U.S. patent application number 14/450957 was filed with the patent office on 2015-02-05 for method of utilization of high dielectric constant (hdc) materials for reducing sar and enhancing snr in mri.
The applicant listed for this patent is Qing X. YANG. Invention is credited to Qing X. YANG.
Application Number | 20150038831 14/450957 |
Document ID | / |
Family ID | 44152049 |
Filed Date | 2015-02-05 |
United States Patent
Application |
20150038831 |
Kind Code |
A1 |
YANG; Qing X. |
February 5, 2015 |
METHOD OF UTILIZATION OF HIGH DIELECTRIC CONSTANT (HDC) MATERIALS
FOR REDUCING SAR AND ENHANCING SNR IN MRI
Abstract
Provided are a spectroscopic method and spectroscopic device for
analyzing a region. Layers or coats of materials with high
dielectric constant or permittivity with very low conductivity are
inserted in between radiofrequency (RF) coil or coil's conductive
elements and the sample to enhance the signal to noise ratio (SNR),
improve image contrast, and reduce the specific absorption rate
(SAR) of magnetic resonance imaging or magnetic resonance
spectroscopy instruments. The embodiments of the present invention
can be used as an auxiliary device to the standard pre-constructed
RF coils or incorporated with RF coil constructions for enhancing
RF coil performances in both transmission and reception.
Inventors: |
YANG; Qing X.; (Hummelstown,
PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
YANG; Qing X. |
Hummelstown |
PA |
US |
|
|
Family ID: |
44152049 |
Appl. No.: |
14/450957 |
Filed: |
August 4, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12958385 |
Dec 1, 2010 |
8798719 |
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14450957 |
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PCT/US2010/058625 |
Dec 2, 2010 |
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12958385 |
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Current U.S.
Class: |
600/421 ;
600/410 |
Current CPC
Class: |
A61B 5/415 20130101;
G01R 33/565 20130101; G01R 33/5659 20130101; A61B 5/055 20130101;
A61B 5/418 20130101; G01R 33/288 20130101 |
Class at
Publication: |
600/421 ;
600/410 |
International
Class: |
G01R 33/565 20060101
G01R033/565; A61B 5/055 20060101 A61B005/055; G01R 33/28 20060101
G01R033/28 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This research was supported by grant number R01 AG02771 from
the NIH. The government has certain rights in the invention.
Claims
1. (canceled)
2. (canceled)
3. (canceled)
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6. (canceled)
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19. (canceled)
20. An analytical spectroscopic device, the device adapted for
analyzing a region and comprising: a high dielectric material
configured to surround the region to be analyzed; a radiofrequency
coil and the high dielectric constant material in an arrangement
such that the radiofrequency coil is configured to apply a
radiofrequency field to the region through the high dielectric
constant material; and a computer configured to gather information
about the region resulting from application of the radiofrequency
field to the region; wherein the radiofrequency field includes an
electric field and a radiofrequency magnetic field, the
radiofrequency magnetic field generated by the electric field;
wherein the arrangement of the radiofrequency coil with respect to
the high dielectric constant material results in the radiofrequency
magnetic field extending through the region and increases the
signal to noise ratio of all the information gathered.
21. The device of claim 20, wherein the high dielectric constant
material conforms to body parts of a mammal.
22. The device of claim 20, wherein the high dielectric constant
material is a radiofrequency field concentrator.
23. The device of claim 20, wherein the radiofrequency coil is a
phase array coil.
24. The device of claim 20, wherein the high dielectric material
increases an efficiency of delivery of the radiofrequency field to
the region as compared to a device not including the high
dielectric material.
25. The device of claim 20, wherein the high dielectric material
increases the radiofrequency field in the region with decreased
input current in the radiofrequency coil as compared to a device
not including the high dielectric material.
