probe relative to the impedance of the power source P and
also to effect fine tuning of the resonant frequencies using
techniques well known to one skilled in the art. The current ratio
is -1.4:1 for both .sup.1H and .sup.31P nuclei.
[0036] In FIG. 2 is shown the plots of the B1 field produced at the
two different resonant frequencies where position X=0 corresponds
to the plane of the ring 14A of the probe. The ring 14B can lie in
the same plane or can be offset slightly away from the plane. The
field was measured at frequencies of 121.5 MHz for .sup.31P and 300
MHz for 1H. It will be noted that the field at each frequency
generates a peak at a position spaced from the plane of the probe
with the field B1 being relatively low immediately adjacent the
plane. At the peak there is provided maximum homogeneity in that
the field is substantially constant over the area adjacent the
distance which is in the example 1.0 cm from the plane. It will be
noted that the distance of the peak for each of the two frequencies
is equal and an example shown is at the 1.0 cm distance.
[0037] The multi ring concept provides high B.sub.1 homogeneity
within a specific VOI. An opposite current in the small ring acts
to compensate for the roll-off of the B.sub.1 field associated with
a single surface coil (large ring). Thus in the selected VOI the
resulting superposition of two RF fields provides improved
homogeneity of B.sub.1. To conduct MRI and MRS on a rat liver it is
necessary to create a desired region of interest 10 to 15 mm deep
inside the rat abdomen. To achieve a homogeneous B.sub.1 field of
proper frequency at a given VOI resonant frequencies and currents
in individual rings were calculated using the Biot-Savart law. It
was found that the following parameters provided the best B.sub.1
homogeneity within VOI: diameters of small and large coils are 15
and 40 mm respectively with current ratio -1.4:1 for both .sup.31P
and .sup.1H, and distance between coils of 4 mm.
[0038] To make the assembly resonate at 121.5 MHz and 300 MHz, the
resonance frequencies of the small ring are calculated, using the
calculations set out hereinafter, to be 113.7 MHz and 291.3 MHz.
The large ring then has to resonate at 118 MHz and 296.3 MHz.
Distributed capacitance reduces conservative electric field losses
within the sample. Placing a metal foil between the capacitors and
the sample reduced the remaining losses. Coupling between the rings
and the double-resonant matching loop is achieved by a mutual
inductance. Since the assembly in one example is constructed for a
narrow bore magnet, a rectangular (40 mm.times.50 mm) loop sliding
along the bore is used for convenient matching. A fine tuning to
one or the other frequency is achieved by using the narrow strip of
a metal foil moving between the matching coil and the dual coil
assembly. All coils can be made of 1.6 mm diameter tinned copper
wire.
[0039] To achieve a double resonance of the coil each ring has to
resonate at two frequencies, therefore the frequency splitting
network is used. For a simple single frequency probe resonance is
established when the capacitor reactance -1/j.omega.C and the coil
reactance j.omega.L compensate one another. Adding the parallel LC
trap in series with such a coil creates the frequency-splitting
network. The reactance of this network, as a function of frequency,
begins being capacitive, then passes through a pole at the
frequency corresponding to the resonance of the trap, and then
becomes capacitive again. The reactance curve crosses the
anti-reactance curve of the coil twice and this results in two
resonance frequencies. Parameters of the splitting circuit must be
chosen such that its frequency .omega..sub.0 lies between
frequencies of interest .omega..sub.lower and .omega..sub.higher
(in the present case, 121.5 MHz for .sup.31P and 300 MHz for
.sup.1H at 7 T).
[0040] In order to preserve high magnitude of Q for both
frequencies, it is desirable to keep the splitter inductance L2 at
about 25% of the inductance of main coil L1. Following this
recommendation, similar values of Q for both resonant frequencies
can be experimentally achieved. It is found that a slight RF field
created by a parallel LC trap does not influence B.sub.1
homogeneity. The whole assembly is mounted on 2 mm plastic (high
density polyethylene) support.
