Title: Conduction cooled passively-shielded MRI magnet
Abstract: A magnetic resonance imaging (MRI) device for imaging a volume is provided with at least one main magnet for generating a magnetic field, and at least one gradient coil for manipulating the magnetic field generated by the at least one main magnet to image the volume. The magnetic fields generated by the at least one gradient coil are substantially unshielded.
Patent Number: 6,995,562 Issued on 02/07/2006 to Laskaris, et al.
| Inventors:
|
Laskaris; Evangelos (Schenectady, NY);
Huang; Xianrui (Clifton Park, NY);
Ogle; Michele Dollar (Burnt Hills, NY)
|
| Assignee:
|
General Electric Company (Niskayuna, NY)
|
| Appl. No.:
|
898182 |
| Filed:
|
July 26, 2004 |
| Current U.S. Class: |
324/318; 324/307 |
| Current Intern'l Class: |
G01V 3/00 (20060101) |
| Field of Search: |
324/318,319,322,309,307,300
335/299
|
References Cited [Referenced By]
U.S. Patent Documents
| 4291541 | Sep., 1981 | Kneip et al.
| |
| 4310799 | Jan., 1982 | Hutchison et al.
| |
| 5225782 | Jul., 1993 | Laskaris et al.
| |
| 5285181 | Feb., 1994 | Laskaris et al.
| |
| 5304934 | Apr., 1994 | Laskaris et al.
| |
| 5406204 | Apr., 1995 | Morich et al.
| |
| 5461873 | Oct., 1995 | Longsworth.
| |
| 5874880 | Feb., 1999 | Laskaris et al.
| |
| 5874882 | Feb., 1999 | Laskaris et al.
| |
| 5883558 | Mar., 1999 | Laskaris et al.
| |
| 5994991 | Nov., 1999 | Laskaris et al.
| |
| 5999075 | Dec., 1999 | Laskaris et al.
| |
| 6150819 | Nov., 2000 | Laskaris et al.
| |
| 6157279 | Dec., 2000 | Laskaris et al.
| |
| 6166617 | Dec., 2000 | Laskaris et al.
| |
| 6172588 | Jan., 2001 | Laskaris et al.
| |
| 6783059 | Aug., 2004 | Laskaris et al.
| |
| Foreign Patent Documents |
| 195 27 150 | Jan., 1997 | DE.
| |
| 0 307 072 | Mar., 1989 | EP.
| |
| 0 416 959 | Mar., 1991 | EP.
| |
| 0 629 875 | Dec., 1994 | EP.
| |
| 2 282 451 | Apr., 1995 | GB.
| |
Other References
"Enhancement of the critical current density and flux pinning of MgB2
superconductor by nanoparticle SiC doping" article by S.X. Dou et al.
|
Primary Examiner: Shrivastav; Brij B.
Attorney, Agent or Firm: Testa; Jean K., Cabou; Christian G.
Claims
What is claimed is:
1. A magnetic resonance imaging (MRI) system, comprising:
a superconductor magnet for generating a magnetic field for imaging a volume;
an unshielded gradient coil for manipulating the magnetic field;
at least one cooling tube mounted adjacent to the superconductor magnet for cooling
the superconductor magnet; and
a cryocooling system thermally coupled to said at least one cooling tube.
2. The MRI system of claim 1, further comprising at least one passive shield
for passively shielding an external fringe magnetic field of said superconductor
magnet, said at least one passive shield being comprised of:
a plurality of laminated magnetizable rings,
wherein said plurality of laminated rings suppress eddy currents generated within
said at least one passive shield.
3. The MRI system of claim 2, wherein said cryocooling system comprises:
a cryorefrigerator for cooling a cooling medium used by said at least one cooling tube.
4. The MRI system of claim 3, wherein said cryorefrigerator is positioned external
to said superconductor magnet.
5. The MRI system of claim 1, further comprising:
at least one cooled thermal spreader, said at least one cooled thermal spreader
comprising one of:
a cooled thermal shield, and
a cooled coil former on which said superconductor magnet is wound.
