Practical Electromagnetic Modeling Methods

Abstract

Electromagnetic modeling plays an important role in the analysis and design of radiofrequency (RF) coils for magnetic resonance imaging (MRI) applications. In this article, we discuss several practical methods for modeling RF coils and their interactions with the imaging subject. We first define the mathematical boundary-value problem to be analyzed. Then, we describe the process to build an electromagnetic model for the analysis of loaded RF coils. This is followed by the analytical solution of two simplified problems, whose results allow a quick qualitative understanding of critical issues such as field penetration into a dielectric object versus frequency, achievable signal-to-noise ratio at higher frequencies, B1-field inhomogeneity due to the increased displacement current at higher frequencies, level of the specific absorption rate (SAR) due to the dielectric and conduction losses, various effects of an RF shield, and field penetration depth of a surface coil. For a more accurate quantitative analysis of these issues, we describe three numerical methods, which have been most widely used for the modeling of RF coils loaded with a dielectric object. These are the finite-difference time-domain (FDTD) method, finite-element method (FEM), and method of moments (MoM). We describe their basic principles at a higher level so that they can be easily understood, their practical use in the modeling of RF coils, and their advantages and limitations. Potential solutions to overcome the major limitations, such as the use of hybrid techniques, are also also discussed. Keywords: electromagnetic modeling; finite difference time domain; finite-element method; method of moments; magnetic resonance imaging; numerical analysis; radiofrequency coil; radiofrequency field