Simulation And Analysis Of Irregular Multicoil B₀ Shimming In C-type Permanent Magnets Using Genetic Algorithm And Simulated Annealing

Permanent magnet magnetic resonance apparatus has a wide application prospect due to its low cost.With the development of technology,the magnetic field strength of permanent magnets has been continuously improved,which raises many new demands for the study of permanent magnet magnetic resonance technology,especially in portable detection.Due to the influence of the magnetic block material and the flat plate structure,the magnetic field generated by the permanent magnet is more inhomogeneous,and the spherical harmonic high-order shimming term is more difficult.On the other hand,the inhomogeneity of the magnetic field in the edge region of the permanent magnet is more pronounced than that of the superconducting magnet.Especially when imaging the shoulder joint,the problem of magnetic field inhomogeneity becomes particularly prominent.The MultiCoil(MC)BO shimming,as a new shimming technology developed in recent years,exhibits unique advantages.Due to the BO field distribution characteristics and the structural characteristics of the magnet,it is of great significance to study the MC shimming method of open permanent magnets.Irregularly shaped coils can further enhance the effect of multiple coil shimming,but increase the difficulty and complexity of multi-coil design.Irregular coils may provide an efficient shimming method for permanent magnet MC shimming.This article has completed the following work:Firstly,the shimming theory of MC in open magnets is analyzed in detail,and the method and formula for calculating magnetic field using Biot-Savart Law are extended and deduced.The calculation of optimized current is performed by using non-linear fitting method,Genetic Algorithm(GA),Simulated Annealing Algorithm(SA),and Genetic Algorithm combine with Simulated Annealing Algorithm(GASA)are described in detail.Secondly.The algorithm flow is described,including the calculation of the magnetic field generated by the coil using the Biot-Savart Law,the current fitting,the GASA algorithm,and the overall algorithm flow.This is followed by the process of establishing a coil model and an inhomogeneous field model.Thirdly,the shimming characteristics of MC in C-type permanent magnets are studied.Aiming at improving the efficiency of finding the optimal shape for irregular MC,this paper presents a method that uses genetic algorithm and simulated annealing(GASA)to optimize irregular MC shapes for shimming.By simulating the shimming of the irregular MC of the flat-plate model and shoulder-joint model in C-type permanent magnets,the effects of parameters such as complexity,size,and number of groups of coils are analyzed.The shimming of MC in superconducting magnets and permanent magnets is compared.The results show that for a locally non-uniform BO field in the sample boundary region of a C-type permanent magnet,an irregular multi-coil can achieve a better shimming effect,and the GASA algorithm provides a more stable and highly efficient optimization method.In addition to the coil shape,the structure and size of the coil arrangement also play an important role in shimming.Therefore,multiple parameters such as coil position and size can be set as the optimization target,and even each coil shape can be different.Multi-objective optimization for constraints is a more complex issue that will be one of the focuses of subsequent research.