6. The MRI system of claim 5, wherein said cooled thermal shield comprises:
a shield cylinder; and
the at least one cooling tube is wrapped around said shield cylinder, said at
least one cooling tube being coupled to the cryocooling system.
7. The MRI system of claim 6, wherein said shield cylinder comprises:
an epoxy-glass copper-wire composite material.
8. A magnetic resonance imaging (MRI) device for imaging a volume, comprising:
at least one superconducting coil operating at cryogenic temperatures for generating
a magnetic field; and
at least one gradient coil for manipulating the magnetic field generated by said
at least one superconducting coil to image said volume,
wherein said at least one superconducting coil includes at least one cooling
tube abutting superconducting coil layers, said at least one cooling tube being
coupled to a cryocooling system.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to magnetic resonance imaging (MRI) devices,
and more particularly to MRI devices including at least one gradient coil for manipulating
the magnetic field generated by the MRI magnet, wherein the magnetic fields generated
by the gradient coil are substantially magnetically unshielded.
MRI devices are widely used in the medical community as a diagnostic tool for
imaging items such as tissue and bone structures. Conventional MRI devices are
described, for example, in U.S. Pat. Nos. 5,225,782; 5,285,181; and 5,304,934 which
are all incorporated by reference herein in their entirety.
As shown in FIG. 1, known superconducting (SC) MRI devices
10 typically
employ windings
30 for generating a homogeneous magnetic field in an image
volume
20, the windings
30 operating in liquid helium to maintain
the temperature at approximately 4° K. The liquid helium pool requires a vessel
40 which is vacuum tight and which meets American Society of Mechanical
Engineering (ASME) pressure vessel requirements; such a vessel
40 is typically
made of welded aluminum alloy cylinders and flanges. Thermal radiation shields
(not shown), of which two are typically used, are also made of welded aluminum
pieces and contain the helium vessel
40.
When the gradient coils
50 in the bore of the MRI device
10 are
electrically pulsed, the resulting time changing magnetic flux in any of the electrically
conducting cylinders surrounding the gradient coils induces eddy currents. These
eddy currents in turn produce their own magnetic fields which degrade the quality
of the desired gradient field in space and time. A second set of gradient coils
60 (i.e., shield gradient coils) in the magnet bore compensate for the aggressive
pulse sequences which are routinely used in MR imaging today. These shield gradient
coils
60 set up fields which counteract those of the main gradient coil
50 in the region outside of the shield coil
60, thus greatly reducing
any mutual inductance with conducting members, such as the thermal shields, and
minimizing the resultant eddy currents. The present inventors have found that,
in a typical implementation, the shield coils
60 generally cancel about
50% of the magnetic field produced by the gradient coils
50.
A need exists, however, for a MRI device
10 which reduces the amount of
resultant eddy currents produced in the MRI device
10 by the gradient coils
50 in systems without the shield coils
60, or, for systems with shield
coils, further reduces the amount of resultant eddy currents in the MRI device.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed at reducing or eliminating one or more of the
problems set forth above, and other problems found within the prior art.
According to one embodiment of the present invention, a magnetic resonance
imaging (MRI) device for imaging a volume is provided comprising at least one main
magnet for generating a magnetic field, and at least one gradient coil for manipulating
the magnetic field generated by the at least one main magnet to image the volume,
wherein the magnetic fields generated by the at least one gradient coil are substantially unshielded.
Preferably, the main magnet comprises at least one superconducting coil
operating at cryogenic temperatures. More preferably, the main magnet further comprises
at least one cooling tube abutting superconducting coil layers, the cooling tube
being coupled to a cryocooler heat exchanger.
According to one aspect of the present invention, the main magnet may include
a composite vacuum vessel enshrouding the at least one superconducting coil, the
composite vacuum vessel being formed of a material wherein eddy currents are not
substantially induced therein by the magnetic fields generated by the at least
one gradient coil.
According to another aspect of the present invention, the main magnet is
inductively isolated from the gradient coil.
According to another aspect of the present invention, the MRI device further
comprises at least one cooled thermal spreader. Preferably, the cooled thermal
spreader comprises at least one of a cooled thermal shield, and a cooled coil former
on which a superconducting coil is wound. The cooled coil former preferably comprises
a composite material including fiberglass, epoxy, and copper wire.