[0041] Plots of the axial B.sub.1 fields produced by a loaded
dual-coil assembly at two frequencies of interest and measured by
Network Analyzer HP 8752A (Hewlett Packard, Santa Rosa, Calif.;
USA) using a small search probe are shown in FIG. 2. The field
strength is measured and presented in arbitrary units. It can be
seen that the regions of B.sub.1 homogeneity for both frequencies
overlap and correlate with the theoretical calculations, used to
optimize ROI depth. The value of Q (where Q=.omega.L/R) for the
unloaded assembly is 160 at both frequencies. When the coil is
loaded with a cylinder of 22 cm diameter and 17 cm high filled with
physiological saline which placed 2 mm above the outermost ring,
corresponding values of Q decreased to 105 and 75 for 121.5 MHz and
300 MHz respectively. In order to operate suitable, it is desirable
for Q to be above 100 and is preferably approximately equal for
both peaks. This can only be obtained by calculation of the
inductance and capacitance values using the formulae set out
hereinafter.
[0042] To obtain the desired current ratio, the inductance and
reactance of both rings must be calculated to find the appropriate
resonance frequencies of each ring. The mutual inductance between
the rings, calculated from the coil geometry (ring radii and
distance between them), then causes the two-ring set to resonate
exactly at the Larmor frequency. To make the coil resonate at two
frequencies, a splitting network reactance can be included.
[0043] The mutual inductance M.sub.12 between two circuits can be
expressed as 1 M 12 = 0 4 1 [ 2 s1 12 ] s 2 [ 1 ]
[0044] where .mu..sub.0 is the permeability of free space, ds.sub.1
and ds.sub.2 are elements of circuit 1 and 2 respectively and
d.sub.12 is the distance between them. Based on the Lenz's law, the
reactance of the first and the second ring is 2 X 1 = V 1 I 1 = - j
0 M 12 I 2 I 1 [ 2 ] X 2 = V 2 I 2 = - j 0 M 12 I 1 I 2 [ 3 ]
[0045] On the other hand the reactance of a ring with inductance
L.sub.p, capacitance C.sub.p, and frequency splitting network with
a choke L.sub.sp1 and capacitance C.sub.sp1 is given by 3 X p = X L
p + X C p + X spl = j 0 L p + 1 j 0 C p + j 0 L spl 1 j 0 C spl j 0
L spl + 1 j 0 C spl . [ 4 ]
[0046] The ring inductance can be approximated as 4 L p 0 r p { ln
( 8 r p b p ) - 2 } [ 5 ]
[0047] where r.sub.p is the radius of the ring and b.sub.p is the
radius of its cylindrical conductor. The choke inductance L.sub.sp1
is chosen to be approximately 25% of the inductance of main ring
L.sub.p (12). The tuning capacitance C.sub.p and splitting network
capacitance C.sub.sp1 required for each ring can be calculated when
comparing the reactance in Eq. [2] and [4] (for a small ring) and
in Eq. [3] and [4] (for a large ring) at two resonance frequencies
(e.g. 121.5 MHz and 300 MHz). The resonance frequency of each ring
is then found by solving the equation
X.sub.L.sub..sub.P+X.sub.C.sub..sub.P+X.sub.sp1=0. [6]
[0048] After tuning each ring to the appropriate frequency the
rings are brought together to form the multi-ring surface coil. The
two rings couple to create four resonant modes, with the second and
the fourth modes producing the desired B.sub.1 profiles at the
Larmor frequencies .omega..sub.0 (e.g. 121.5 MHz and 300 MHz for
.sup.31P and .sup.1H at 7 T).
[0049] To compare the performance of the doubly tunable double ring
surface coil with single frequency one, two additional dual ring
probes with the same ring sizes and current ratios (-1.4:1), tuned
to either 121.5 MHz or 300 MHz were made. Experimental SNR
measurements were obtained from a cylindrical (2.7 cm inner
diameter) flask filled with 0.2 M phosphate buffer (pH 7.4). The
selected VOI of 1 cm.sup.3 was centered 1 cm from the bottom of the
sample.
