Selective cutting (e.g. – punching) – With means to impose randomly actuated control of... – Embodying means to detect indicium in work or pattern
Reexamination Certificate
2002-12-23
2004-08-31
Shrivastav, Brij B. (Department: 2859)
Selective cutting (e.g., punching)
With means to impose randomly actuated control of...
Embodying means to detect indicium in work or pattern
C324S309000
Reexamination Certificate
active
06783059
ABSTRACT:
BACKGROUND OF 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 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.
REFERENCES:
patent: 4782671 (1988-11-01), Breneman et al.
patent: 5225782 (1993-07-01), Laskaris et al.
patent: 5285181 (1994-02-01), Laskaris et al.
patent: 5304934 (1994-04
Huang Xianrui
Laskaris Evangelos
Ogle Michele Dollar
Foley & Lardner LLP
General Electric Company
Shrivastav Brij B.
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