26. An analytical spectroscopic device, the device adapted for
analyzing a region and comprising: an imaging device configured to
apply a radiofrequency field to the region to be analyzed; a high
dielectric constant material arranged and disposed between the
imaging device and the region to be analyzed; and a computer
configured to gather information about the region resulting from
application of the radiofrequency field to the region; wherein the
high dielectric constant material increases a signal to noise ratio
of the information gathered.
27. The device of claim 26, wherein the high dielectric constant
material includes a dielectric constant value that is higher than a
dielectric constant value of a sample in the region.
28. The device of claim 26, wherein the high dielectric constant
material decreases a radiation loss from the imaging device as
compared to a device not including the high dielectric
material.
29. The device of claim 26, wherein the high dielectric constant
material is selected from the group consisting of ceramics, water,
deuterium-based jell, water-based jell, suspensions including
dielectric additives, and combinations thereof.
30. The device of claim 26, wherein the high dielectric constant
material is a pad.
31. The device of claim 26, wherein the high dielectric constant
material conforms to body parts of a mammal.
32. The device of claim 31, wherein the high dielectric constant
material comprises a helmet conforming to at least a portion of a
human head.
33. The device of claim 26, wherein the high dielectric constant
material includes a uniform thickness.
34. The device of claim 26, wherein the high dielectric constant
material between the imaging device and the region to be analyzed
increases the signal to noise ratio of the information gathered
with decreased radiofrequency field power as compared to a device
not including the high dielectric constant material.
35. The device of claim 34, wherein the radiofrequency field power
is decreased by at least about 50%.
36. The device of claim 35, wherein the signal to noise ratio is
increased by at least about 27%.
37. The device of claim 26, wherein the high dielectric constant
material includes an adjustable volume.
38. The device of claim 26, wherein the high dielectric constant
material is integral with the imaging device.
39. An article adapted for use with an imaging device, the article
comprising: a high dielectric constant material; and a geometry
conforming to body parts of a mammal; wherein the high dielectric
constant material is arranged and disposed to increase a signal to
noise ratio of information gathered from a region during
application of a radiofrequency field.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of U.S.
patent application Ser. No. 12/958,385, now allowed, filed Dec. 1,
2010, and entitled Method Of Utilization Of High Dielectric
Constant (HDC) Materials For Reducing SAR And Enhancing SNR In MRI,
which claims priority to, and the benefit of, U.S. Provisional
Patent Application No. 61/266,109, filed Dec. 2, 2009, and PCT
Patent Application No. PCT/US2010/058625 filed Dec. 2, 2010,
entitled A Method of Utilization of High Dielectric Constant (HDC)
Materials for Reducing SAR and Enhancing SNR in MRI, all of which
are hereby incorporated by reference in their entirety.
FIELD OF THE INVENTION
[0003] This invention relates generally to the radiofrequency (RF)
electromagnetic field, denoted as B1, used for magnetic resonance
imaging (MRI) and magnetic resonance spectroscopy (MRS)
spectroscopy instruments. Particularly, the invention relates to
the use of materials with high dielectric constant (HDC) or
permittivity with low electrical conductivity for RF field
generation in the MRI or NMR instruments.
BACKGROUND OF THE INVENTION
[0004] RF coils are used to produce RF magnetic field (B.sub.1) for
excitation and detection of the magnetization signal from an object
such as parts of human body during MRI and MRS data collections.
The RF field can be generated by a coil or a set of coils. The RF
field is transmitted into the sample or human body to excite the
nuclear spins. Subsequently, the RF signals from the nuclear spins
are received by the same or a different set of RF coils. The
present invention improves the efficiency of RF coils for both
transmission and reception of the B.sub.1 field. The efficiency of
a RF coil includes B.sub.1 field uniformity and intensity in a
region of interest (ROI) produced by unit input current. In
general, a stronger B.sub.1 field generated by a RF coil with unit
input current translates to better reception sensitivity and lower
RF heating of the tissue samples that may be hazardous to human
body when it reaches to a certain level. The reception sensitivity
of a RF coil is experimentally determined by measured image signal
to noise ratio (SNR) from an ROI obtained with standardized imaging
protocols, and the heating effect by RF field can be assessed
numerically with calculation of Specific Absorption Rate (SAR)
distribution in the human body (1).
