The Passive Shimming Technique Research For A Special Application Of The Permanent Magnet

In order to match imaging standard, it's required that to bring main magnetic field homogeneity of magnetic resonance imaging (MRI) apparatus to an acceptable level. However, even for an ideal design, magnetic field homogeneity doesn't reach expected level after magnet assemblies. Hence, special methods are needed to improve the homogeneity, that's shimming. At present, two common methods have been used to shim: active shimming and passive shimming. The basic theory for shimming is based on spherical coordinate analytic method. It brings homogeneity of field to match imaging standard by counteracting unwanted spherical harmonic component.At beginning, the thesis introduces domestic and foreign shimming development, analyze the existing shimming method, and probe into the possible method. Then key questions, difficulties and main content of this research are presented.Secondly, in the process of study shimming methods of permanent magnet for open MRI system, active shimming is unsuitable due to its specialties. If common methods based on spherical harmonic function were adopted, nonlinear equations will be solved at last. Hence a high efficient method is needed. In this paper, a passive shimming method based on integer programming and basic theories of shimming is developed, and build up a shimming mathematic model that is solved by using LINGO8.0 and MATLAB6.5. The results are displayed in the form of graphics.Thirdly, the experimental verification is performed in order to test correctness of the model and results. Shimming experiments are performed in queen-pole magnet and traditional C-shape magnet, and it turned out that the trend of the model is reasonable. In the process of measure, a method based on harmonic function was adopted to obtain mass experiment data which are foundation for the research. Detailed experimental errors and some defects are analyzed in this thesis. Last, the summary which including some weakness and questions need to be solved is given about the whole paper and suggestions for the further research are proposed.