According to another aspect of the present invention, the MRI device further
comprises a cryocooler heat exchanger thermally coupled to the at least one magnet,
and a cryorefrigerator for cooling a cooling medium used by the cryocooler heat
exchanger. Preferably, the cryorefrigerator is positioned substantially outside
of the magnetic fields generated by the at least one gradient coil. The cooling
medium may comprise one of liquid helium, liquid hydrogen, liquid nitrogen, and
liquid neon.
According to another aspect of the present invention, the gradient coil
comprises a plurality of epoxy-glass layers, and a plurality of insulated copper
wire layers.
According to another aspect of the present invention, the MRI device further
comprises at least one passive shield for passively shielding an external fringe
magnetic field of the at least one magnet, the at least one passive shield being
comprised of a plurality of laminated magnetizable rings. The plurality of laminated
rings suppress eddy currents generated within the at least one passive shield.
According to another embodiment of the present invention, a magnetic resonance
imaging (MRI) device for imaging a volume is provided comprising means for generating
a main magnetic field, and means for manipulating the main magnetic field to image
the volume, wherein the means for manipulating generates magnetic fields which
are substantially unshielded.
According to one aspect of the present invention, the MRI device further
comprises means for cryocooling the means for generating a main magnetic field
without substantially inducing eddy currents within the means for cryocooling.
According to another aspect of the present invention, the MRI device further
comprises means for passively shielding the means for generating a main magnetic field.
According to another embodiment of the present invention, a magnetic resonance
imaging (MRI) system is provided comprising a superconductor magnet for generating
a magnetic field for imaging a volume, an unshielded gradient coil for manipulating
the magnetic field, and a cryocooling system thermally coupled to the superconductor magnet.
According to one aspect of the present invention, the cryocooling system
comprises a cryocooler heat exchanger thermally coupled to the superconductor magnet,
and a cryorefrigerator for cooling a cooling medium used by the cryocooler heat
exchanger. Preferably, the cryorefrigerator is positioned external to the superconductor magnet.
According to another embodiment of the present invention, a magnetic resonance
imaging (MRI) device for imaging a volume is provided comprising at least one superconducting
coil operating at cryogenic temperatures for generating a magnetic field, and at
least one gradient coil for manipulating the magnetic field generated by the at
least one main magnet to image the volume. The at least one superconducting coil
includes at least one cooling tube abutting superconducting coil layers, the at
least one cooling tube being coupled to a cryocooler heat exchanger.
According to another embodiment of the present invention, a magnetic resonance
imaging (MRI) device for imaging a volume is provided comprising at least one main
magnet for generating a magnetic field, at least one gradient coil for manipulating
the magnetic field generated by the at least one main magnet to image the volume,
and at least one cooled thermal spreader. The cooled thermal spreader comprises
at least one of a cooled thermal shield, and a cooled coil former on which the
at least one main magnet is wound.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional view of a known MRI device.
FIG. 2 is a cross sectional view of a MRI device according to an embodiment
of the present invention.
FIG. 3 is an enlarged view of a portion of a cooled coil former according to
an embodiment of the present invention.
FIG. 4 is an end view of a MRI device according to an embodiment of the present invention.
FIG. 5 is a cross sectional view of a cylindrical cooled thermal shield according
to an embodiment of the present invention.
FIG. 6 is a cross sectional view of a MRI device with both a cooled coil former
and a cylindrical cooled thermal shield according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Reference will now be made in detail to presently preferred embodiments
of the present invention. Wherever possible, the same reference numbers will be
used throughout the drawings to refer to the same or like parts.
The reduction of eddy currents according to an embodiment of the present invention
may be accomplished through the implementation of eddy current free components
in the MRI device
200. Such eddy current free components can be used to
eliminate or reduce any residual eddy currents generated, as present shield coils
still allow about 50% of the magnetic field produced by the gradient coils through.
By way of example, eddy current free components can be used as part of a cooled
thermal spreader, such as a cooled thermal shield or a cooled coil former. Other
components may also be made wholly or partly from eddy current free/resistant materials.
