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Abstract: Deterioration of radiofrequency (RF) inhomogeneity with increasing static magnetic field in magnetic resonance imaging (MRI) is one of the fundamental challenges preventing their clinical rendition and posing safety hazards. Variation in RF coil designs could help redistribute RF energy absorption over the imaged object. This work is intended to determine experimentally the difference in RF heating produced within a human head phantom by in situ measurement of RF inhomogeneity as a function of coil design utilized at 8 T. The heating patterns of 1/4 wavelength (long) and 1/8 wavelength 11-cm (short) transverse electromagnetic (TEM) coils loaded with a homogeneous human head phantom at 340 MHz were evaluated. In addition, different transmit/receive (T/R) configurations were used in search for the possibility of “hot-spot” formation. Fluoroptic thermometry was used to measure temperatures in multiple positions in a head phantom made of ground turkey breast for RF powers corresponding to a specific absorption rate (SAR) of 4.0 W/kg for 10 min. Numerical simulations were performed to study the general RF power deposition patterns in phantoms at 340 MHz including the effects of field polarization. The temperature increases varied from 0 to 0.8°C for the long RF coil, while the short RF coil produced a maximum temperature change of 0.5°C. Similar to ultra high-field electromagnetic simulations, these measurements revealed low peripheral and high deep-tissue heating at 8 T. The findings indicated that the largest temperature changes for both cases were less than 1°C. While these results showed an increase in localized heating due to RF pulses at 8 T, they highlight that RF inhomogeneity could be redistributed using different RF coil designs through which the hot spots could be made cooler. [Copyright &y& Elsevier]

Abstract: While magnetic resonance images essentially contain a map of the both circularly polarized components of the RF transverse magnetic fields (B1 field), the thermal heat and electromagnetic power deposition is generated by the associated electric fields. Measurement of electric field distributions/intensities across a sample yields an indirect indication of possible cause of heating within the sample and potentially enables the detection of “hot spots,” which can be present within inhomogeneous radiofrequency (RF) fields, such as the case with magnetic resonance imaging at high field strength. As a result, establishing a valid technique for direct measurements of the electric field and its correlation, obtained using computational electromagnetics, is essential in assessing (1) the safety of the RF coil designs and (2) the validity of the calculations. In this work, a probe was built and used to measure the transverse electric field (E1 field) distributions within an empty 8 T (tuned to 340 MHz) RF head coil and within a saline water phantom loaded in the same coil. The measured E1 field distributions were favorably compared to the distributions obtained utilizing a finite difference time domain in-house package. [Copyright &y& Elsevier]

A four-channel Tic-Tac-Toe (TTT) transmit RF coil was designed and constructed for foot and ankle imaging at 7T MRI. Numerical simulations using an in-house developed FDTD package and experimental analyses using a homogenous phantom show an excellent agreement in terms of B 1 + field distribution and s-parameters. Simulations performed on an anatomically detailed human lower leg model demonstrated an B 1 + field distribution with a coefficient of variation (CV) of 23.9%/15.6%/28.8% and average B 1 + of 0.33 μT/0.56 μT/0.43 μT for 1 W input power (i.e., 0.25 W per channel) in the ankle/calcaneus/mid foot respectively. In-vivo B 1 + mapping shows an average B 1 + of 0.29 μT over the entire foot/ankle. This newly developed RF coil also presents acceptable levels of average SAR (0.07 W/kg for 10 g per 1 W of input power) and peak SAR (0.34 W/kg for 10 g per 1 W of input power) over the whole lower leg. Preliminary in-vivo images in the foot/ankle were acquired using the T2-DESS MRI sequence without the use of a dedicated receive-only array. [ABSTRACT FROM AUTHOR]