[0005] The SNR of an MR image is critically important for the
quality of the image as MRI is an intrinsically low SNR instrument.
Significant amount of efforts and resources have been invested in
order to gain higher image SNR in a given image data acquisition
time. In particularly, this includes the use of higher static
magnetic field strength from current 1 to 1.5 Tesla to 3 to 7
Tesla, which is extremely expensive. Unfortunately, an increase in
static magnetic field strength leads to a higher frequency of RF
field, which, in turn, dramatically increases RF heating effect of
the tissue (1) and creates RF field inhomogeneity artifacts (2-5).
These two problems pose serious challenges for high field MRI
development in human imaging. The present invention introduces a
novel method of use of HDC materials that increases B.sub.1 field
intensity inside the sample and reduces SAR in the sample during
image acquisition.
[0006] The recent experimental data in high field (3-8 Tesla) human
imaging systems demonstrated that high dielectric constant of human
body plays an important role for RF field behavior in human body in
high field. The electrical properties, geometry, and relative
position of the sample in the coil become important factors in
determining the B.sub.1 field distribution inside the sample
(6-11). Consequently, adjustment of B.sub.1 field distribution
inside the sample or human body and the coupling between the sample
and coil can be facilitated with HDC materials. Foo, et al.
proposed a method of correcting for the RF inhomogeneity in human
body observed on a 4 Tesla MRI system by "dielectric loading of the
coil-to-shield space in an RF resonator (coil and shield assembly)"
(10). Based on Foo's theoretical analysis, he proposed to adjust
the RF homogeneity by loading the coil-to-shield space with
dielectric material of suitable relative permittivity so as to
alter the propagation constant of the coil. With theoretical
calculations, Foo predicted that a value of between 30 and 40 for
the relative permittivity of the dielectric material in the
coil-to-shield space would reduce the RF field inhomogeneity from
+/-15% to about +/-3% over a central 30-cm-diameter region of a
homogeneous 40-cm-diameter body at both 64 MH and 170 MHz
corresponding to a 1.5 and 4 Tesla MRI system respectively.
However, their experimental results at 4 Tesla showed that "the
improved RF field homogeneity would be accompanied by increased RF
power requirements and reduced coil sensitivity." There at least
three distinctive differences in Foo's work from the present
invention. 1) The dielectric material is inserted in the
coil-to-sample space in the present invention, while it was loaded
in "the coil-to-shield space" in Foo's work. In fact, the
dielectric material is placed in the opposite side of the RF coil
of the present invention. As demonstrated in their experimental
results, Foo's approach produced totally opposite outcome i.e. a
"increased RF power requirements and reduced coil sensitivity". 2)
The theoretical bases are totally different. In the present
invention, the HDC material is used to couple the RF field produced
by the RF coil with the sample. In Foo's work, the dielectric
material was used to manipulate "the axial propagation constant of
RF resonator" itself. 3) In Foo's work, the choice of dielectric
material was based on the specific design and geometry of the RF
coil or resonator. In principle, the present invention requires no
knowledge of the RF coil configuration. It is more effective,
however, that HDC-pads of the present invention are developed to
fit a specific coil design for optimal effect.
[0007] To address the same RF field inhomogeneity issue in the head
image taken at 4Tesla, Alsop et al. presented a novel spiral RF
volume coil design for high field MRI use (2). Images acquired with
his spiral coil design showed a signal drop on the top of the human
head. It was attributed as the abrupt change in dielectric property
between tissue and air at the top of head since his coil design
theoretical analysis was based on a mathematical model of an
infinite long cylinder. To mitigate this additional problem
associated with the spiral coil design, a dielectric pad at the end
of the coil was included during image data collection. In his work,
the introduction of a dielectric pad was specifically used only for
compensation of the signal drop caused by the use of spiral coil.
