Study of Shimming Method Based on the Central Magnetic Field for MRI Permanent Main Magnet

　　Study of Shimming Method Based on the Central Magnetic Field for MRI Permanent Main Magnet
　　The research of this thesis is supported by the Doctoral Program Foundation of Chinese Higher Education Institutions, Ministry of Education of China—Study of the shimming technique of the permanent magnet for open magnetic resonance imaging system(under Grant 20050142003).The system of open MRI magnet is one of the significant development aspects of medical MRI device at the present time. Its image quality is closely related with the shimming technique. The http://www.999magnet.com/products/108-alnico-magnets passive shimming with permanent MRI magnet is still a hot and difficult investigation problem. In this thesis, the research on shimming problem for open permanent MRI magnet has been presented in detail. The main work is as follows.The way of passive shimming is to shim the magnet passively, using pieces of the permanent magnetic material placed at specified locations on the pole surfaces to bring an additional magnetic field which corrects the initial field. Therefore, the numeric analysis method, analytic solving and practical measuring are presented to determine the additional magnetic field created by a single shimming sheet in this paper, which lays a foundation on the passive alnico magnets shimming process.An improved method for allowing an MRI magnet to meet its demand to the field homogeneity has been developed. The method is denoted as central shimming based on the theory of integer linear programming, which involves procedure of practical shimming and experience from shimming engineers.
　　The method proposed in the paper uses the value of central magnetic field in the imaging region to approximately correct the magnetic field of sampling points. After building the passive shimming model, the branch and bound method is adopted to optimize the shim locations and the number of shims at each allowable location which will bring the magnetic field uniformities to an acceptable level. Firstly, the numerical analysis method is proposed to compute the distribution of the magnetic field in the imaging region, which created by the shim arrays on the pole surface. Secondly, the related software is programmed to fulfill shimming steps. Thirdly, shimming is repeated based on the magnetic field corrected in the second step. The theoretical shimming scheme is obtained through combining process of the coarse and fine shimming, which is completed from outer rings to inner rings on the pole surface. In order to check the precision of the shimming algorithm proposed in this paper, shimming experiment of the queen-post permanent magnet is completed in the enterprise that produces the magnet used for MRI. According to the results comparison of measuring and computing, the validity of the method is testified finally. 更多还原