[0050] Measurements of the RF field strength demonstrated that the
regions of B.sub.1 homogeneity for both frequencies overlap and
correlate with the theoretical calculations used to optimize
B.sub.1 within the VOI (FIG. 2). When operating at the .sup.1H
resonance frequency (300 MHz), the SNR of the doubly tunable double
ring surface coil was 22, identical to that of the double ring
single frequency coil. At the .sup.31P resonant frequency (121.5
MHz) the SNR for the doubly tunable coil was about 72% of that of
the single tuned coil.
[0051] The advantages of the probe include a low power requirement,
a high sensitivity close to that of a traditional surface coil, and
good field homogeneity within a desired VOI comparable to that of a
volume coil. Suppression of signal from the surface, which is
particularly important for .sup.31P liver spectra, also eliminates
a high phosphocreatine (PCr) signal from the abdominal wall thereby
providing high quality spectra from liver in vivo. The performance
of the coil for .sup.1H MR is very similar to that of previously
described dual ring single frequency coils, where an improvement in
B.sub.1 homogeneity and a reduction of outer voxel contamination
compared to a standard surface coil were reported. The introduction
of the frequency splitting network creates an additional resonance
mode and thus broadens the spectrum of NMR experiments that can be
performed. The doubly tunable coil produces better B.sub.1 field
homogeneity than standard double resonance surface coils and has a
lower power requirement and a better SNR than double resonance
volume coils (14-17).
[0052] The homogeneous B.sub.1 region produced by a multi-ring coil
can be designed for any depth and frequencies of interest.
Construction is relatively simple and robust, as theoretical
calculations are in good agreement with practice and the set-up
ensures practically identical B.sub.1 homogeneity at both
frequencies. Another good feature is the suppression of signal from
the surface, which is especially important for .sup.31P liver
spectra since a high PCr signal from abdominal wall can thus be
avoided.
[0053] Although specifically designed for proton and phosphorus,
the doubly tunable double ring surface coil can be adapted for a
wide range of applications, and can be designed to have a region of
high B.sub.1 homogeneity at different depths and frequencies of
interest.
[0054] Further improvements in the coil design are also possible.
Introduction of more than one frequency splitting network will
create additional resonance frequencies, giving for example a
triply tunable coil. With a larger diameter for the large ring, the
larger and deeper VOI can be sampled, but at the cost of reduced
SNR.
[0055] The doubly tunable double ring surface coil presented in
this study could be considered a "local volume coil" due to its
ability to produce a homogenous B.sub.1 over a selected VOI. While
it provides the spatially localized homogeneous field of a volume
coil, it requires considerably less RF power and produces a higher
SNR than a volume coil. The two-resonance nature of the coil will
facilitate a wide range of applications, particularly where
sequential MR imaging and spectroscopy examinations are important.
It would by particularly useful for the routine application of
spectroscopic techniques in medical diagnosis to perform quickly
and routinely sequential MR imaging and spectroscopy.
[0056] The doubly tunable double ring surface coil was also tested
in MRI and MRS studies of rat liver in vivo. Experiments were
carried out on a 7 T 21-cm horizontal bore magnet equipped with an
MSLX Bruker console (Bruker, Karlsruhe, Germany). A FLASH proton
image was acquired in the axial plane with TR/TE=3.7/2.2 ms, 2 mm
slice thickness, 8.times.8 cm.sup.2 field of view (FOV) and a
matrix size of 128.times.128. A phosphorus spectrum was selected
from a 2-D CSI experiment acquired with FOV 8.85 cm
(horizontal).times.8.0 cm (vertical), TR=1 s, matrix size 8.times.8
zero-filled to 16.times.16, acquisition size 1k zero-filled to 4k,
sweep width 4000 Hz, and processed with exponential line-broadening
of 12 Hz and manual phase correction. Localized shimming on the
liver was performed using a proton VOSY sequence with a
15.times.15.times.25 mm.sup.3 (lateral, vertical and axial
dimensions respectively) voxel with TE=20 ms, TM=30 ms. As can be
seen from the spectrum, virtually no PCr contamination from
abdominal muscle is observed.
[0057] Since various modifications can be made in my invention as
herein above described, and many apparently widely different
embodiments of same made within the spirit and scope of the claims
without departing from such spirit and scope, it is intended that
all matter contained in the accompanying specification shall be
interpreted as illustrative only and not in a limiting sense.
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