Designs Of Novel Permanent Magnet And Bi-planar Shielded Longitudinal Gradient Coils

MRI system is an important applied direction of NUMR. In the past several years, MRI technology of permanent magnet typed system is developed tremendous quickly. Both the permanent magnet and gradient system are two pivotal components in the permanent typed MRI system. Nowadays, multi-functional MRI requires relevant magnet with higher magnetic field and stronger gradient system, however, the much stronger magnetic field, the more complicated design difficulty. The main purpose of this thesis is to design both a novel C-typed permanent magnet with 0.4Tesla amplitude's magnetic field in its air-gap and a pair of longitudinal shielded bi-planar gradient coils used in the MRI system. This thesis consists of two parts .The first part begins with the general structure of permanent magnet to establish both the physical model of such magnet and optimized objective function due to the performances to be designed. The simulation is finished with the electromagnetic module and optimization module in ANSYS. Based on the result, some important concepts, such as suitable working point of NdFeB and the sensitive influences to gap magnetic field from the ferromagnetic pole-piece and shim ring, are discussed. Based on the analysis, optimized design pattern is obtained after the modification. Whether the optimizing procedure is continued in three-dimensional step is determined by numerical computation of the 3D magnet converted from the two-dimensional magnet mentioned above. The design work of gradient coils, which belongs to the domain of inverse electromagnetism problem, is a difficult problem in the open MRI system of permanent type. The second part of this thesis combines discrete lead design method with distributed current density methodology to propose a new concept to design gradient coil. This new method arises from the transformation from line current carried in discrete leads to continuously distributed current density using Dirac function. The complete expression of magnetic field stimulated by shielded bi-planar gradient coil is obtained based on the matured target field together with the concept of distributed current density, and relevant routines are also provided by way of improved genetic algorithm. Proved by the experiment of a real prototype, this design method for magnet proposed in this thesis is successful in the design work of 0.4Tesla magnet, and more important, a series of magnets from 0.23 Tesla to 0.35 Tesla can also be designed with this technique. The tested result of gradient coils indicates that the arrangement of gradient magnetic field stimulated by the designed gradient coils shows a good agreement with ideal field, which proves the correctness of the new design method.