U.S. patent application number 13/927617 was filed with the patent office on 2015-01-01 for rf shield for reducing eddy current heating in a pet-mr imaging system.
The applicant listed for this patent is General Electric Company. Invention is credited to Saikat Saha.
Application Number | 20150005616 13/927617 |
Document ID | / |
Family ID | 52116260 |
Filed Date | 2015-01-01 |
United States Patent
Application |
20150005616 |
Kind Code |
A1 |
Saha; Saikat |
January 1, 2015 |
RF SHIELD FOR REDUCING EDDY CURRENT HEATING IN A PET-MR IMAGING
SYSTEM
Abstract
An imaging apparatus is disclosed that includes an MRI system,
either as a stand-alone system or hybrid PET-MRI system. The MRI
system includes gradient coils positioned about a patient bore, an
RF coil former comprising an inner surface and an outer surface, an
RF shield positioned on the outer surface of the RF coil former so
as to be formed about the RF coil former, and an RF coil positioned
on the inner surface of the RF coil former and about the patient
bore, with the RF coil coupled to a pulse generator to emit an RF
pulse sequence and receive resulting MR signals from a subject of
interest. The RF shield includes a plurality of slits formed
therein configured to disrupt the formation of gradient field
induced eddy currents on the RF shield, so as to prevent the
generation of high temperature profiles on the surface of the
shield.
Inventors: |
Saha; Saikat; (Waukesha,
WI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
General Electric Company |
Schenectady |
NY |
US |
|
|
Family ID: |
52116260 |
Appl. No.: |
13/927617 |
Filed: |
June 26, 2013 |
Current U.S.
Class: |
600/411 ;
324/322 |
Current CPC
Class: |
G01R 33/422 20130101;
A61B 6/4258 20130101; G01R 33/56518 20130101; A61B 6/037 20130101;
A61B 6/4417 20130101; A61B 5/0035 20130101; A61B 6/4275 20130101;
G01R 33/481 20130101; A61B 5/055 20130101 |
Class at
Publication: |
600/411 ;
324/322 |
International
Class: |
G01R 33/422 20060101
G01R033/422; A61B 6/03 20060101 A61B006/03; G01R 33/48 20060101
G01R033/48 |
Claims
1. An imaging apparatus comprising: a magnetic resonance imaging
(MRI) system comprising: a plurality of gradient coils positioned
about a patient bore; an RF coil former comprising an inner surface
and an outer surface; an RF shield positioned on the outer surface
of the RF coil former so as to be formed about the RF coil former;
and an RF coil positioned on the inner surface of the RF coil
former and about the patient bore, with the RF coil coupled to a
pulse generator to emit an RF pulse sequence and receive resulting
MR signals from a subject of interest; and wherein the RF shield
includes a plurality of slits formed therein configured to disrupt
the formation of gradient field induced eddy currents on the RF
shield.
2. The imaging apparatus of claim 1 wherein the plurality of slits
includes at least one of: a plurality of longitudinal slits
extending in a z-direction along the RF shield; and a plurality of
circumferential slits formed in a circumferential direction along
the RF shield.
3. The imaging apparatus of claim 2 wherein the RF coil former
comprises a pair of raised portions positioned on opposing sides of
an indented portion that is indented in a radially inward
direction, with the RF shield conforming to the RF coil former so
as to also have an a pair of raised portions and an indented
portion.
4. The imaging apparatus of claim 3 wherein the plurality of
longitudinal slits are formed on the indented portion of the RF
shield.
5. The imaging apparatus of claim 3 wherein the plurality of
longitudinal slits are formed on the pair of raised portions of the
RF shield.
6. The imaging apparatus of claim 3 wherein the plurality of
circumferential slits are formed between the indented portion of
the RF shield and the pair of raised portions of the RF shield.
7. The imaging apparatus of claim 6 further comprising a plurality
of capacitive devices positioned to bridge across the plurality of
circumferential slits formed between the indented portion of the RF
shield and the pair of raised portions of the RF shield so as to
provide an RF short.
8. The imaging apparatus of claim 3 wherein the plurality of
circumferential slits are formed on the pair of raised portions of
the RF shield.