In the present invention, HDC-ads are placed inside an RF volume
coil of any design to improve the efficiency of a given RF
coil.
BRIEF DESCRIPTION OF THE INVENTION
[0008] In one embodiment, a spectroscopic method of analyzing a
sample includes identifying a region to be analyzed, applying a
radiofrequency field from a radiofrequency coil through a high
dielectric constant material to the region, and gathering
information about the region with a computer, the information
gathered from the application of the radiofrequency field to the
region. The radiofrequency field includes an electric field and a
radiofrequency magnetic field, the radiofrequency magnetic field
generated by the electric field. The radiofrequency coil is
positioned with respect to the high dielectric constant material,
such that the radiofrequency magnetic field extends through both
the region and the high dielectric constant material and increases
the signal to noise ratio of all the information gathered.
[0009] In another embodiment, a spectroscopic method of analyzing a
sample includes identifying a region to be analyzed, applying a
radiofrequency field from a radiofrequency coil through a high
dielectric constant material to the region, and gathering
information about the region from the application of the
radiofrequency field to the region with a computer. The
radiofrequency field includes an electric field and a
radiofrequency magnetic field, the radiofrequency magnetic field
generated by the electric field. The radiofrequency coil is
positioned with respect to the high dielectric constant material,
such that the radiofrequency magnetic field extends through both
the region and the high dielectric constant material and reduces
required radiofrequency power.
[0010] In another embodiment, an analytical spectroscopic device
adapted for analyzing a region includes a high dielectric material
configured to surround the region to be analyzed, a radiofrequency
coil and the high dielectric constant material in an arrangement
such that the radiofrequency coil is configured to apply a
radiofrequency field to the region through the high dielectric
constant material, and a computer configured to gather information
about the region resulting from application of the radiofrequency
field to the region. The radiofrequency field includes an electric
field and a radiofrequency magnetic field, the radiofrequency
magnetic field generated by the electric field. The arrangement of
the radiofrequency coil with respect to the high dielectric
constant material results in the radiofrequency magnetic field
extending through the region and increases the signal to noise
ratio of all the information gathered.
[0011] It is desirable to have a strong and uniform B.sub.1
distribution in entire sample being imaged. It is an object of the
present invention that materials (liquids, solids or mixtures) with
high dielectric constant (HDC) or permittivity are incorporated
into RF coils to change the RF field to a desirable distribution
inside the sample. Here the word "high" refers to the value of
dielectric constant of a material that is higher than that of the
tissue of the sample under investigation at the frequency
transmitted and received by the RF coil. The presently preferred
embodiments are in forms of a pad or pads filled with HDC
materials, referred as HDC-pad thereafter.
[0012] It is another object of this invention that HDC materials
are incorporated into RF coils to reduce the transmission power for
a given excitation flip-angle in a region of interest (ROI) inside
the sample.
[0013] It is another object of this invention that HDC materials
are incorporated into a set of RF receiving coils to enhance the
image SNR of the sample.
[0014] For certain applications in the art, it is desirable to have
a strong and uniform B.sub.1 only in a part of the sample or a ROI
in the sample. It is another object that HDC materials are
incorporated into the coil or inside the sample to enhance an RF
coil performance in term of an increase in SNR in a ROI near or
around HDC materials in the sample and a decrease of input RF
power, therefore, SAR in the sample during excitation of
magnetization.
[0015] It is another object of this invention that HDC materials
are used as a RF field trap to reduce radiation loss of high field
MRI coils. In high field MRI (3T, 4T, 7T and above), radiation loss
becomes more significant as the corresponding RF field frequency
increases.
[0016] It is anther object of this invention that HDC materials are
used as contrast enhancement agents.
