Dyadic Green Function/MoM based Method for the Analysis of Electromagnetic Fields inside a Lossy, Multilayered Spherical Head Phantom Excited By RF Coil

F. Liu, S. Crozier, B. Xu, I. Gregg School of Information Technology and Electrical Engineering, Brisbane, Queensland, Australia Synopsis The precise evaluation of RF coil-tissue interactions and the electromagnetic fields (EMFs) inside biological samples is becoming an increasingly critical design requirement for high-field MRI engineering. Suitable phantom-based analysis is essential to coil design and evaluation. This paper presents an analysis model of RF coils in the presence of a multi-layered lossy sphere, roughly resembling the human head. The advantage of using this construct is that the fields can be calculated relatively quickly while maintaining realistic overall loading. In this model, the EMFs are handled by the dyadic Green’s function (DGF) and the coil currents are evaluated by method of moments (MoM). Test examples show the capability of the proposed model. Introduction In high-frequency RF resonator design and analysis, a problem that has been of significant concern is electromagnetic interactions (EMI) with the human head. To investigate the safety issue and the field characteristics, a number of invaluable head models/phantoms have been developed [1-4]. As the human head is more like a stratified spheral structure, a multilayered spherical model might effective in addressing this EMI problem while maintaining computational speed. We therefore introduce a generic spherical model which can be divided to be an arbitrary number of concentric spherical layers each with different electrical properties. This method accounts for the coupling between the coils and head phantom, which is essential in near field analyses. After the EMFs are calculated, various imaging or antenna parameters can be straightforwardly investigated. Dyadic Green’s Function (DGF) /MoM Based Solutions for Arbitrarily shaped/positioned RF coils The EMFs inside a layered spherical head phantom as shown in Fig.1 are modelled using the DGF technique. The DGF is a dyad that relates a vector field to a vector current source [5], and is constructed on spherical wave functions that satisfy vector wave functions in spherical coordinates. In this work, all the DGFs refer, by example, to the electric-fields. The magnetic fields can be determined by invoking duality. According to the principle of superposition, the EMFs inside and outside the layered sphere can all be