9. The imaging apparatus of claim 3 further comprising a positron
emission tomography (PET) system integrated with the MRI system,
the PET system having a detector array encircling the patient bore
and the RF shield with the detector array being controlled to
acquire PET emissions of the subject of interest; and wherein the
PET detector array is positioned in the indented portion of the RF
coil former.
10. The imaging apparatus of claim 2 further comprising a heat
removal device positioned on the RF shield at an end of a
respective longitudinal slit to remove heat therefrom, the heat
removal device comprising one or more components configured to
provide electrical insulation and thermal conductivity.
11. The imaging apparatus of claim 1 wherein the plurality of slits
are shaped as linear slits, teardrop-shaped slits or a combination
thereof.
12. An RF coil assembly for use in a stand-alone or hybrid magnetic
resonance imaging (MRI) system, the RF coil assembly comprising: a
generally cylindrical RF coil former having an inner surface and an
outer surface; an RF shield affixed to the outer surface of the RF
coil former and configured to conform to the outer surface thereof;
and an RF coil affixed to an inward facing surface of the RF coil
former; wherein the RF shield includes: a plurality of longitudinal
slits cut in the RF shield extending in a z-direction along the RF
shield; and a plurality of circumferential slits cut in the RF
shield extending in a circumferential direction along the RF
shield; wherein the plurality of longitudinal slits and the
plurality of circumferential slits are configured to disrupt the
formation of gradient field induced eddy currents on the RF shield,
so as to thereby reduce a surface temperature of the RF shield.
13. The RF coil assembly of claim 12 wherein the RF coil former
comprises raised portions formed on opposing ends of the RF coil
former and an indented portion formed between the raised portions
and extending radially inward toward the inner surface; and wherein
the RF shield includes raised portions and an indented portion
corresponding to the raised portions and the indented portion of
the RF coil former.
14. The RF coil assembly of claim 13 wherein the plurality of
longitudinal slits comprises at least one of: longitudinal slits
formed on the indented portion of the RF shield; and longitudinal
slits formed on the raised portions of the RF shield.
15. The RF coil assembly of claim 13 wherein the plurality of
circumferential slits comprises at least one of: circumferential
slits formed between the indented portion of the RF shield and the
raised portions of the RF shield; and circumferential slits formed
on the raised portions of the RF shield.
16. The RF coil assembly of claim 15 further comprising a plurality
of capacitive devices positioned to bridge across the plurality of
circumferential slits cut in the RF shield.
17. The RF coil assembly of claim 12 further comprising a thermal
conductor and an electrical insulator positioned on the RF shield
at an end of a respective longitudinal slit to remove heat
therefrom, the thermal conductor and the electrical insulator being
integrated into a single component or formed as separate
components.
18. A PET-MRI apparatus comprising: a magnetic resonance imaging
(MRI) system having a plurality of gradient coils positioned about
a patient bore, an RF coil former having inner and outer surfaces,
an RF shield formed about the outer surface of the RF coil former,
and an RF coil positioned on the inner surface of the RF coil
former, with the RF coil coupled to a pulse generator to emit an RF
pulse sequence and receive resulting MR signals from a subject of
interest; and a positron emission tomography (PET) system having a
detector array positioned to encircle the bore, with the detector
array being controlled to acquire PET emissions of the subject of
interest; wherein the RF shield includes: a pair of raised portions
formed on opposing ends of the RF shield; and an indented portion
formed between the pair of raised portions, the indented portion
being indented in the radial direction inwardly toward the patient
bore; a plurality of longitudinal slits formed in the RF shield and
extending in a z-direction along the RF shield; and a plurality of
circumferential slits formed in the RF shield and extending in a
circumferential direction along the RF shield.
19. The PET-MRI apparatus of claim 18 wherein the plurality of
longitudinal slits comprises at least one of: longitudinal slits
formed on the indented portion of the RF shield; and longitudinal
slits formed on the pair of raised portions of the RF shield.
20. The PET-MRI apparatus of claim 18 wherein the plurality of
circumferential slits comprises at least one of: circumferential
slits formed between the indented portion of the RF shield and the
pair of raised portions of the RF shield; and circumferential slits
formed on the pair of raised portions of the RF shield.
