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Method for Using Steel to Correct Field Inhomogeneities in a Superconductive NMR Magnet

IP.com Disclosure Number: IPCOM000037541D
Original Publication Date: 1989-Mar-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Schwall, RE: AUTHOR

Abstract

Nuclear Magnetic Resonance Imaging (MRI) is a medical imaging technique which requires a very homogeneous magnetic field. Field homogeneities on the order of 1 ppm over the imaging volume are desirable. Fields of 0.5 to 2.0 Tesla and above are supplied for this purpose by superconductive solenoids. The inherent homogeneity of such solenoids is usually 10 to 100 times worse than required, said inhomogeneities being due to manufacturing tolerances in the coil itself and the presence of magnetic material in the environment. It is common practice to include shim coils in the magnet system to correct the perturbations. It has been shown, however, that corrections can be obtained by placing steel at suitable locations in the magnet bore.

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Method for Using Steel to Correct Field Inhomogeneities in a Superconductive NMR Magnet

Nuclear Magnetic Resonance Imaging (MRI) is a medical imaging technique which requires a very homogeneous magnetic field. Field homogeneities on the order of 1 ppm over the imaging volume are desirable. Fields of 0.5 to 2.0 Tesla and above are supplied for this purpose by superconductive solenoids. The inherent homogeneity of such solenoids is usually 10 to 100 times worse than required, said inhomogeneities being due to manufacturing tolerances in the coil itself and the presence of magnetic material in the environment. It is common practice to include shim coils in the magnet system to correct the perturbations. It has been shown, however, that corrections can be obtained by placing steel at suitable locations in the magnet bore. The present invention presents a systematic method for locating and attaching the steel in the magnet bore.

The inner surface of the magnet bore is divided into cells by marking the surface of the bore tube with axial lines and circles. The spacing of the lines and circles is chosen to optimize the method for computing the location of the steel. This computation is normally performed by a linear optimization technique. Each intersection of a line and circle defines one cell of a grid, and the weight or thickness of steel placed in the cell is the independent variable in the optimization. A continuous strip of one half of an adhesive backed fabric fa...