Improvement of the Field Homogeneity with a Permanent Magnet Assembly for MRI

In the last few years, MRI (magnetic resonance imaging) has become one of the most useful and important radiological and diagnostic methods. MRI requires a strong uniform magnetic field in the area of the patient to be examined. The requirement of a highly homogeneous field grows more stringent with rapid progress in tomographic techniques. At the same time, low-cost magnets are also desired. We previously developed and mass-produced a permanent magnet-type circuit using high-energy Nd-Fe-B meterial. This paper describes a newly developed 15-plane measuring method to replace the previous 7-plane method, for precise evaluation of the homogeneous field. Using this method of analysis and a linear programming method, a pole piece with a new shape has been developed. As a consequence, the homogeneity was doubled and the magnetic weight was reduced by 10% as compared with existing pole pieces.