Description
BACKGROUND OF THE INVENTION
[0001] Embodiments of the invention relate generally to magnetic
resonance (MR) imaging, and more specifically, to an RF shield
configured to prevent the generation of high temperature profiles
on the surface thereof resulting from eddy current heating, so as
to minimize impact on the performance of thermally sensitive parts,
such as a positron emission tomography (PET) detector array in a
hybrid PET-MRI system.
[0002] MR imaging involves the use of magnetic fields and
excitation pulses to detect the free induction decay of nuclei
having net spins. When a substance such as human tissue is
subjected to a uniform magnetic field (polarizing field B.sub.0),
the individual magnetic moments of the spins in the tissue attempt
to align with this polarizing field, but process about it in random
order at their characteristic Larmor frequency. If the substance,
or tissue, is subjected to a RF magnetic field (excitation field
B.sub.1) which is in the x-y plane, i.e. perpendicular to the DC
magnetic field (B0) direction, and which is near the Larmor
frequency, the net aligned moment, or "longitudinal magnetization",
M.sub.z, may be rotated, or "tipped", into the x-y plane to produce
a net transverse magnetic moment M.sub.t. A signal is emitted by
the excited spins after the excitation signal B.sub.1 is terminated
and this signal may be received and processed to form an image.
[0003] When utilizing these signals to produce images, magnetic
field gradients (G.sub.x, G.sub.y, and G.sub.z) are employed.
Typically, the region to be imaged is scanned by a sequence of
measurement cycles in which these gradients vary according to the
particular localization method being used. The resulting set of
received NMR signals are digitized and processed to reconstruct the
image using one of many well known reconstruction techniques.
[0004] PET imaging involves the creation of tomographic images of
positron emitting radionuclides in a subject of interest. A
radionuclide-labeled agent is administered to a subject positioned
within a detector ring. As the radionuclides decay, positively
charged particles known as "positrons" are emitted therefrom. As
these positrons travel through the tissues of the subject, they
lose kinetic energy and ultimately collide with an electron,
resulting in mutual annihilation. The positron annihilation results
in a pair of oppositely-directed gamma rays being emitted at
approximately 511 keV.
[0005] It is these gamma rays that are detected by the
scintillators of the detector ring. When struck by a gamma ray,
each scintillator illuminates, activating a photovoltaic component,
such as a photodiode. The signals from the photovoltaics are
processed as incidences of gamma rays. When two gamma rays strike
oppositely positioned scintillators at approximately the same time,
a coincidence is registered. Data sorting units process the
coincidences to determine which are true coincidence events and
sort out data representing deadtimes and single gamma ray
detections. The coincidence events are binned and integrated to
form frames of PET data which may be reconstructed into images
depicting the distribution of the radionuclide-labeled agent and/or
metabolites thereof in the subject.
[0006] In combination PET-MRI systems, the RF shield associated
with the MRI scanner is positioned in between the RF body coil and
the gradient coil to help prevent the high amplitude RF field being
radiated out--with the PET detector array being placed outside the
RF shield in order to shield the sensitive detector array from the
RF field. Depending on the proximity of the RF shield to the
gradient coil and the type of gradient pulsing sequence applied,
large amount of eddy-currents are created on the RF shield
surface--with the pattern of these eddy current more or less
mirroring the primary gradient current pattern. The eddy current
generated on the RF shield produces heat that create high
temperature profiles that affect the performance of any thermally
sensitive parts located on or near the RF shield, such as the PET
detector modules. The eddy current generated on the RF shield also
raises the overall temperature of the patient bore, which may
potentially cause discomfort to a subject being imaged.
[0007] It would therefore be desirable to provide an RF shield that
prevents the generation of high temperature profiles on the surface
of the RF shield resulting from eddy current heating, such as by
disrupting larger eddy current profiles and any azimuthal
generation of eddy current and by preventing the build-up of axial
currents on the shield. It would also be desirable for the RF
shield to still provide the necessary amount of shielding to the
PET detector array and maintain the RF coil performance and image
quality.
BRIEF DESCRIPTION OF THE INVENTION
[0008] Embodiments of the invention provide an RF shield for use in
a stand-alone or hybrid MRI system.
