U.S. patent application number 11/376506 was filed with the patent office on 2007-09-20 for methods and apparatus for mri shims.
This patent application is currently assigned to General Electric Company. Invention is credited to Longzhi Jiang, Edwin Lawrence Legall.
Application Number | 20070216413 11/376506 |
Document ID | / |
Family ID | 38481783 |
Filed Date | 2007-09-20 |
United States Patent
Application |
20070216413 |
Kind Code |
A1 |
Legall; Edwin Lawrence ; et
al. |
September 20, 2007 |
METHODS AND APPARATUS FOR MRI SHIMS
Abstract
A method includes heating a MRI shim with a heater to reduce a
temperature rise per unit of time during a gradient operation of a
MRI system.
Inventors: |
Legall; Edwin Lawrence;
(Menomonee Falls, WI) ; Jiang; Longzhi; (Florence,
SC) |
Correspondence
Address: |
FISHER PATENT GROUP, LLC
700 6TH STREET NW
HICKORY
NC
28601
US
|
Assignee: |
General Electric Company
|
Family ID: |
38481783 |
Appl. No.: |
11/376506 |
Filed: |
March 15, 2006 |
Current U.S.
Class: |
324/320 |
Current CPC
Class: |
G01R 33/389 20130101;
G01R 33/3873 20130101 |
Class at
Publication: |
324/320 |
International
Class: |
G01V 3/00 20060101
G01V003/00 |
Claims
1. A method comprising: heating a MRI shim with a heater to reduce
a temperature rise per unit of time during a gradient operation of
a MRI system, wherein said heating further comprises heating with a
heater under control by a PID control.
2. (canceled)
3. A method in accordance with claim 1 wherein said heating further
comprises heating with a foil heater.
4. A method in accordance with claim 3 wherein said heating further
comprises heating with a double electrically isolated heater.
5. A method in accordance with claim 4 wherein said heating further
comprises heating a heater on a tray that is non-electrically
conductive, non-magnetic, and a poor heat conductor.
6. A method in accordance with claim 5 wherein said heating further
comprises heating a heater below a backing plate that is
non-electrically conductive, non-magnetic, and a good heat
conductor.
7. A method in accordance with claim 6 wherein the backing plate
and foil heater both have holes for attaching the MRI shim to the
tray.
8. A method in accordance with claim 7 wherein said heating further
comprises heating the shim to its operating temperature in no
greater than five minutes.
9. A method in accordance with claim 8 wherein after the shim is
brought to its operating temperature the PID control maintains the
shim within 0.1 C of the operating temperature with a single or
multiple zone control system.
10. A MRI shim system comprising: a shim; a heater in thermal
communication with said shim; and a tray positioned on one side of
the heater that is non-electrically conductive, non-magnetic, and a
poor heat conductor, wherein the shim is on another side of the
heater.
11. (canceled)
12. (canceled)
13. A system in accordance with claim 10 further comprising a
backing plate that is non-electrically conductive, non-magnetic,
and a good heat conductor, wherein the backing plate is positioned
between said heater and said shim.
14. A system in accordance with claim 10 further comprising a PID
control controlling said heater.
15. A system in accordance with claim 12 further comprising a PID
control controlling said heater such that said shim is maintained
within 0.1.degree. C. of said shim's operating temperature.
16. An imaging apparatus for producing Magnetic Resonance (MR)
images of a subject having a magnet assembly for producing a static
magnetic field and a gradient coil assembly disposed within said
magnet assembly for generating a magnetic field gradient for use in
producing MR images, said apparatus comprising: a MRI shim system
configured to maintain a shim within 0.1.degree. C. of said shim's
operating temperature, wherein said MRI shim system comprises a
heater in thermal communication with said shim, and said heater
comprises a double electrically isolated film heater.
17. (canceled)
18. An imaging apparatus in accordance with claim 16, wherein said
MRI shim system further comprises a PID control controlling said
heater.
19. (canceled)
20. An imaging apparatus in accordance with claim 16, wherein said
double electrically isolated film heater comprises a plurality of
shim mounting openings.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to methods and apparatus
for magnetic resonance imaging (MRI) systems, and more particularly
to methods and apparatus that facilitate maintaining a consistent
shim temperature.
