Development of a Permanent Magnet Assembly for MRI

A permanent magnet assembly for MRI (magnetic resonance imaging) has been developed using a high-energy product sintered Nd-Fe-B magnet. There are three types of magnet assembly for use in MRI: superconductive electromagnets, conductive electromagnets, and permanent magnets. The last does not require liquid helium, electricity or cooling water, and can be installed in small spaces. Despite these advantages, however, permanent magnet assemblies have not been widely adopted, mainly because of the difficulty of attaining uniformity on the order of ppm, and the tremendous weight of such magnets. However, a high-energy product Nd-Fe-B magnet developed in 1983 has made it possible to realize MRI based on permanent magnets. Using such a magnet and investigating the optimum configuration of pole pieces with shims, we obtained a Pk/Pk deviation of 30 ppm over a 35 cm diameter spherical volume at the center of the air gap.