[0017] The principle of this invention is to insert layers or coats
of materials with high dielectric constant and very low
conductivity in between RF coil or coil conductive elements and
sample. The HDC materials stated herein can be solids, such as
ceramics, or liquids, such as water, deuterium or water-based jell
or suspensions of any dielectric additives to adjust the dielectric
constant. The geometry, amount, mechanical and electric properties
of the HDC material best suited for the embodiments of the present
invention will depend on specific applications, RF coil
configuration and the resonance frequency of the RF field. For
water-based jell pad, the signal from the pad can be removed by
addition of a small amount of Manganese Chloride (MnCl.sub.2) or
any other NMR signal relaxing compound or by replacing water with
deuterium (D.sub.2O).
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The foregoing and other novel features and advantages of the
invention will become more apparent and more readily appreciated by
those skilled in the art after consideration of the following
description in conjunction with the associated drawings, of
which:
[0019] FIG. 1A shows computer models of a bird-cage coil.
[0020] FIG. 1B shows a bird-cage coil surrounding a human head with
an HDC-pad.
[0021] FIG. 2 shows examples of elliptical ROI used to evaluate
T2-weighted image intensity for SNR measurement and uniformity
assessment on mid-axial (left), sagittal (center), and coronal
(right) planes.
[0022] FIG. 3 shows addition of HDC-pad resulted in dark regions in
central regions of all brain images due to overtipping (left
columns) with the RF power level optimized without the HDC-pad. The
images on right columns were acquired using input RF power with
attenuation values listed below. The images on the far right column
were obtained with final power attenuation for 90.degree. flip
angle was 14.5 dB, 3-dB more attenuation than that of without the
HDC-pad.
[0023] FIG. 4 shows axial, sagittal, and coronal images through
brain with (top) and without (bottom) HDC-pad. Measured SNR values
in regions indicated on FIG. 1 are given under each image.
[0024] FIG. 5 shows calculated B.sub.1 maps at 128 MHz in three
orthogonal planes with (top row) and without the HDC-pad (middle).
The B.sub.1.sup.+ distributions along the dotted lines in the
corresponding maps are plotted on the bottom. All the B.sub.1 maps
are scaled to 1 W input power.
DETAILED DESCRIPTION OF THE INVENTION
[0025] A preferred embodiment of the present invention will now be
described in detail with reference to FIGS. 1-5. Those skilled in
the art will appreciate that the description given herein with
respect to those figures is for exemplary purposes only and is not
intended in any way to limit the scope of the invention. All
questions regarding the scope of the invention may be resolved by
referring to the appended claims.
Theoretical Considerations
[0026] For conductive dielectric materials such as human brain
tissues, the RF field inside the sample is perturbed by conductive
current (J.sub.c) and displacement current (J.sub.d) according to
Ampere's Law with Maxwell's correction,
.gradient..times.B=.mu.J.sub.c+.mu.J.sub.d=.mu..sigma.E+i.mu..epsilon..s-
ub.r.epsilon..sub.o.omega.E [1]
where B is magnetic flux density, E is electric field, .omega. is
angular frequency, .epsilon..sub.r is relative electric
permittivity (dielectric constant), .epsilon..sub.o is the electric
permittivity in vacuum, .sigma. is electrical conductivity, m is
magnetic permeability, and i= {square root over (-1)} is the
complex unit which introduces a 90-degree phase difference between
conductive current and displacement current (Johnk 1988). For plane
waves travelling in a homogeneous medium, the conductive current
leads to decay of the RF field in the direction of propagation,
while the displacement current with a 90.degree. phase shift acts
as a secondary field source facilitating RF wave propagation. In
this case, the opposing contributions of the two sources to B.sub.1
can be considered using ratio J.sub.c and J.sub.d given by
J.sub.c/J.sub.d=.sigma./.omega..epsilon..sub.o.epsilon..sub.r.