[0002] Typical superconducting magnets may use one or the
combination of two independent systems to adjust the magnetic field
homogeneity in the imaging volume, in order to meet image quality
requirements. The first system is an active system that consists of
superconducting correction coils embedded in the superconducting
magnet cartridge. The second is a passive system that consists of
the placement of carbon steel plates at specified axial and radial
location from the magnet isocenter.
[0003] Both systems have design capability limitations that are
driven by parameters such as: for active systems; maximum current,
superconductor type, coils size, coils location and former
structural integrity, and for passive systems; steel shim size,
steel chemical composition homogeneity, steel shim location, and
temperature. For gradient embedded magnet passive shimming system,
steels shims are placed inside of the gradient coil in a tray and
rail configuration that allows easy shim removal or shim
replacement during magnet shimming operation. Heat provided by the
gradient coil operation produces temperature fluctuation in the
shims during imaging acquisition. For the embedded gradient shim
configuration, heat travels from the coil structure to the shim due
to conduction contact, by convection only if air is allowed at the
shim location, and by radiation. For a magnet inner bore
configuration, heat travels from the outside gradient surface to
the shim by radiation and conduction thru supporting members if
vacuum is present between the components.
[0004] Changes to the temperature of the shims causes a change to
the magnet field center frequency due to changes in the
magnetization properties of the steel with temperature (approximate
0.4%), this is called B0 intensity drift. The actual drift
experienced by a magnet during an imaging scan, directly depends on
the amount of steel used for the passive shimming, and the amount
of heat that the shims are exposed to during gradient operation
(shim temperature change). For good image quality, application
specialists desire that the magnet drift cause less than a 1 pixel
shift during a fifteen minute scan (a typical is 1 Hz/min). In
order to achieve good image quality, one desire is that the
temperature of the shim must not exceed a certain temperature rise
per unit time. Therefore, an additional temperature control system
or an active cooling system is typically employed to ensure that
the magnet performance is maintained during any operation condition
of the gradient coil. The additional cooling typically employed
raises costs and adds complexity. Therefore, it would be desirable
to maintain the shim's temperature without an expensive cooling
system.
BRIEF DESCRIPTION OF THE INVENTION
[0005] In one aspect, a method includes heating a MRI shim with a
heater to reduce a temperature rise per unit of time during a
gradient operation of a MRI system.
[0006] In another aspect, a MRI shim system includes a shim and a
heater in thermal communication with the shim.
[0007] In still another aspect, an imaging apparatus for producing
Magnetic Resonance (MR) images of a subject having a magnet
assembly for producing a static magnetic field and a gradient coil
assembly disposed within the magnet assembly for generating a
magnetic field gradient for use in producing MR images is provided.
The apparatus includes a MRI shim system configured to maintain a
shim within 0.1.degree. C. of the shim's operating temperature.
DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a block diagram of an embodiment of a magnetic
resonance imaging (MRI) system.
[0009] FIG. 2 illustrates an exploded shim system.
[0010] FIG. 3 illustrates a PID control controlling a heater.
[0011] FIG. 4 is a cross-sectional view of the system shown in FIG.
3.
DETAILED DESCRIPTION OF THE INVENTION
[0012] Herein described are methods and apparatus that provide a
simple design to control the temperature of the shims during any
gradient operational conditions.
[0013] As used herein, an element or step recited in the singular
and proceeded with the word "a" or "an" should be understood as not
excluding plural said elements or steps, unless such exclusion is
explicitly recited. Furthermore, references to "one embodiment" of
the present invention are not intended to be interpreted as
excluding the existence of additional embodiments that also
incorporate the recited features.
[0014] FIG. 1 is a block diagram of an embodiment of a magnetic
resonance imaging (MRI) system 10 in which the herein described
systems and methods are implemented. MRI 10 includes an operator
console 12 that includes a keyboard and control panel 14 and a
display 16. Operator console 12 communicates through a link 18 with
a separate computer system 20 thereby enabling an operator to
control the production and display of images on screen 16. Computer
system 20 includes a plurality of modules 22 which communicate with
each other through a backplane. In the exemplary embodiment,
modules 22 include an image processor module 24, a CPU module 26
and a memory module 28, also referred to herein as a frame buffer
for storing image data arrays. Computer system 20 is linked to a
disk storage 30 and a tape drive 32 to facilitate storing image
data and programs. Computer system 20 communicates with a separate
system control 34 through a high speed serial link 36.
