Metamaterial-Inspired Radiofrequency (RF) Shield With Reduced Specific Absorption Rate (SAR) and Improved Transmit Efficiency for UHF MRI

To prevent the interferences between radiofrequency (RF) coils and other components in the magnetic resonance imaging (MRI) system such as gradient coils, it is essential to place an RF shield between the RF coils and gradient coils. However, the induced currents on conventional RF shields have negative influences on the RF coil performance. To reduce these influences, metamaterial absorbers (MA), a class of metamaterials exhibiting nearly unity absorption rate for the incident electromagnetic fields, can be employed for the design of a novel RF shield. However, the adoption of metamaterials in MRI systems is usually problematic because of the bulkiness of the metamaterial structure. In this work, capacitors and metallic interconnectors are used to miniaturize the MA so that the unit MA cell can operate at the Larmor frequencies of 7T and 9.4T MRI and stay compact. This MA-RF shield is used to improve the transmit efficiency of RF surface coils and reduce the specific absorption rate (SAR) in the region of interest (ROI). It is successfully demonstrated by simulations and experiments that, compared with conventional RF shield structure, the transmit efficiency can be enhanced by more than 32% and the peak SAR value can be reduced by 22% using the MA-RF shield. Moreover, it is observed that the transmit field penetration is improved when the surface coil is used with the MA-RF shield. This proof-of-concept study suggests a new practical way for the utilization of metamaterials in ultra-high field MRI applications.

Automated summary

In this study, capacitors and metallic interconnectors were used to miniaturize metamaterial absorbers (MA) for use as a novel RF shield in MRI systems. The MA-RF shield significantly improved transmit efficiency of RF surface coils and reduced specific absorption rate (SAR) compared to conventional RF shields. The proof-of-concept study demonstrated an innovative practical way for utilizing metamaterials in ultra-high field MRI applications.