[0009] In accordance with one aspect of the invention, an imaging
apparatus includes a MRI system comprising a plurality of gradient
coils positioned about a patient bore, an RF coil former comprising
an inner surface and an outer surface, an RF shield positioned on
the outer surface of the RF coil former so as to be formed about
the RF coil former, and an RF coil positioned on the inner surface
of the RF coil former and about the patient bore, with the RF coil
coupled to a pulse generator to emit an RF pulse sequence and
receive resulting MR signals from a subject of interest. The RF
shield includes a plurality of slits formed therein configured to
disrupt the formation of gradient field induced eddy currents on
the RF shield.
[0010] In accordance with another aspect of the invention, an RF
coil assembly for use in a stand-alone or hybrid MRI system
includes a generally cylindrical RF coil former having an inner
surface and an outer surface, an RF shield affixed to the outer
surface of the RF coil former and configured to conform to the
outer surface thereof, and an RF coil affixed to an inward facing
surface of the RF coil former. The RF shield of the RF coil
assembly further includes a plurality of longitudinal slits cut in
the RF shield extending in a z-direction along the RF shield and a
plurality of circumferential slits cut in the RF shield extending
in a circumferential direction along the RF shield, wherein the
plurality of longitudinal slits and the plurality of
circumferential slits are configured to disrupt the formation of
gradient field induced eddy currents on the RF shield, so as to
thereby reduce a surface temperature of the RF shield.
[0011] In accordance with yet another aspect of the invention, a
PET-MRI apparatus includes a MRI system having a plurality of
gradient coils positioned about a patient bore, an RF coil former
having inner and outer surfaces, an RF shield formed about the
outer surface of the RF coil former, and an RF coil positioned on
the inner surface of the RF coil former, with the RF coil coupled
to a pulse generator to emit an RF pulse sequence and receive
resulting MR signals from a subject of interest. The PET-MRI
apparatus also includes a positron emission tomography (PET) system
having a detector array positioned to encircle the bore, with the
detector array being controlled to acquire PET emissions of the
subject of interest. The RF shield or the MRI system further
includes a pair of raised portions formed on opposing ends of the
RF shield, an indented portion formed between the pair of raised
portions that is indented in the radial direction inwardly toward
the patient bore, a plurality of longitudinal slits formed in the
RF shield and extending in a z-direction along the RF shield, and a
plurality of circumferential slits formed in the RF shield and
extending in a circumferential direction along the RF shield.
[0012] Various other features and advantages will be made apparent
from the following detailed description and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The drawings illustrate embodiments presently contemplated
for carrying out the invention.
[0014] In the drawings:
[0015] FIG. 1 is a schematic block diagram of an exemplary PET-MR
imaging system for use with an embodiment of the invention.
[0016] FIGS. 2 and 3 are perspective views of an RF coil assembly,
including an RF shield, for use in the PET-MR imaging system of
FIG. 1 according to an embodiment of the invention.
[0017] FIG. 4 is a perspective view of the RF coil assembly of
FIGS. 2 and 3 with a PET detector array positioned on the RF shield
according to an embodiment of the invention.
[0018] FIG. 5 is a perspective view of an RF coil assembly,
including an RF shield, for use in the PET-MR imaging system of
FIG. 1 according to another embodiment of the invention.
[0019] FIGS. 6A and 6B are views of heat removal devices for use
with the RF shield of FIG. 5 according to embodiments of the
invention.
DETAILED DESCRIPTION
[0020] An RF coil assembly is provided that includes an RF shield
having a plurality of slits formed therein that are configured to
prevent the generation of high temperature profiles on the surface
of the RF shield. Longitudinal slits and/or circumferential slits
are formed in the RF shield that disrupt the formation of eddy
currents on the RF shield surface, so as to reduce the heat
produced from such eddy currents and prevent the generation of the
high temperature profiles that affect the performance of any
thermally sensitive parts located on or near the RF shield, such as
PET detector modules.
[0021] According to embodiments of the invention, the RF coil
assembly can be implemented in a variety of imaging systems or
apparatuses. For example, the RF coil assembly can be incorporated
into a stand-alone MR imaging system or can be incorporated into a
hybrid MR imaging system, such as a hybrid PET-MR imaging system,
for example. Thus, while embodiments of the invention are set forth
here below with respect to a hybrid PET-MR imaging system, it is
recognized that other stand-alone and hybrid MR imaging systems are
considered to be within the scope of the invention.
