超导核磁共振成像主磁体无源数值匀场技术的研究

Numerical Method for the Passive Shimming of Superconducting NMR Imaging Magnets

本文提出了超导核磁共振成像主磁体无源数值匀场的概念,发展了一种优化无源数值匀场方法。将优化无源数值匀场方法应用于中国科健有限公司研制的国产第一台超导核磁共振成像主磁体,其结果令人满意。

The magnetic field in the imaging volume of superconducting NMR imaging mag- nets is given by Where P_n^m are the associated Legendre functions of the first type , A_n^m and B_n^m are constants. Supposing B_o=A_o^o To improve the homogeneity of the field , Bz'must be compensated . Based on the magnetic field caused by a magnetic moment,a distribution of the magnetic moments in a superconducting NMR imaging magnet can be determined through some pseudoinversion algorithm directly from Bz' . The magnetic moments are used to compensate Bz'. Introducing optimization into the pseudoinversion algorithm , this paper presents a method of optimum passive numerical shiming . The main point of the method is a optimization problem as follows Where Am_z is the field caused by the magnetic moments used to compensate Bz', and m_z are the magnetic moments, A represent the relations between source and field points of the magnetic moments . Solving the optimization problem , the optimum distribution of the magnetic moments in the magnet is obtained . Realizing the distribution, Bz' is expected to be compensated efficiently and the homogeneity of the field to be high . With the method of optimum passive numerical shimming, the inhomogeneous field in the imaging volume of the first superconducting NMR imaging magnet produced in China was compensated by the magnetic moments in iron planes, plated with zinc. The homogeneity of the field was improved from 98.18 ppm to 14.93 ppm on a 50 cm DSV, the result was nearly the same with the calculating.