[0015] System control 34 includes a plurality of modules 38
electrically coupled using a backplane (not shown). In the
exemplary embodiment, modules 38 include a CPU module 40 and a
pulse generator module 42 that is electrically coupled to operator
console 12 using a serial link 44. Link 44 facilitates transmitting
and receiving commands between operator console 12 and system
command 34 thereby allowing the operator to input a scan sequence
that MRI system 10 is to perform. Pulse generator module 42
operates the system components to carry out the desired scan
sequence, and generates data which indicative of the timing,
strength and shape of the RF pulses which are to be produced, and
the timing of and length of a data acquisition window. Pulse
generator module 42 is electrically coupled to a gradient amplifier
system 46 and provides gradient amplifier system 46 with a signal
indicative of the timing and shape of the gradient pulses to be
produced during the scan. Pulse generator module 42 is also
configured to receive patient data from a physiological acquisition
controller 48. In the exemplary embodiment, physiological
acquisition controller 48 is configured to receive inputs from a
plurality of sensors indicative of a patient's physiological
condition such as, but not limited to, ECG signals from electrodes
attached to the patient. Pulse generator module 42 is electrically
coupled to a scan room interface circuit 50 that is configured to
receive signals from various sensors indicative of the patient
condition and the magnet system. Scan room interface circuit 50 is
also configured to transmit command signals such as, but not
limited to, a command signal to move the patient to a desired
position with a patient positioning system 52.
[0016] The gradient waveforms produced by pulse generator module 42
are input to gradient amplifier system 46 that includes a G.sub.X
amplifier 54, a G.sub.Y amplifier 56, and a G.sub.Z amplifier 58.
Amplifiers 54, 56, and 58 each excite a corresponding gradient coil
in gradient coil assembly 60 to generate a plurality of magnetic
field gradients used for position encoding acquired signals. In the
exemplary embodiment, gradient coil assembly 60 includes a magnet
assembly 62 that includes a polarizing magnet 64 and a whole-body
RF coil 66.
[0017] In use, a transceiver module 70 positioned in system control
34 generates a plurality of electrical pulses that are amplified by
an RF amplifier 72 that is electrically coupled to RF coil 66 using
a transmit/receive switch 74. The resulting signals radiated by the
excited nuclei in the patient are sensed by RF coil 66 and
transmitted to a preamplifier 76 through transmit/receive switch
74. The amplified NMR (nuclear magnetic resonance) signals are then
demodulated, filtered, and digitized in a receiver section of
transceiver 70. Transmit/receive switch 74 is controlled by a
signal from pulse generator module 42 to electrically connect RF
amplifier 72 to coil 66 during the transmit mode and to connect
preamplifier 76 during the receive mode. Transmit/receive switch 74
also enables a separate RF coil (for example, a surface coil) to be
used in either the transmit or receive mode.
[0018] The NMR signals received by RF coil 66 are digitized by
transceiver module 70 and transferred to a memory module 78 in
system control 34. When the scan is completed and an array of raw
k-space data has been acquired in the memory module 78, the raw
k-space data is rearranged into separate k-space data arrays for
each cardiac phase image to be reconstructed, and each of these
arrays is input to an array processor 80 configured to Fourier
transform the data into an array of image data. This image data is
transmitted through serial link 36 to computer system 20 where it
is stored in disk memory 30. In response to commands received from
operator console 12, this image data may be archived on tape drive
32, or it may be further processed by image processor 24 and
transmitted to operator console 12 and presented on display 16.
[0019] FIG. 2 illustrates a tray 100 with a base or bottom portion
102, and a plurality of side walls 104 and 106 which together form
a channel shape. Positioned in the channel are a plurality of shim
systems 108. FIG. 2 also shows one shim system 108 exploded at
reference 110. Each shim system 108 includes a heater 112, a
backing plate 114 and a shim 116. For mounting purposes, a
plurality of openings or holes 118 are provided.