[0022] Referring to FIG. 1, the major components of an exemplary
hybrid PET-MR imaging system 10 that may incorporate embodiments of
the present invention are shown. The operation of the system may be
controlled from an operator console 12 which includes a keyboard or
other input device 13, a control panel 14, and a display screen 16.
The console 12 communicates through a link 18 with a separate
computer system 20 that enables an operator to control the
production and display of images on the display screen 16. The
computer system 20 includes a number of modules, such as an image
processor module 22, a CPU module 24 and a memory module 26. The
computer system 20 may also be connected to permanent or back-up
memory storage, a network, or may communicate with a separate
system control 32 through link 34. The input device 13 can include
a mouse, keyboard, track ball, touch activated screen, light wand,
or any similar or equivalent input device, and may be used for
interactive geometry prescription.
[0023] The system control 32 includes a set of modules in
communication with one another and connected to the operator
console 12 through link 40. It is through link 34 that the system
control 32 receives commands to indicate the scan sequence or
sequences that are to be performed. For MR data acquisition, an RF
transmit/receive module 38 commands the scanner 48 to carry out the
desired scan sequence, by sending instructions, commands, and/or
requests describing the timing, strength and shape of the RF pulses
and pulse sequences to be produced, to correspond to the timing and
length of the data acquisition window. In this regard, a
transmit/receive switch 44 and amplifier 46 control the flow of
data to scanner 48 from RF transmit module 38 and from scanner 48
to RF receive module 38. The system control 32 also connects to a
set of gradient amplifiers 42, to indicate the timing and shape of
the gradient pulses that are produced during the scan.
[0024] The gradient waveform instructions produced by system
control 32 are sent to the gradient amplifier system 42 having Gx,
Gy, and Gz amplifiers. Amplifiers 42 may be external of scanner 48
or system control 32, or may be integrated therein. Each gradient
amplifier excites a corresponding physical gradient coil in a
gradient coil assembly generally designated 50 to produce the
magnetic field gradients used for spatially encoding acquired
signals. The gradient coil assembly 50 forms part of a magnet
assembly 52 which includes a polarizing magnet 54 and an RF coil 56
(i.e., whole-body RF coil). Alternatively, the gradient coils of
gradient coil assembly 50 may be independent of the magnet assembly
52. The coils 56 of the RF coil may be configured for both
transmitting and receiving, or for transmit-only or receive-only. A
pulse generator 57 may be integrated into system control 32 as
shown, or may be integrated into scanner equipment 48, to produce
pulse sequences or pulse sequence signals for the gradient
amplifiers 42 and/or the RF coil 56. In addition, pulse generator
57 may generate PET data blanking signals synchronously with the
production of the pulse sequences. These blanking signals may be
generated on separate logic lines for subsequent data processing.
The MR signals resulting from the excitation pulses, emitted by the
excited nuclei in the patient, may be sensed by the whole body coil
56 or by separate receive coils and are then transmitted to the RF
transmit/receive module 38 via T/R switch 44. The MR signals are
demodulated, filtered, and digitized in the data processing section
68 of the system control 32.
[0025] An MR scan is complete when one or more sets of raw k-space
data has been acquired in the data processor 68. This raw k-space
data is reconstructed in data processor 68 which operates to
transform the data (through Fourier or other techniques) into image
data. This image data is conveyed through link 34 to the computer
system 20 where it is stored in memory 26. Alternatively, in some
systems computer system 20 may assume the image data reconstruction
and other functions of data processor 68. In response to commands
received from the operator console 12, the image data stored in
memory 26 may be archived in long term storage or may be further
processed by the image processor 22 or CPU 24 and conveyed to the
operator console 12 and presented on the display 16.
[0026] In combined MR-PET scanning systems, PET data may be
acquired simultaneously with the MR data acquisition described
above. Thus, scanner 48 also contains a positron emission detector
array or ring 70, configured to detect gamma rays from positron
annihilation radiations emitted from a subject. Detector array 70
preferably includes a plurality of scintillators and photovoltaics
arranged about a gantry. Detector array 70 may, however, be of any
suitable construction for acquiring PET data. In addition, the
scintillator packs, photovoltaics, and other electronics of the
detector array 70 are shielded from the magnetic fields and/or RF
fields applied by the MR components 54, 56 by way of an RF shield
(not shown), as will be explained in detail below.