In principle, materials with low .sigma. and high .epsilon..sub.r
can enhance the local B.sub.1 field strength for a RF field
frequency range high enough to induce the displacement current much
stronger conducting current. This equation describes the
relationship within the dielectric materials. Subsequently, the RF
wave propagates into the sample with stronger amplitude enhanced by
the HDC materials. Thus, in general, placement of HDC-pads near a
ROI in MRI should result in enhanced local B.sub.1 field strength
with concomitant improvement of SNR and reduction of overall
SAR.
HDC-Pad Design for Human Head Imaging
[0027] FIG. 1b illustrates geometry and configuration related to
the RF coil and the human head of the preferred embodiment of
present invention. The embodiment of present invention is a HDC-pad
in a form of a helmet, containing approximately 6 liters of
distilled water, was conformed around the superior portion of the
head, extending to just above the level of the eyes in front, just
above the ears on the sides, and just below the end of the skull in
the back of the head. The thickness of the HDC-pad is 3 mm.
Computer Modeling
[0028] The quantitative evaluation on how placement of a HDC-pad
changes the B.sub.1 field distribution depends on the detailed
geometries and sizes of the coil and sample, the embodiment of
present invention, thus, must be determined numerically with
computer modeling. In the following a computer modeling on a
HDC-helmet is used to demonstrate the efficacy of the
invention.
[0029] A numerical model with finite difference time domain (FDTD)
method was used to calculate the RF field distribution in the
sample and coil model shown in FIG. 1 a at 128 MHz corresponding to
a static magnetic field strength of 3 Tesla. All the FDTD
calculations were performed with commercially-available software
(XFDTD; Remcom, Inc, State College, Pa.), and post-processing of
the simulation results was performed with home-built programs in
MATLAB (The Mathworks, Inc., Natick, Mass.). A 30 mesh with
isometric 2 mm resolution was created within a region of
57.times.50.times.42 cm.sup.3. The calculation was performed with
-35 dB convergence to ensure that the steady state was reached. A
Liao boundary condition was used for the outer boundary truncation
of the grid (12). The coil was modeled after that used in
experiment: a copper 12-rung high-pass birdcage coil (32.8 cm i.d.
and 25.4 cm length, shield diameter of 40.0 cm). The coil model was
driven using 24 current sources spaced evenly on the two end rings,
at the locations of capacitors in the actual coil. The human head
model used for the FDTD calculation included 16 types of tissue and
the corresponding electric properties at 128 MHz were HDC-pad was
modeled with a 3 cm thick uniform layer of water (.sigma.=0.0047
S/m.sup.2, .epsilon..sub.r=78) over the head excluding the face as
shown in FIG. 1b (13).
Experimental Measurements
[0030] Human brain images were acquired on a 3 T whole body system
(Bruker, Biospin, Ettlingen, Germany) using a quadrature 12-element
high-pass birdcage coil with 26 cm inner diameter and 29 cm length.
Axial brain images were acquired with identical imaging parameters
with and without a HDC-pad placed around the head and after the
coil was tuned and matched, and RF power was calibrated for each
condition. The subject remained in the magnet during the process of
placing and/or removing the HDC-pad, re-tuning the coil and
adjustment of RF power for 90.degree./180.degree. flip angle. Input
power for the flip angle was adjusted manually with and without the
HDC-pad while maximizing the total signal on 5 axial slices
covering a 2.5 cm slab through the center of the brain. Fast
spin-echo (RARE) images with slice thickness=5 mm, matrix=128
.times.128, FA=180.degree., and FOV=30 cm were acquired on five
axial, sagittal, and coronal planes spaced 5 mm apart through the
cerebrum. The experiment was repeated four times with two human
subjects. All of the subjects provided written informed consent
prior to participation, in accord with the requirements of the
Institutional Review Board of the Pennsylvania State University
College of Medicine.
[0031] Signal-to-noise ratio (SNR) was measured using the magnitude
images acquired under the above two conditions. The average signal
intensity was calculated in an elliptical ROI covering most of the
cerebrum in each of the 15 images acquired with and without the
HDC-pad. Examples of the elliptical region in each orientation are
shown in FIG. 2. The noise was measured by calculating the standard
deviation in an ROI covering 29 cm.sup.2 in a region of no visible
signal or artifact across the top of each image.
