Improving B1 homogeneity in abdominal imaging at 3 T with light and compact metasurface

TL;DR

This paper introduces a lightweight, flexible metasurface that enhances B1 homogeneity in abdominal MRI at 3 T, matching the performance of traditional dielectric pads while being more practical and cost-effective.

Contribution

The study presents a novel, compact metasurface design that improves RF homogeneity in abdominal MRI, offering an alternative to bulky dielectric pads with comparable effectiveness.

Findings

Metasurface removes B1+ dark voids in simulations.

In vivo results show similar homogeneity improvements as dielectric pads.

Metasurface is stable across different body mass indices.

Abstract

Radiofrequency field inhomogeneity is a significant issue in imaging large fields of view in high- and ultrahigh-field MRI. Passive shimming with coupled coils or dielectric pads is the most common approach at 3 T. We introduce and test light and compact metasurface, providing the same homogeneity improvement in clinical abdominal imaging at 3 T as a conventional dielectric pad. The metasurface comprising a periodic structure of copper strips and parallel-plate capacitive elements printed on a flexible polyimide substrate supports propagation of slow electromagnetic waves similar to a high-permittivity slab. We compare the metasurface operating inside a transmit body birdcage coil to the state-of-the-art pad by numerical simulations and in vivo study on healthy volunteers. Numerical simulations with different body models show that the local minimum of B1+ causing a dark void in the abdominal domain is removed by the metasurface with comparable resulting homogeneity as for the pad without noticeable SAR change. In vivo results confirm similar homogeneity improvement and demonstrate the stability to body mass index. The light, flexible, and cheap metasurface can replace a relatively heavy and expensive pad based on the aqueous suspension of barium titanate in abdominal imaging at 3 T.