Design Method Of MRI Head RF Coil Based On Electromagnetic Field Inverse Problem Theory

Hemorrhagic stroke is the main health threat faced by the people of our country.It has the characteristics of rapid onset,rapid change of disease,high disability rate and high fatality rate.Bedside monitoring for Early prehospital diagnosis has always been a requirement for patients.Compared with CT,magnetic resonance imaging technology has the advantages of non-invasive and non-radiation in the clinical diagnosis of stroke medicine.Compared with high-field magnetic resonance,low-field magnetic resonance has the characteristics of small size and light weight,all of these determine that low-field magnetic resonance is very suitable for bedside monitoring.Low-field magnetic resonance equipment is miniaturized and lightweight,but it also causes a sharp drop in magnetic field strength,which in turn weakens the magnetic resonance signal.The function of the RF receiving coil is to receive the magnetic resonance signal.The weak magnetic resonance signal is easy to be interfered by noise,so the performance of the radio frequency coil is very important.This article uses the inverse boundary element method(BEM).Calculating the mapping relationship between the stream function distribution of the coil wiring area and the magnetic field distribution of the imaging target area.According to the target magnetic field distribution,combined with the design requirements of the head radio frequency receiving coil the wire diameter of the radio frequency coil is calculated by solving the inverse problem.This article has formed a complete set of RF coil design methods.After the performance of the coil was verified by simulation,the designed radio frequency receiving coil was produced,and the imaging experiment was completed in the 50 m T ultra-low field magnetic resonance imaging system,and the imaging results were analyzed.The main work of the thesis is as follows:(1)Research on the boundary element design method of RF coil.This article introduces the application of the inverse boundary element method in the design of radio frequency coils.The mapping matrix between the flow function and the target magnetic field is directly established,and the coil structure is calculated from the perspective of the inverse problem according to the required RF field requirements.In the calculation,the regularization parameter and the regularization matrix are introduced the variance between the expected magnetic field and the calculated magnetic field is used as the optimization target,the smoothness of the stream function solution is used as the penalty function,and the weight is adjusted by the regularization parameter to obtain the ideal RF coil structure.(2)Research on the RF coil that locates the imaging target area in the lenticulostriate hemorrhage area.In order to be able to accommodate the heads of all patients,the RF coil is designed to be larger in size.Deviate the imaging target area from the geometric center of the coil and locate it in the lenticular artery hemorrhage area.Focusing the radio frequency energy from the entire brain in this area can enhance the signal of the area and improve the uniformity of the target area.The simulation of the calculated coil confirms the improvement of the uniformity of the target area,and verifies the calculation effect of the boundary element method.(3)Manufacture the designed RF coil.After impedance matching of the coil,the imaging verification experiment is carried out first,and the water model is selected for imaging,and the imaging result of the cross section of the water model sample is obtained.After verifying the performance of the RF coil,a human head imaging experiment was carried out.The imaging results of the inverse boundary element method coil and the solenoid coil were compared,and it was found that the uniformity of the imaging target area has been improved.In the imaging comparison experiment,the uniformity is improved,which proves that the boundary element method is effective in designing special radio frequency coils in the imaging target area.The RF coil designed in this article can be used for imaging in a 2MHz low-field magnetic resonance imaging system,but the resulting image has a bright background,and the signal-to-noise ratio needs to be further enhanced in future research.