Results
[0032] FIG. 3 shows a set of axial images of human brain after
addition of the HDC-pad and retuning of the coil, but before
re-optimization of the RF power levels (using power values
optimized without the HDC-pad) on the left columns, and images with
the HDC-pad using the RF power levels indicated at the bottom of
columns of the images. After addition of the pad, the RF power
levels that previously generated 90.degree./180.degree.
excitation/refocusing pulses in the brain created much larger flip
angles due to enhancement of the B.sub.1 field. A 3-dB reduction of
power for both the excitation and refocusing pulses was required to
maximize the signal and imagins were acquired shown on the far
right column in FIG. 3.
[0033] FIG. 4 shows images acquired with and without the HDC-pad on
axial, sagittal, and coronal planes. The SNR in the brain measured
in the elliptical regions indicated in FIG. 1 is listed below each
corresponding image. For all 15 planes acquired, the SNR increased
by 20 to 40% (in average, 27%) with addition of the HDC-pad while
the overall uniformity of image intensities appear to be similar in
both cases. The SNR of repeated experiments are within the same
range.
[0034] FIG. 5 shows the calculated B.sub.1.sup.+ distributions with
and without the HDC-pad in three orthogonal planes arranged as in
the experimental results. All the fields correspond to 1 W of power
delivered to the coil. The calculated B.sub.1.sup.+ distributions
under the two conditions presented similar characteristics as the
in vivo images in FIG. 4. The RF field appears to be greater inside
the brain regions surrounded by the HDC-pad but lower in the
regions in the face and neck outside of the padded region.
B.sub.1.sup.+ outside the upper portion of the head and pad is
lower when the HDC-pad is present and the same input power is
maintained. This can be seen in the line plots at the bottom of the
figure, which present B.sub.1.sup.+ along the dotted lines in the
maps above in FIG. 5. The black and gray lines indicate
B.sub.1.sup.+ for cases with and without the HDC-pad respectively.
The RF field around the coil elements and in between the coil
element and shield (seen most clearly in the middle line plot) was
reduced more than 70%, indicating less current is required in the
coil to maintain the same RF power level when the pad is present.
In ROIs similar to those used in the in vivo results, the average
B.sub.1.sup.+ in the simulation is seen to increase by 12-20% with
HDC-pad while the source current in the coil is reduced. These
computer modeling results demonstrated that the HDC-pad improves
the efficiency of generation of B.sub.1 field by a given RF coil,
resulting in an increase in image SNR and decrease of RF power
required for a given flip angle.
Discussion
[0035] Addition of a HDC-pad surrounding the head resulted in a
reduction of required RF power by approximately 50% and an increase
in image SNR by approximately 27% with a transmit/receive volume
coil at 3 T. No obvious local bias field induced by HDC-pad in the
entire cerebrum was observed in the images in FIG. 4. In addition,
image uniformity within the cerebrum appeared to be somewhat
improved. The standard deviation of the signal intensity
distribution in the elliptical regions on all 15 images acquired
from approximately the same brain volume decreased by an average of
12% with addition of the HDC-pad. The uniformity in this brain area
that is normally interference by the so-called bright center spot
due to the RF wave effect was reduced markedly.
[0036] Comparing images in FIG. 4 acquired with and without the
HDC-pad showed that the image intensity in the cerebrum relative to
the neck and face is much greater with the HDC-pad than without it.
