中文摘要：

目的用磁性材料改善永磁MRI的主磁体极面设计以增加其开放度并减小极面上的涡流.方法用磁性材料代替常用的铁磁材料作为极面上的匀场环,并讨论匀场效果.结果通过在软件MAXWELL上进行模拟,发现磁环结构可以减小边缘凸起程度,从而增加开放度.此外,与铁磁材料相比,永磁材料如NdFeB合金的磁化系数非常小,使得磁环上几乎不产生涡流.结论匀场环改用磁性材料可以在场强不变的情况下增加开放度,让病人更舒适;或是在开放度不变的情况下增大场强,同时极面上涡流的减小为提高成像速度创造了条件.

英文摘要：

Objective An improvement of MRI pole pieces is presented to increase the open degree and decrease the eddy current of the pole pieces. Methods The ferromagnetic ring is displaced by magnetic ring and the shimming effect is discussed. Results Simulation indicates that pole pieces with magnetic shimming ring lead a higher open degree because of less convexity than ones with ferromagnetic ring. Besides, compared with ferromagnetic materials, permanent magnet such as NdFeB alloy has very small susceptibility which leads to almost no eddy current in magnetic ring. Conclusion This work will improve patient comfort with the same magnitude of magnetic field or increase the magnitude of the magnetic field with the same open degree. In addition, the decreasing eddy current of this kind pole pieces will provide the possibility for increasing the imaging speed.