Simulation Design And Optimization Of The Permanent Magnet For The Small-sized MRI System

In recent years, magnetic resonance imaging (MRI) has become an important medical imaging technique which is widely used in clinical diagnosis and treatment. MRI system is mainly composed of the magnet system, the gradient system, the radio frequency (RF) system and the computer-aided system. As the heart of MRI system, the main magnet is one of the most important and expensive parts, which creates a static magnetic field that determines the final imaging quality. The permanent magnet is usually used in the low field and portable small-sized MRI system. Our research work in this thesis focuses on the simulation design and optimization of the permanent magnet for the small-sized MRI system. There are two different types of the permanent magnet, namely H-type magnet and Halbach magnet respectively considered in this thesis. In summary, the research work of this thesis mainly includes the following three aspects.(1) Design of shimming rings for small-sized permanent MRI magnet using sensitivity analysis and particle swarm optimization algorithmIn this work, sensitivity analysis (SA) and particle swarm optimization algorithm (PSO) algorithms have been successfully used to analyze and optimize the shimming rings in a small-sized permanent MRI magnet system. First of all, the sensitivity of the structure parameters of the shimming rings has been analyzed, and the results of SA indicate that the inner radius of the shimming rings is a crucial factor that can affect the magnetic field quality significantly, and should be carefully considered in the optimization and practical engineering. The height of the shimming ring can be strategically fixed in the optimization process to reduce the computation time and also to increase the optimization accuracy. In addition, PSO is used to solve the nonliner optimization problem, and the PSO results show that the optimized shimming rings can significantly improve the magnetic field uniformity. The proposed method can be effectively used for the production of the high quality permanent MRI magnet systems.(2) Optimization and design of multi-ring pole piece for small-sized permanent MRI magnetA novel configuration of multi-ring pole piece for a small-sized permanent MRI magnet is proposed in this work. In MRI system, one-ring pole pieces of pure-iron are commonly used to improve the field homogeneity in the imaging region. More rings are taken into consideration in the model of a small H-type permanent magnet in this work. The multi-ring constructions were designed and optimized by 3D finite element method (FEM) calculation and PSO algorithm. The results show that the optimal multi-ring pole pieces can effectively improve the uniformity, with a better performance compared with the flat and traditional one-ring pole piece configuration. The proposed novel configuration of multi-ring pole pieces can be effectively performed for various permanent MRI magnet systems. The scope of the theoretical work presented here can provide a reference to the actual magnet design and manufacture.(3) Optimization and design of Halbach magnet for portable MRI deviceIn this work, we have proposed some improved 2D and 3D constructions of the Halbach magnet, which can improve the magnetic field uniformity greatly. Firstly, a Halbach magnet model of the portable NMR device has been built for small animals based on the basic theory. Two improved magnet construtions are proposed based on the 2D model simulation and optimization, and then the model is extended to 3D. We propose some improved constructions of the Halbach magnet with more homogeneous magnetic field than the initial magnet model. At the same time, the weight of these improved magnets is estimated and compared to each other, which is one of the important factors for the portable NMR device in the design.