It is likely that this is due to the enhancement of the B.sub.1
field for a given input power in the regions that were surrounded
by the HDC-pad. B.sub.1 field distribution of the coil is altered
by the strong displacement currents in the region of the pad,
resulting in stronger B.sub.1 fields in the vicinity of the
HDC-pad. These experimental data suggested that the regional
enhancement of the B.sub.1 fields in the ROI can be extended to the
entire cerebrum with proper coverage by the HDC-pad while the
B.sub.1 field outside the sample is decreased. As a direct result,
average SAR levels for the brain imaging will be lowered with the
HDC-pad, and because SNR is proportional to the ratio of the
B.sub.1 field strength to the square root of the corresponding RF
power, SNR is increased with addition of the HDC-pad. The numerical
modeling results summarized in FIG. 5 are consistent with the
experimental data and clearly demonstrate the enhancement of the
B.sub.1 field in the brain by the HDC-pad. The B.sub.1 field is
increased in the entire cerebrum region while outside the brain
appears to be reduced by the placement of the HDC-pad when the
B.sub.1.sup.+ maps are normalized to unit input power. Under this
condition, B.sub.1.sup.+ is increased in the cerebrum with reduced
input current in the coil. This is apparent in the middle line
plot, where the two ends of the line pass through coil current
elements. Since the magnetic field in the upper portion of the head
is relatively high compared to elsewhere, a greater percentage of
the input power is also dissipated in that region. To maintain the
same total input power as is the case in FIG. 5, the coil requires
less current and produces lower B.sub.1.sup.+ in regions outside
the HDC-pad compared to the case when the HDC-pad is not present.
Thus, overall, the presence of the HDC-pad improves efficiency of
delivery of both B.sub.1 field and RF energy to the section of the
head surrounded by the HDC-pad.
[0037] Those skilled in the art will appreciate that further
development of present invention will lead to an even greater
improvement in reducing SAR and improving regional SNR in MRI. The
in vivo data presented here at 3 T suggested that HDC-pads around
the head or other parts of the anatomy could be used to enhance
performance of an RF coil in a variety of cases. A HDC-pad with
adjustable volume could be used to enhance RF coil performance
while simultaneously providing comfort and reduction of patient
motion. This could be particularly beneficial for pediatric
patients since most RF coils are designed to accommodate larger
adult anatomies. In some cases, there may also be advantages to
incorporating dielectric material directly into RF coil
constructions. Further developments of presently preferred
embodiment include determination of the locations, dimensions,
geometries and permittivity distributions of the material for
optimal B.sub.1 enhancement.
[0038] Water was used in this embodiment as a dielectric medium to
demonstrate the desired effect of present invention as water has
relatively high dielectric constant and low conductivity, is
readily available, inexpensive, and nontoxic. From a technical
point of view, however, water is unlikely to be the most suitable
dielectric material for many intended applications in the art
because it produces strong signal that saturates the receiver and
decreases the dynamic range of the digitizer and its movements and
geometry are difficult to control. Deuterium (D.sub.2O) and high
dielectric constant material such as barium titanates slurry
suspension in the deuterium can be used in replace the water. It is
known to the art also that certain ceramic materials has dielectric
constant as high as a few thousands, which can be used for the
embodiments of the present invention.
[0039] Those skilled in the art will appreciate that strategic
placement of HDC-pads around the head within a given RF coil at 3 T
can result in reduced RF transmission power and improved image SNR
throughout the cerebrum, and that with further exploration and
development, use of HDC-pads may provide a relatively simple and
low-cost method for improving quality and safety of MRI in a
variety of applications.
[0040] Those skilled in the art will also appreciate that numerous
other modifications to the preferred embodiment and other
embodiments of present invention are possible within the scope of
the invention. These include further developments in optimization
of the size, shape, thickness and volume of HDC-pads for specific
applications to given body parts or organs for MRI systems with
various static magnetic field strengths; in formulation and
processing of high dielectric materials used for the HDC-pads; in
selection of the values of dielectric constant (permittivity) and
in incorporation HDC material in RF coil constructions. Other
developments would be implementing HDC-pads and other embodiments
of the present invention to the MRI systems with different static
magnetic field strengths available.
[0041] While the invention has been described with reference to one
or more embodiments, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiment disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include
all embodiments falling within the scope of the appended
claims.
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