One such MRI magnet
200 is shown in the block diagram of FIG. 2. According
to this embodiment, a thermosiphon convection cooled coil former
210 is
provided with the main MRI magnet (i.e., superconducting coils
230) for
thermally shielding and/or cooling the main MRI magnet. Preferably the coil former
210 is made of a composite material comprising an epoxy-glass copper-wire
composite material; however, other materials such as replacing copper with aluminum,
etc. could also be used. Hence, it should be appreciated that the coil former
210
(and thermal shield cylinder
810 in FIGS. 5 and 6) are preferably made of
a material in which eddy currents are not substantially produced during operation
of the unshielded gradient coils
295. The bore
285 of the vacuum
vessel is also made substantially of eddy current free materials.
As shown in FIGS. 2 and 3, one or more cooling tubes
270 are mounted on
the coil former
210. Similarly, as shown in FIGS. 5 and 6, one or more cooling
tubes
270 may also be mounted on the thermal shield
810 (if provided).
Preferably, the cooling tubes
270 are wrapped substantially around the coil
former
210 and/or the thermal shield cylinder
810.
The cooling tubes
270 are configured to pass a cooling medium (e.g., liquid
helium, liquid hydrogen, liquid nitrogen, liquid neon, etc.) about the coil former
210 and/or thermal shield cylinder
810, thereby cooling the coil
former
210 and/or thermal shield cylinder
810, the cooling medium
chosen to have a temperature lower than the superconductor critical temperature
required by the combination of current density and magnetic field at which the
superconductor will be operating in. Hence, the cooling tubes
270 are coupled
to a cryorefrigerator
280 (FIG. 4) via a coolant flow circuit (not shown).
It should be appreciated that the size and number of tubes of the cooling tubes
270 depends on many heat transfer design details including, but not limited
to, overall size, flow rate and resistance, materials, and the heat load of the
MRI device
200.
Cooling is provided by circulating the cooling medium through the MRI device
200, where cold cooling medium heat exchanges with the coil former
210
and/or thermal shield cylinder
810 via the cooling tubes
270, and
returns back to the cryorefrigerator
280 at a higher temperature. As known
cryorefrigerator designs are limited on the types of materials employable therein
due, in part, to the extreme thermal operating conditions, the use of non-metallic
"eddy current free" materials is also limited. Hence, the cryorefrigerator
280
is preferably positioned external to the MRI device
200 as shown in FIG.
4. This isolates the cryorefrigerator
280 from the fluctuating electric
and magnetic fields generated by the MRI device
200, thereby preventing
generation of eddy currents within the cryorefrigerator
280. Fluid circulation
may be generated by utilizing the difference in gravitational forces between the
cold and the warm ends, or alternatively by way of a pump (not shown).
Cooling medium container
940 is used to store the liquid cooling medium
needed for operation. To minimize any generated eddy current, the container
940
is preferably composed of a thin stainless steel shell to form the pressure boundary
and a fiberglass composite structure wrapped around the stainless steel to support
the pressure load. Details of the coldhead
930 shown in FIG. 4 are dependent
upon the particular implementation.
The aforementioned configuration has reduced eddy currents generated therein
by moving a portion of the cryorefrigeration system (e.g., the cryorefrigerator
280) outside of the MRI magnet
200, and by using non-metallic materials
(e.g., composite materials) for at least some of the MRI components. The reduced
eddy current generation allows for elimination of the shield gradient coils
60
(FIG. 1), or for further reduced eddy currents in configurations with shield gradient
coils
60. Hence, any one of or a combination of the aforementioned features
may be utilized to improve upon a known MRI device
10.
The foregoing description of preferred embodiments of the invention has been
presented for purposes of illustration and description. It is not intended to be
exhaustive or to limit the invention to the precise form disclosed, and modifications
and variations are possible in light of the above teachings or may be acquired
from practice of the invention. The embodiments were chosen and described in order
to explain the principles of the invention and its practical application to enable
one skilled in the art to utilize the invention in various embodiments and with
various modifications as are suited to the particular use contemplated. It is intended
that the scope of the invention be defined by the claims appended hereto, and their equivalents.
*