[0027] Gamma ray incidences detected by detector array 70 are
transformed, by the photovoltaics of the detector array 70, into
electrical signals and are conditioned by a series of front-end
electronics 72. These conditioning circuits 72 may include various
amplifiers, filters, and analog-to-digital converters. The digital
signals output by front end electronics 72 are then processed by a
coincidence processor 74 to match gamma ray detections as potential
coincidence events. When two gamma rays strike detectors
approximately opposite one another, it is possible, absent the
interactions of random noise and signal gamma ray detections, that
a positron annihilation took place somewhere along the line between
the detectors. Thus, the coincidences determined by coincidence
processor 74 are sorted into true coincidence events and are
ultimately integrated by data sorter 76. The coincidence event
data, or PET data, from sorter 76 is received by the system control
32 at a PET data receive port 78 and stored in memory 26 for
subsequent processing 68. PET images may then be reconstructed by
image processor 22 and may be combined with MR images to produce
hybrid structural and metabolic or functional images. Conditioning
circuits 72, coincidence processor 74 and sorter 76 may each be
external of scanner 48 or system control 32, or may be integrated
therein.
[0028] Referring now to FIGS. 2 and 3, an RF coil assembly 80 that
is included in the hybrid PET-MR imaging system 10 is shown,
although it is recognized that RF coil assembly 80 could also be
implemented for use in other stand-alone MRI systems or other
hybrid MRI systems. The RF coil assembly 80 includes an RF coil
former or tube 82, an RF shield 84, and the RF body coil 56.
According to an embodiment of the invention, the RF shield 84 is
formed of stainless steel mesh and the RF coil former 82 is
composed of fiberglass or fiber reinforced plastic (FRP) cylinders
on the radially inner and radially outer surfaces, with a foam
material sandwiched between the inner and outer surfaces, although
it is recognized that other suitable materials could also be used.
The RF shield 84 is positioned on the outer surface 88 of RF coil
former 82 and is formed there about. The RF coil 56 is formed on an
inner surface 92 of RF coil former 82 with an annular receiving or
imaging area 90 (i.e., patient bore), and is separated radially
from gradient coils 50 by RF shield 84, with the RF shield 84
functioning to de-couple the RF coils 56 from the gradient coils 50
(FIG. 1) in the PET-MR imaging system 10.
[0029] As shown in FIG. 2, the RF coil former 82 is generally
cylindrical in shape but includes an indentation or indented
portion 94 formed therein in a radial direction and in an area that
corresponds to the PET detector array 70 (FIG. 1) of the PET-MR
imaging system 10. Thus, the indented portion 94 will be formed in
a generally central area lengthwise on the RF coil former 82, with
a pair of raised or stepped-up portions 96 of the RF coil former 82
being formed on opposing sides of the indented portion 94 and at
opposing ends of the RF coil former 82. The RF shield 84 is applied
over the outer surface 88 of RF coil former 82 and conforms to the
RF coil former 82, such that the RF shield 84 has an identical
shape as the outer surface 88 of the RF coil former 82. The RF
shield 84 thus also includes an indentation/indented portion 98
formed therein in the area that corresponds to the PET detector
array 70 (FIG. 1), with the indented portion 98 being between
raised portions 100 of the RF shield 84.
[0030] According to an exemplary embodiment of the invention, the
indented portions 94, 98 in RF coil former 82 and RF shield 84 have
a stepped configuration. As shown in FIG. 2, a first step 102 and a
second step 104 are formed in the indented portions 94, 98 of RF
coil former 82 and RF shield 84, with the second step 104 being
further indented from raised portions 96, 100 of the RF coil former
82 and RF shield 84 than the first step 102. The first and second
steps 102, 104 accommodate positioning of the detector array 70 and
an accompanying mechanical support frame 106 therein, as shown in
FIG. 4. That is, mechanical support frame 106 can be positioned on
first step 102 of the indented portions 94, 98, such that an outer
surface of the support frame is flush with the raised portions 100
of the RF shield 84. Similarly, detector array 70 may be positioned
on second step 104 of the indented portions 94, 98, such that the
detector array is flush with the raised portions 100 of the RF
shield 84.
