Design Of 3.0T Cranial MRI Active Shimming

Cerebrovascular disease is the primary factor that endangers the lives of our people,and its main detection method is Magnetic Resonance Imaging.Imaging.In order to improve imaging quality and achieve precision medicine,it is necessary to carry out specialized research and development of cranial Magnetic Resonance Imaging.Compared with the whole body type,cranial Magnetic Resonance Imaging has better magnetic field uniformity and smaller structure,which can greatly improve the imaging quality and reduce the cost of production and use.The magnetic field of the magnet is affected by the production technology and environment,and the uniformity of the magnetic field is poor,so it needs to be shimmed.Shimming is divided into passive shimming and active shimming.Passive shimming can only be roughly shimmed due to its own conditions,while active shimming can be finely corrected.There have been many researches on active shimming,but most of them are limited to less than 1.5T,and the magnet structure is mostly biplane structure and long cylindrical structure,which cannot be applied to the short cylindrical structure of cranial Magnetic Resonance Imaging.Based on the existing research at home and abroad,this paper studies the active shimming of 3.0T cranial Magnetic Resonance Imaging.Through the research on the structure of cranial Magnetic Resonance Imaging magnet and the improvement of the active shimming system,the purpose of improving the magnetic field uniformity of cranial Magnetic Resonance Imaging is achieved.This paper firstly studies the principle and shimming process of Magnetic Resonance Imaging.Point out the structural constraints of the active shimming system in the design process and the shimming target and shimming process of active shimming.This paper then proceeded with the design of the 3.0T cranial Magnetic Resonance Imaging main magnet to determine the structural constraints of the shimming system.Through the analysis of human brain size and patient comfort,the structural constraints of the main magnet are determined,and then genetic algorithms are used to optimize calculations to design a main magnet structure suitable for cranial Magnetic Resonance Imaging.Through the simulation of the magnetic field model,the feasibility of the main magnet scheme is verified,and the boundary constraint parameters are provided for the design of the active shimming system.In this paper,the magnetic field in the imaging space area is measured and harmonically analyzed.Through the transformation and processing of the magnetic field measurement data,a fitting function that can be used to describe the magnetic field is obtained;through harmonic analysis,the correction of the magnetic field is transformed into the filtering of the magnetic field,and the target of active shimming is also transformed into a harmonic magnetic field.Then the objective function of active shimming is determined.Based on the above constraints and objective function research,this paper designs an active shimming system.Through the integration of constraint conditions and objective function,a mathematical model of the active shim coil is obtained.By solving the mathematical model,the design parameters of the shim coil are obtained.The magnetic field simulation verification is carried out according to the design parameters.From the simulation results It can be seen that the active shimming system designed in this paper has good shimming performance and can greatly improve the uniformity of the magnetic field.