Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
1999-07-29
2001-11-27
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S216000, C335S300000
Reexamination Certificate
active
06323749
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to superconducting magnetic resonance imaging (“MRI”) devices for producing images used in medical diagnostics.
BACKGROUND OF THE INVENTION
Medical magnetic resonance studies are typically carried out in strong magnetic fields. In an electromagnet employing normal conductors, a portion of the electrical power used to generate the magnetic field is consumed in heating of the resistive coil conductor. Thus, many kilowatts of power may be required to produce the magnetic field strength for a volume sufficient to develop a magnetic resonance image for medical diagnostics.
A coil of a conductive material, which is wound into a solenoid, generates the magnetic field for the magnet. The use of superconducting coils in MRI systems greatly reduces the amount of power consumed since a superconductor has a resistance which approaches zero and thus does not expend as much power to produce the same magnetic field. One of the controlling factors in superconductivity is the critical temperature of the conductive material of the coil. In a typical superconducting MRI magnet, a cryogenic fluid is used to cool a superconducting coil to a temperature at or below the critical temperature so that the coil exhibits superconducting properties.
One type of MRI magnet incorporates a frame formed from a ferromagnetic material, disclosed in U.S. Pat. Nos. 4,766,378 and 5,754,085, the disclosures of which are hereby incorporated by reference herein.
The MRI magnet disclosed in certain embodiments of U.S. Pat. No. 5,754,085 includes a first superconducting coil assembly supported on an upper support and a second superconducting coil assembly supported on a lower support. Superconducting coil assemblies typically include a container for the superconducting coil and the coil is disposed inside the container and immersed in a bath of liquid helium held in the container.
A substantial field is created by the primary magnet assembly. The net forces acting on the coils results in attraction of the coils to each other or attraction between the individual coils and the adjacent ferromagnetic components. The direction and magnitude of the force depends on the location of the coils with respect to the median plane of the magnet. The forces created by the field tends to spread the coil in a radial direction, tending to unravel the coil, or causes the coils to be attracted to the adjacent ferromagnetic structure. Thus, mounting and supporting the superconducting coils within an MRI magnet is a substantial problem. In addition, one of the issues which must be addressed is the tendency of the support structure for the superconducting coil to add heat to the coil by conduction from the outside environment.
The cryogenic fluid utilized to cool a superconducting coil is costly, which is a drawback as compared to a non-superconducting electromagnet. An MRI magnet including support structure for the superconducting coil which minimizes the amount of heat introduced into the superconducting coil, and which utilizes a minimum amount of cryogenic fluid is desirable for reasons of operating cost. Moreover, it is desirable to minimize the amount of cryogenic fluid in proximity to the coil for other reasons. A superconducting magnet coil can become normally-conducting in certain unusual circumstances known as a “quench”. When this happens, energy stored in the flowing current is dissipated rapidly, and the surrounding cryogenic liquid is converted to a gas. While a properly designed superconducting magnet should not quench, safety precautions should be provided to take into account possible quenching and gas evolution. With less cryogenic fluid in the superconducting coil assembly, this problem is minimized.
SUMMARY OF THE INVENTION
The present invention addresses these needs.
A magnetic resonance imaging magnet for examining a patient comprises a first ferromagnetic pole piece and a second ferromagnetic pole piece, the pole pieces being disposed facing each other and defining a patient-receiving gap therebetween. The pole pieces are arranged for producing a magnetic field within the patient-receiving gap and have a central axis extending through the pole pieces. The magnet also comprises a ferromagnetic yoke for supporting the first pole piece and the second pole piece in their respective positions facing each other. The magnet includes a first superconducting coil assembly and a second superconducting coil assembly. Each assembly has an insulated housing and is disposed adjacent one of the pole pieces for producing a magnetic field which passes through the pole pieces and the patient-receiving gap. Each coil assembly includes at least one coil encircling the associated pole piece, at least one thermally conductive member mounted adjacent to the at least one coil, and means for maintaining a cryogenic fluid. The at least one thermally conductive member is in heat transfer relationship with the cryogenic fluid, thereby maintaining the at least one coil at a temperature at which the at least one coil exhibits superconducting properties.
The superconducting coil is cooled by conduction through the thermally conductive member which removes heat from the superconducting coil. The coil is not immersed in a cryogenic fluid such as helium so that the volume of helium required in the superconducting coil assembly is reduced.
The ferromagnetic yoke preferably comprises a magnetic flux conduit to minimize leakage magnetic fields.
The superconducting coil assembly of the magnet also preferably includes a plurality of suspension members connected to the coil assembly for restraining the superconducting coil assembly. The substantial forces acting on the superconducting coil assembly when the coils are generating a magnetic field are counteracted by the plurality of suspension members. The suspension members carry less heat to the superconducting coil assembly than a compression-type mounting having a greater cross-sectional area. However, a compression-type mounting may be used in some embodiments of the invention. Accordingly, the superconducting coil assembly may include a plurality of supporting legs connected to the ferromagnetic yoke for supporting the superconducting coil assembly within the magnet.
In preferred embodiments, the housing for the superconducting coil assembly includes at least one layer of superinsulation surrounding the at least one coil. The housing may also include at least one heat shield within the housing. The heat shield or shields must be optimized to reduce the heat transfer from the hot environment to the cold mass. In certain preferred embodiments, the housing may enclose an inner anti-buckling ring, an outer clamping ring, a top support ring and a bottom support ring for supporting the at least one coil and restraining the at least one coil in the coil assembly. The coil assembly preferably includes a housing having a toroidal shape and being arranged concentrically with the central axis.
In certain preferred embodiments, the magnet includes a first support for supporting and restraining the at least one coil adjacent the at least one thermally conductive member, the first support having a U-shaped cross section. The first support may have any number of cross-sectional shapes. A second housing is preferably included for mounting the at least one thermally conductive member adjacent the at least one coil, the second support having a U-shaped cross-section. The second support also may have a number of cross-sectional shapes.
In preferred embodiments, the means for maintaining the cryogenic fluid for the magnet includes a cryocooler. The cryocooler preferably includes a cryogenic fluid. The at least one thermally conductive member may include at least one tube, carrying a p-cryogenic fluid. The at least one tube may comprise three tubes, for example, depending on the size of the coil. The means for maintaining the cryogenic fluid preferably comprises a cryocooler, re-condensing the cryogenic fluid to a liquid phase, connected to the at least one tube. The cry
Donovan Lincoln
Fonar Corporation
Lerner David Littenberg Krumholz & Mentlik LLP
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