Liu Feng, Crozier Stuart
School of Information Technology and Electrical Engineering, The University of Queensland, St Lucia, Brisbane, Queensland 4072, Australia.
Phys Med Biol. 2004 May 21;49(10):1835-51. doi: 10.1088/0031-9155/49/10/001.
The precise evaluation of electromagnetic field (EMF) distributions inside biological samples is becoming an increasingly important design requirement for high field MRI systems. In evaluating the induced fields caused by magnetic field gradients and RF transmitter coils, a multilayered dielectric spherical head model is proposed to provide a better understanding of electromagnetic interactions when compared to a traditional homogeneous head phantom. This paper presents Debye potential (DP) and Dyadic Green's function (DGF)-based solutions of the EMFs inside a head-sized, stratified sphere with similar radial conductivity and permittivity profiles as a human head. The DP approach is formulated for the symmetric case in which the source is a circular loop carrying a harmonic-formed current over a wide frequency range. The DGF method is developed for generic cases in which the source may be any kind of RF coil whose current distribution can be evaluated using the method of moments. The calculated EMFs can then be used to deduce MRI imaging parameters. The proposed methods, while not representing the full complexity of a head model, offer advantages in rapid prototyping as the computation times are much lower than a full finite difference time domain calculation using a complex head model. Test examples demonstrate the capability of the proposed models/methods. It is anticipated that this model will be of particular value for high field MRI applications, especially the rapid evaluation of RF resonator (surface and volume coils) and high performance gradient set designs.
精确评估生物样本内部的电磁场（EMF）分布，正日益成为高场磁共振成像（MRI）系统一项重要的设计要求。在评估由磁场梯度和射频发射线圈所引起的感应场时，提出了一种多层介质球形头部模型，与传统的均匀头部模型相比，该模型能更好地理解电磁相互作用。本文给出了基于德拜势（DP）和并矢格林函数（DGF）的、在一个头部尺寸大小的分层球体内部的电磁场解决方案，该分层球体具有与人类头部相似的径向电导率和介电常数分布。DP方法是针对对称情况制定的，其中源是一个在很宽频率范围内承载谐波形式电流的圆形回路。DGF方法是针对一般情况开发的，其中源可以是任何一种射频线圈，其电流分布可使用矩量法进行评估。然后，计算出的电磁场可用于推导MRI成像参数。所提出的方法虽然没有体现头部模型的全部复杂性，但在快速原型制作方面具有优势，因为计算时间比使用复杂头部模型进行完整的时域有限差分计算要低得多。测试示例展示了所提出的模型/方法的能力。预计该模型对于高场MRI应用将具有特别的价值，尤其是在射频谐振器（表面和体线圈）的快速评估以及高性能梯度集设计方面。
