Abstract
In this paper an approach of ferromagnetic shimming for permanent MRI magnet is presented. It is designed to reduce unhomogeneity of magnetostatic fleld of permanent magnet to meet the stringent requirement for MRI applications. An optimal conflguration of ferromagnetic pieces is generated through calculation according to the initial fleld map and the demanded flnal homogeneity speciflcations. This approach uses a minimization technique that makes the sum of squared magnetic moment minimum to restrict the amount of the ferromagnetic material used and the maximal thickness of shim pieces stacked at each position on the shimming boards. Simulation results verify that the method is efiective and e-cient. DOI: 10.2529/PIERS060826010243 As well known there are two kinds of MRI (1) magnets: closed cylindrical superconductive(SC) magnet and open biplanar-pole permanent magnet such as C-shaped one. The open space of the C-magnet helps the patient overcome any feelings of claustrophobia that may be experienced in a closed designed magnet. Both superconductive and permanent magnet must be shimmed to reduce the unhomogeneity of the magnetic fleld in the working magnetic fleld volume to within a predetermined speciflcation, i.e., within a few parts per million for use in medical diagnosis. However, to the permanent magnet due to approximation in the design as well as magnetizing and fabricating tolerances, the flnal homogeneity of main magnetic fleld is often far away from the acceptable level. Therefore, shim technology is of major importance in the design and manufacture of permanent MRI magnet. Many papers (2{4) and patents (5,6) published focus on passive steel shimming of the SC magnet. As for active coil shimming of the SC magnet, likewise there are many papers (7,8) published. Recently target fleld method (9{11) has been used for active shimming of SC magnet. Conventional electromagnetic shimming follows the approach based on representing the fleld as a spherical harmonics series:
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