In-depth study of the electromagnetics of ultrahigh-field MRI

In this work, numerical and experimental studies of the transverse electromagnetic (TEM) resonator modes at ultrahigh-field (UHF) MRI are performed using an in-house finite difference time domain package at 340 MHz and using an 8 T whole-body MRI system. The simulations utilized anatomically detailed human head mesh and a spherical head-sized phantom, while the experiments included an electromagnetically equivalent (to simulations) phantom and in vivo human head studies. An in-depth look at the homogeneity of the transmit-and-receive fields and local and global polarization of the electromagnetic waves inside the cavity of the head coil, and also the current distribution obtained on the resonator elements, is provided for several coil modes when the coil is empty and loaded. Based on the numerical and experimental results, which are in excellent agreement, an electromagnetic characterization of loading radio-frequency (RF) head coils during a UHF MRI experiment is provided. The possibility of using the aforementioned modes for specific types of imaging application is briefly reviewed.