[0020] FIG. 3 illustrates a PID (Proportional-Integral-Derivative)
control 120 connected to and controlling heater 112. A temperature
sensor 122 provides PID 120 with the temperature of shim 116. Tray
100 provides structural support and is manufactured with a
non-electrically conductive, non-magnetic and poor heat conductive
material (thermal conductivity k.apprxeq.0.01 to 5 W/mK). Tray 100
is provided with holes 118 to fix steel shims 116 in place and a
small recess at the center of the tray to place foil heater 112. In
one embodiment, heater 112 is a double electrically insulated foil
heater with a power total power of 10 to 15 Watts and approximately
3300 W/m2, and provides enough heating power to raise the
temperature of shims 116 to the their operating temperature in
approximately 3 to 5 minutes. Metal shims 116 are carbon steel 1010
to 1020 with dimensions of 16.times.18.times.1 mm. Backing plate
114 is non electrically conductive, non magnetic, and highly heat
conductive material (k.apprxeq.60 to 100 W/mK plate that is placed
above foil heater 112 and isolates shims 116 from foil heater 112
at the tray center location. Temperature sensor 122 is located
between heater 112 and backing plate 114 for temperature control.
The temperature control provides temperature control of the shim to
less than a 0.1 C variance about the shim's operating
temperature.
[0021] FIG. 4 is a cross-sectional view of the system shown in FIG.
3. FIG. 4 shows that heater 112 is placed directly on tray 100 and
then temperature sensor 122 is placed on top of heater 112.
However, there could be another element between tray 100 and heater
112. On top of temperature sensor 122 sits the backing plate 114,
and then the shim 116 is placed last on top. Although shown in FIG.
4 with the sidewalls 104 and 106 extending much past shim 116,
other embodiments place the side wall even with or below shim 116.
FIG. 4 also illustrates a bolt 130 that attaches all components to
tray 100. However, other fasteners may be used.
[0022] The system is applicable to both embedded gradient passive
shim and inner bore magnet configuration that may require
additional passive shim temperature control to avoid B0 intensity
drift. The system works in maintaining the temperature of the shims
at a constant elevated temperature (this temperature is based on
the maximum operating gradient coil temperature) using the PID
control system. The primary heat necessary to elevate the
temperature of the shim and keep them at constant temperature is
provided by the foil heater attached to the high thermal conductive
plate that provides an isothermal surface for the shims with
reduced temperature fluctuations, driven by secondary heat source
(heat provided by the gradient coil) and the performance of the
temperature control unit. The system is designed so shims reach
steady operation temperature in three to five minutes with peak
power consumption at the starting condition. During gradient
operation the time constant of the temperature rise at the pocket
location is small compared to the time constant of temperature rise
of the shim due to the heat provided by the foil heater, allowing
good conditions for a PID temperature control system, in which the
temperature of the shim is controlled to be within 0.1.degree. C.
from the operating temperature. By controlling in real time the
temperature of the shims, the temperature rise per unit time is
minimized and so is the associated effect on the magnet B0
intensity drift performance during any gradient operating
conditions.
[0023] Technical effects include that the active system provides
uniform temperature of the shims regardless of the gradient coil
operating conditions, the system is applicable for both the
embedded gradient configuration and the magnet inner bore passive
shim system configuration, the system is upgradeable and includes
easily replaceable components in case of component failure, and the
tray system provides easy shim assembly and shim replacement during
magnet shimming operations. Other technical effects include a
single or multiple control zone and single or multiple control unit
per tray, temperature control units and heater power supplies are
the only supporting equipment for the system (no chiller, air flow
systems, or any other cooling system are required).
[0024] Exemplary embodiments are described above in detail. The
assemblies and methods are not limited to the specific embodiments
described herein, but rather, components of each assembly and/or
method may be utilized independently and separately from other
components described herein.
[0025] While the invention has been described in terms of various
specific embodiments, those skilled in the art will recognize that
the invention can be practiced with modification within the spirit
and scope of the claims.
* * * * *