[0031] As shown in FIG. 2, according to one embodiment of the
invention, longitudinal slits 108 are cut/formed in RF shield 84
extending in the z-direction 110. The slits 108 may be formed to
have a width of 1 mm, for example, and are configured to reduce
heating caused by gradient field induced eddy currents in the RF
shield 84 during operation of the PET-MR imaging system 10 by
increasing impedance for the gradient eddy currents due to
increased path length. That is, the longitudinal slits along the
z-axis of the RF shield 84 are strategically cut to disrupt the
larger eddy current profiles and prevent any azimuthal generation
of eddy current. According to the embodiment of FIG. 2, the slits
108 are formed in the indented portion 98 of RF shield 84--and more
specifically in the region of the indented portion 98 that
accommodates the detector array 70, i.e., in the region of second
step 104.
[0032] As further shown in FIG. 2, in addition to the longitudinal
slits 108 formed in RF shield 84, circumferential slits 112 are
also strategically cut/formed in the RF shield 84. The
circumferential slits 112 extend in a circumferential direction 114
along the RF shield 84 and function to prevent the build-up of
axial currents on the shield. According to the embodiment of FIG.
2, circumferential slits 112 are formed in the RF shield 84 between
the indented portion 98 of the shield and the raised portions 100.
The gap between the raised portions 100 of the RF shield and the
indented portion 98 formed by the circumferential slits 112 are
bridged by capacitive devices 116 that act as an RF short, so as to
minimize heat generation on the RF shield 84 and hence the
temperature rise thereof. According to embodiments of the
invention, the capacitive devices 116 may be provided as disc
capacitors, jumpers, dielectric double layered PCB capacitors, or
lumped capacitors, for example.
[0033] Referring now to FIG. 5, the RF shield 84 is shown according
to another embodiment of the invention. In the embodiment shown in
FIG. 5, longitudinal slits 108 are formed in RF shield 84 extending
in the z-direction, with the longitudinal slits 108 being formed
both in the indented portion 98 of RF shield 84 (i.e., in the
region of second step 104) and in the raised portions 100 of RF
shield 84. The slits 108 may be formed to have a width of 1 mm, for
example, and are configured to reduce heating caused by gradient
field induced eddy currents in the RF shield 84 during operation of
the PET-MR imaging system 10 by increasing impedance for the
gradient eddy currents due to increased path length.
[0034] According to an exemplary embodiment, the slits 108 are
formed in an outer region of each of raised portions 100 of RF
shield 84, with a heat removal device 118 being positioned on the
RF shield 84 at an end of a respective longitudinal slit 108 on
raised portions 100 to remove heat therefrom. According to
embodiments of the invention, the heat removal device 118 includes
electrically insulating and thermally conductive elements that may
either be integrated into a single component or formed as separate
components. For example, referring to FIGS. 6A and 6B, various
embodiments of heat removal devices 118 are shown. In the
embodiment of FIG. 6A, the heat removal device 118 includes an
electrically insulating base plate 120 affixed to the RF shield 84
(i.e., on raised portion 100 of the shield) at the end of
longitudinal slit 108, with a thermally conductive plate 122 that
exhibits high thermal conductivity being positioned on the
electrically insulating base plate 120 and adjacent the end of
longitudinal slit 108. In the embodiment of FIG. 6B, the heat
removal device 118 is formed of a single heat spreader 124 formed
of a material that exhibits poor electrical conductivity and high
thermal conductivity, with the heat spreader 124 being affixed to
the RF shield 84 (i.e., on raised portion 100 of the shield) at the
end of longitudinal slit 108. It is recognized that the shape,
size, and location of the heat removal devices 118 can be varied
and optimized to achieve a desired level of heat
removal/performance.
[0035] Referring again to FIG. 5, the embodiment of the RF shield
84 shown therein also includes circumferential slits 112 cut/formed
in the RF shield 84. More specifically, the circumferential slits
112 are cut/formed in the raised portions 100 of RF shield 84 so as
to extend in a circumferential direction 114 along the RF shield
84, with the slits 112 functioning to prevent the build-up of axial
currents on the shield. As further shown in FIG. 5, capacitive
devices 116 can be positioned at desired locations on raised
portions 100 of the RF shield to bridge the circumferential slits
112 to act as an RF short and thereby minimize heat generation on
the RF shield 84 and the temperature rise thereof. According to
embodiments of the invention, the capacitive devices 116 may be
provided as disc capacitors, jumpers, dielectric double layered PCB
capacitors, or lumped capacitors, for example.
