Monopole Antenna Array with Individual Shields for Ultra High Field Magnetic Resonance Imaging

Abstract

Magnetic resonance imaging (MRI) requires the use of radio frequency (RF) coils for transmission and signal reception. In ultra-high field MRI, i.e., operating at 298 MHz and above, the design and development of RF coils becomes challenging due to a relatively short wavelength, inhomogeneity in the transmit RF field, and interference and coupling between coil elements. Designs which can overcome existing limitations can potentially lead to an improvement in magnetic resonance image quality, due to a stronger and more homogeneous signal across the image. We investigated the potential utility of a shielded monopole antenna array tuned to 298 MHz for 7 T MRI head imaging. We aimed to develop a 298 MHz monopole antenna array with four monopole elements. We considered a number of different shielding arrangements, included single and three sided designs. Using COMSOL simulations, we examined how mutual decoupling can be minimised whilst maximising array sensitivity and RF field homogeneity. We fabricated an unshielded and a shielded prototype four element monopole array and performed bench testing to validate simulation findings and also to empirically evaluate frequency of operation, shielding performance and array sensitivity. The use of shielding monopole array elements was able to improve field homogeneity. Without the use of shielding as much as 10.75% increase in field variability was observed when the field-of-view extended across the entire brain, however when monopole elements were shielded the increase in variation reduced to 0.86%. Shielding also improved the decoupling between individual monopole array elements (−11.72 dB versus −22.38 dB). Field intensity increased on average by 33.77% using the optimised three sided shields on each monopole. Based on our findings, appropriately shielded monopole antenna arrays can potentially produce RF coil sensitivity and field uniformity above the benchmark required for ultra-high field MRI applications. Such arrays may find use in head imaging applications, wherein RF coil insensitivity and field inhomogeneity hinder image interpretation.