Optimal-permittivity Dielectric Liners for a 4.7T Transceiver Array

Placing dielectric pads adjacent to the imaging region is an effective method to increase the signal locally and also increase the radio frequency magnetic field homogeneity in magnetic resonance imaging. The use of local high permittivity pads is becoming more common, and this work focuses on the effect of larger dielectric pads on the transmit/receive performance of an array (e.g., coupling, efficiency and safety) having 8 channels, used to image a cylindrical phantom at 4.7T (200MHz). We investigate the effects of a dielectric liner surrounding the whole volume of interest both with and without an air gap. The simulations reveal that high permittivities are not recommended because they substantially degrade the longitudinal homogeneity, resulting in hot spots of specific absorption rate at the driven end of the array. Furthermore, high permittivities lead to dielectric resonances in the liner at frequencies close to the Larmor frequency, potentially degrading the performance of the array. Indeed, simulations and measurements confirm that a compromise must be made between improvements in field homogeneity and transmit performance, and that an optimal permittivity exists which is much lower than those commonly used in the literature. The optimal permittivity achieves minimal coupling (<-23dB) between array elements, exhibits an intrinsic electromagnetic impedance equal to the geometric mean of those of the coil former and phantom and can be realized with inexpensive materials. For this permittivity the performance with an air gap of thickness equal to that of the liner is equivalent to that without the air gap.