Shimming Method And Optimization Design Of Main Magnet For Open-type Magnetic Resonance Imaging

As medical Magnetic Resonance Imaging (MRI) are harmless to patients when detecting the human body, it is more and more widely used in medical diagnosis, and has become one of the most important imaging devices. With the increasing demand for MRI devices, its imaging quality receives much concern. According to imaging principles of MRI, the imaging quality is closely related to magnetic field strength and magnetic field homogeneity of the main magnetic field.In this paper, the method of improving magnetic field homogeneity are studied for two type main magnets of open-type MRI. For open permanent MRI system, in the condition of no changes to the magnet structure, small shims are placed on the magnet planes to correct the magnetic field in imaging space, to achieve the purpose of improving magnetic field homogeneity, that is passive shimming. Based on integer linear programming of operations research and the central shimming idea obtained from actual shimming process, a mathematical model of passive shimming is proposed in this paper. Simplex method and branch and bound program is compiled in FORTRAN to solve the model, and the number of shims at special position on the pole faces is obtained. The correctness of mathematical model needs to be verified. In order to save resources, commercial software ANSOFT is used to verify the shimming scheme. According to shimming scheme, the ANSOFT model of shims array is established, and then the magnetic field of imaging space produced by the shims is calculated. By comparing the magnetic field distribution in imaging space before and after shimming, we can know the correctness of the mathematical model. Simulation research is done on two different double-column type magnets. The magnetic field inhomogeneity of the two magnets reduce significantly, which proves the correctness and validity of the model.For open superconducting MRI, the magnetic field homogeneity in imaging space is improved by optimizing the magnet structure, meanwhile, the magnetic field strength is ensured. Preliminary research of optimization and design for a 1.5T open superconducting MRI main magnet is carried out in this paper. Without considering special optimization algorithm, based on the basic theory of electromagnetic and magnetic circuit theory, the magnet is designed with several schemes using commercial software ANSOFT to find optimization direction for improving the magnetic field homogeneity in imaging space. The variable parameters for design schemes are as following:core length, yoke position, pole face shape and coil current distribution. In this paper, several single and mixed design schemes are simulated and analyzed, and the results provide some research foudation and direction for open superconducting MRI magnet design.