# Comparison of B1+ and SAR efficiency for a high-impedance metamaterial shield with different remote RF arrays at 7 T MRI: A simulation study

## TL;DR

This study uses simulations to evaluate how high-impedance metamaterial shields affect MRI performance at 7 T, focusing on transmit field and safety.

## Contribution

The paper introduces a simulation-based analysis of metamaterial shields for 7 T MRI, comparing B1+ and SAR efficiency across different impedances and antenna designs.

## Key findings

- Higher impedance values (above 1000 Ω) improved SAR efficiency and enabled higher-order wave modes.
- Intermediate impedances (10⁻2–103 Ω) caused significant losses and potential heating issues.
- Fractionated dipole arrays with inductances showed increased SAR efficiency and a larger field of view.

## Abstract

This study explores high-impedance surface (HIS) metamaterial shields for enhancing the transmit field in whole-body MRI at 7 T. We studied the possibility of placing a metamaterial layer between the gradient coil and bore liner using electromagnetic simulations to evaluate B1+ and SAR efficiency across different impedances.

Simulations were performed in three stages, first metamaterial design and characterization, then single-element dipole simulations with a homogenous phantom, and finally, simulations including a four-element arrays with a virtual body model, including the whole scanner geometry. Four antenna types were evaluated for B1+ and SAR efficiency.

Due to space constraints the metamaterial does not reach high enough impedance, resulting in minimal performance gains for most antennas. However, fractionated dipole arrays with inductances showed increased SAR efficiency and a larger field of view. Higher impedance values (above 1000 Ω) reduced losses and enabled higher-order wave modes, improving efficiency. Intermediate impedances (10⁻2–103 Ω) introduced significant losses, potentially causing heating and detuning.

HIS metamaterials can enhance transmit performance in 7 T MRI but require careful optimization of impedance, material losses, and antenna design. These factors must be considered to ensure both efficacy and safety in ultra-high-field applications.