RF magnetic field, specific energy absorption rate, and signal to noise ratio in MRI experiments and numerical calculations with finite difference time domain method /

by Liu, Wanzhan.

iii When MRI moves towards higher fields for higher signal to noise ratio (SNR), one of the problems is that the complicated interaction between the radiofrequency (RF) field and the biological tissues degrades the performance of the system. The finite difference time domain (FDTD) numerical method for electromagnetism, verified by experiments, is a valuable tool to study the RF field in high field MRI. The RF magnetic (B1) field distribution and the SNR for different end-ring/shield configurations in birdcage-type RF coils are examined numerically at 64 and 125 MHz and experimentally at 125 MHz. With a previously developed male body model, a new anatomically accurate female body model is created to study B1 field distribution, SNR, and specific energy absorption rate (SAR) in different body types at 64 MHz and 128 MHz. The RF radiation loss, which is associated with SNR and SAR, in a surface coil, in a head size birdcage coil, and in a head size TEM coil loaded with phantoms at a frequency range from 64 MHz to 600 MHz is also evaluated numerically. It is found that a) the end-ring/shield configuration in a birdcage coil affects B1 homogeneity and SNR in a head, b) loading a larger, more muscular subject results in significantly less homogeneous + B 1 distribution, lower SNR, higher SAR levels, and c) the radiation becomes significant at high fields and the interaction between the RF field and the dielectric material in the sample helps to reduce the radiation. These results provide useful information for RF design and MRI safety guideline at high fields.