Simulation Comparison of Birdcage Coil and Metamaterial Liner for MRI at 3T and 4.7T

Abstract

The application of a thin metamaterial (MTM) liner to the RF shield of the magnetic resonance imaging (MRI) system is proposed to improve whole-body imaging compared to conventional "birdcage" body volume coils. The MTM liner facilitates travelling wave (TW) MRI, where the radio-frequency (RF) magnetic field propagates within the MRI bore, which acts as circular waveguide. Full-wave electromagnetic simulations were performed comparing a conventional birdcage coil to a novel MTM liner design in terms of key MR RF performance metrics: transmit efficiency, homogeneity and specific absorption rate (SAR). The designs are compared at 3T (128 MHz) and 4.7T (200MHz), two high-field strengths where the RF field homogeneity and SAR constraints are critical. The ring-structure design of the MTM liner is described and simulations with an empty MRI bore and a homogeneous human body model are performed, including realistic losses for the lumped components used in practical construction. The results show that the MTM liner has a similar mean transmit efficiency, while having reduced 10g averaged local SAR compared to the conventional birdcage.