Applied Superconetics, Inc.

General Atomics, Inc.

Ga Technologies, Inc.

Yen-Hwa L Hsu

Bruce C Breneman

Raymond E Sarwinski

John R Purcell

Richard L Creedon

Jang-Yu Hsu

Wilkie Y Chen

A method for using ferromagnetic shims to reduce the inhomogeneity of the magnetic field in a Diameter Spherical Volume (DSV) in the bore of a Magnetic Resonance Imaging (MRI) device is provided. More specifically, a magnetic field is established within the bore of the MRI device. An operator defines the DSV. The strength of the magnetic field is measured at the surface of the DSV. A Legendre polynomial is derived to model the inhomogeneity in the magnetic field in the DSV. A set of coefficients for the harmonic terms of the polynomial is calculated from the field strength measurements and segregated into three separate groups for evaluation. Each group of terms expresses field inhomogeneity in a unique set of spatial dimensions. Ferromagnetic shims are attached to the surface of the bore in individual dipole, linear bar, and ring arrangements to separately reduce the first, second, and third groups of harmonic coefficients to acceptable levels corresponding to a predetermined peak-to-peak inhomogeneity range.

Method for determining shim placement on tubular magnet

A MRI superconducting magnet having an open access frame with spaced parallel end plates, a superconducting coil assembly associated with each end plate, and gradient coils associated with each end plate includes an apparatus for minimizing hysteresis. The apparatus for minimizing hysteresis comprises a layer of nonferromagnetic conducting material interposed between the gradient coils and end plates. The interposed layer carries eddy currents induced by time varying magnetic fields produced by the gradient coils, thereby minimizing AC eddy current hysteresis from the end plates. The layer preferably has a thickness of at least one skin depth at the frequency associated with the time varying magnetic field. In another embodiment, each end plate has a plurality of slits formed therein for eliminating eddy currents in the end plates.

Open access superconducting MRI magnet having an apparatus for reducing

An open access MRI magnet includes a ferromagnetic frame open on at least two sides and having upper and lower end plates and at least two support columns. A superconducting coil assembly is mounted to each end plate for generating a magnetic flux field in a patient receiving area located between the end plates. Each superconducting coil assembly includes a toroidal vacuum tight vessel, insulation, and one or more temperature shields mounted within the vacuum tight vessel. One or more coils of superconducting wire is wound within the vacuum vessel, and is coupled to a power source and to a persistent switch for maintaining a constant flow of current with no power consumption. A return path for the flux is provided by the end plates and support posts of the frame. The superconducting coils, the thermal shields, and the persistent switch are cooled by a cryocooler, by means of conduction through solid thermal conductors or by a combination of conduction and convection, through a fluid cooling circuit, without the consumption of a liquid cryogen.

Open access superconducting MRI magnet

An open access superconducting MRI magnet includes a ferromagnetic frame open on at least two sides and having upper and lower end plates and at least two support posts. A superconducting coil assembly is mounted to each end plate for generating a magnetic flux field in a patient receiving area located between the end plates. A magnetic flux field shaping means is associated with each end plate. The flux field shaping means includes rose shims, inner rings mounted within the rose shims, and removable segments mounted to the rose shims. Transition plates are located between the support ports and end plates for minimizing flux leakage. In addition the end plates are formed with a varying thickness such that a cross section taken perpendicular to a return path of magnetic flux is substantially constant.

Superconducting MRI magnet with magnetic flux field homogeneity control

A method for manufacturing a stress controlled wire comprises the step of twisting superconductor fibers together into a bundle of fibers. A plurality of bundles are then twisted together and disposed within the lumen of a tube and the lumen is flooded with solder which is allowed to harden. The tube and its contents are subsequently heated as they are wound upon a drum to melt the solder and allow realignment of the superconductor fibers within the solder. The subsequent solidification of solder after the tube is wound onto the drum provides an unstressed support for the fiber bundles.

Stress controlling superconductor wire

An apparatus for minimizing hysteresis in a magnetic resonance imaging device of the type utilizing a pair of oppositely charged parallel pole faces forming a magnetic field therebetween, and gradient coils spaced apart from each pole face for producing time varying magnetic fields, comprises a layer of nonferromagnetic conducting material interposed between the pole face and the gradient coil. The interposed layer carries eddy currents induced by time varying magnetic fields produced by the gradient coils, thereby minimizing AC induced eddy current hysteresis from the pole faces. The layer preferably has a thickness of at least one skin depth at the frequency associated with the time varying magnetic field. The layer is preferably contiguous with the pole face, and comprises copper or aluminum. In another embodiment, each pole face has a plurality of slits therethrough radiating inward from the circumference for eliminating the eddy currents in the pole face. The slits in one pole face are preferably symmetrical to those in the other. There are preferably twenty-four (24) slits spaced fifteen degrees (15.degree.) apart. There is also disclosed the method for accomplishing the same.

Method and apparatus of reducing magnetic hysteresis in MRI systems

A magnet system for providing a substantially homogeneous magnetic field in a region. The magnet system includes a main coil and a pair of correcting coils with the main coil generating a magnetic field in a zone including the region. The correcting coils are at first and second locations in the zone and are connected to one another so that a magnetic field generated by the main coil will induce currents in the correcting coils which oppose one another. The correcting coils are constructed so that there will be no resultant current in them when there is generated substantially the same field strength of both of the first and second locations. Thus, the generation of differing magnetic field strengths at the respective locations effects generation of a resultant current in the correcting coil causing strengthening of the magnetic field at the location having the lesser magnetic field strength and weakening of the magnetic field at the other location, making the net field relatively homogeneous.

Magnet system providing a region of substantially homogeneous field

A switch comprises a length of superconductive wire having a plurality of normally conductive stabilizing segments thereon.

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Solana Beach, CA, United States of America

Yen-Hwa L Hsu