[0036] While a number of embodiments of an RF shield 84 have been
shown and described here above, it is recognized that various other
arrangements and configurations of longitudinal and circumferential
slits 108, 112 are considered to be within the scope of the
invention. That is, it is recognized that the specific location and
width of the slits 108, 112 formed in the RF shield 84 are
optimized based on the design of the RF coil 56 present in the
system. Thus, various combinations of slits 108, 112 shown in FIGS.
2 and 5 can be implemented to minimize the eddy current, with the
physical dimensions (size, shape) and locations of the slits being
optimized to reduce the eddy current heating. For example,
according to one embodiment, the slits 108, 112 can be staggered
along the circumference/length of the RF shield 84 to further
disrupt the eddy current pattern.
[0037] Beneficially, embodiments of the invention thus provide an
RF shield 84 for use in an MR or hybrid PET-MR imaging system that
is configured to prevent the generation of high temperature
profiles on the surface of the RF shield resulting from eddy
current heating. The RF shield 84 includes an arrangement of
longitudinal and circumferential slits 108, 112 formed therein that
disrupt eddy current profiles by disrupting any azimuthal
generation of eddy current and by preventing the build-up of axial
currents on the RF shield.
[0038] Therefore, according to one embodiment of the invention, an
imaging apparatus includes a MRI system comprising a plurality of
gradient coils positioned about a patient bore, an RF coil former
comprising an inner surface and an outer surface, an RF shield
positioned on the outer surface of the RF coil former so as to be
formed about the RF coil former, and an RF coil positioned on the
inner surface of the RF coil former and about the patient bore,
with the RF coil coupled to a pulse generator to emit an RF pulse
sequence and receive resulting MR signals from a subject of
interest. The RF shield includes a plurality of slits formed
therein configured to disrupt the formation of gradient field
induced eddy currents on the RF shield.
[0039] According to another embodiment of the invention, an RF coil
assembly for use in a stand-alone or hybrid MRI system includes a
generally cylindrical RF coil former having an inner surface and an
outer surface, an RF shield affixed to the outer surface of the RF
coil former and configured to conform to the outer surface thereof,
and an RF coil affixed to an inward facing surface of the RF coil
former. The RF shield of the RF coil assembly further includes a
plurality of longitudinal slits cut in the RF shield extending in a
z-direction along the RF shield and a plurality of circumferential
slits cut in the RF shield extending in a circumferential direction
along the RF shield, wherein the plurality of longitudinal slits
and the plurality of circumferential slits are configured to
disrupt the formation of gradient field induced eddy currents on
the RF shield, so as to thereby reduce a surface temperature of the
RF shield.
[0040] According to yet another embodiment of the invention, a
PET-MRI apparatus includes a MRI system having a plurality of
gradient coils positioned about a patient bore, an RF coil former
having inner and outer surfaces, an RF shield formed about the
outer surface of the RF coil former, and an RF coil positioned on
the inner surface of the RF coil former, with the RF coil coupled
to a pulse generator to emit an RF pulse sequence and receive
resulting MR signals from a subject of interest. The PET-MRI
apparatus also includes a positron emission tomography (PET) system
having a detector array positioned to encircle the bore, with the
detector array being controlled to acquire PET emissions of the
subject of interest. The RF shield or the MRI system further
includes a pair of raised portions formed on opposing ends of the
RF shield, an indented portion formed between the pair of raised
portions that is indented in the radial direction inwardly toward
the patient bore, a plurality of longitudinal slits formed in the
RF shield and extending in a z-direction along the RF shield, and a
plurality of circumferential slits formed in the RF shield and
extending in a circumferential direction along the RF shield.
[0041] This written description uses examples to disclose the
invention, including the best mode, and also to enable any person
skilled in the art to practice the invention, including making and
using any devices or systems and performing any incorporated
methods. The patentable scope of the invention is defined by the
claims, and may include other examples that occur to those skilled
in the art. Such other examples are intended to be within the scope
of the claims if they have structural elements that do not differ
from the literal language of the claims, or if they include
equivalent structural elements with insubstantial differences from
the literal languages of the claims.
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