Comparison of new element designs for combined RF-shim arrays at 7 T

Abstract

In this work we identify novel concepts for RF-shim loop architectures suitable for 7 T made of 2 concentric conducting loops fulfilling RF and DC functions, respectively, and to determine their relative SNR performance. The goal is to minimize interference between the 2 systems while making efficient use of the space closest to the body. We show by means of theoretical derivation of the frequency spectrum that the proposed 2-loop structure exhibits an anti-resonant null and a resonant peak in the frequency domain. The proposed structure is comprised of 2 concentric wire loops either arranged as nested loops or in the form of a coaxial cable, in which the 2 conductors carry the RF and shim signals, respectively. We use theory, simulation, and phantom measurements to obtain frequency spectra and SNR maps for the proposed structures. Retained SNR is found to be 75% in the coaxial loop and ranges from 57% to 67% in 3 different coaxial configurations. We have found both implementations to be a viable concept for the use in RF-shim devices if remaining SNR limitations can be overcome. We have investigated 2 new design modalities in 7 T RF-shim coil design that separate the RF and shim conductors such that the previously proposed toroidal chokes are eliminated—thereby removing undesired additional loss, bulk, and design complexity.