Shimming Permanent Magnet of MRI Scanner

In this paper an approach of ferromagnetic shimming for permanent MRI magnet is presented. It is designed to reduce unhomogeneity of magnetostatic field of permanent magnet to meet the stringent requirement for MRI applications. An optimal configuration of ferromagnetic pieces is generated through calculation according to the initial field map and the demanded final homogeneity specifications. 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 effective and efficient. 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 field in the working magnetic field volume to within a predetermined specification, 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 final homogeneity of main magnetic field 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 field method [9–11] has been used for active shimming of SC magnet. Conventional electromagnetic shimming follows the approach based on representing the field as a spherical harmonics series: