多层球状头模型中MRI线圈激励下射频场的求解——并矢格林函数/矩量法

介绍一种磁共振成像系统(MRI)线圈激励下的人体头部电磁场计算方法.先构造一个与人头实际尺寸相当的多层球状头模型，并赋以实测电导率和介电常数，然后基于并矢格林函数(DGF)来求解MRI线圈激励下的头模型内部电磁场.只要射频线圈的激励电流源分布可由矩量法(MoM)求出，都可用该法计算由其激发的电磁场，由此计算出来的电磁场分布可以用于MRI成像参数的确定.结果表明，该方法比有限时域差分法(FDTD)的计算速度提高了10倍以上.仿真试验结果显示，人头中的磁场和电场受频率高低影响，随着频率的增高，磁场与人体组织的相互作用越大，人头中的磁场越不均匀，从而造成了磁共振信号的失真.所提出的DGF/MoM法可用于高场MRI的线圈优化设计，例如射频线圈(表面线圈和体线圈）的快速评价以及高性能的梯度线圈的快速优化设计.

On evaluation of MRI coil excited radio-frequency fields inside multilayered spherical head phantom——DyadicGreen''''s function/MoM method

This work presented a method for computation of magnetic resonance imaging(MRI) coil excited electromagnetic fields(EMFs) inside the human head.A head-sized,multilayered dielectric spherical head phantom with measured radial conductivity and permittivity profiles was constructed,and then DyadicGreen's function(DGF)-based method was nsed to calculate the EMFs inside the head phantom.This solution is suitable for calculation of EMFs excited by any radio-frequency(RF) resonators whose current distributions can be calculated by method of moment(MoM).The calculated EMFs can then be used to deduce MRI imaging parameters.Results show that the computation time of the proposed method is much lower than that of a full finite difference time domain(FDTD) calculation using a complex head model,and the computation speed is increased by more than ten-fold.Simulation demonstrates that the magnetic and electric fields in human head are affected by the frequency,and that field/tissue interactions increase with the frequency increases,and that the inhomogeneity of the magnetic field in human head increases,thus resulting in the distortion of MR signals.The proposed DGF/MoM method can be used for high field MRI coil design,for example,the rapid evaluation of RF resonator(surface and volume coils) and high performance gradient system designs.