Rapid field calculations forthe effect of ferromagnetic material in MRI magnet design

Abstract

Magnetic resonance imaging (MRI) magnets have very stringentconstraints on the homogeneity of the static magnetic fieldthat they generate over desired imaging regions. The magnetsystem also preferably generates very little stray fieldexternal to its structure, so that ease of siting and safety areassured. This work concentrates on deriving means of rapidlycomputing the effect of `cold' and `warm' ferromagneticmaterial in or around the superconducting magnet system, so asto facilitate the automated design of hybrid material MRmagnets.A complete scheme for the direct calculation of the sphericalharmonics of the magnetic field generated by a circular ring offerromagnetic material is derived under the conditions ofarbitrary external magnetizing fields. The magnetic fieldproduced by the superconducting coils in the system is computedusing previously developed methods. The final, hybrid algorithm isfast enough for use in large-scale optimization methods. Theresultant fields from a practical example of a 4 T, clinical MRImagnet containing both superconducting coils and magneticmaterial are presented.