Metamaterial Liner for MRI Excitation—Part 1: Theory, Modeling and Design

Abstract

This work details the practical and theoretical concepts underlying the design of a metamaterial (MM) liner for magnetic resonance imaging (MRI) radio frequency (RF) excitation. The Part 2 companion to this paper details the practical design and performance comparison to a conventional birdcage coil. The operation of the MM liner relies on specifically engineered effective permittivity and permeability of a thin liner on the inner surface of the MRI bore, to enable the propagation of electromagnetic (EM) waves at frequencies below the natural-cutoff of the bore itself. Here, the effective anisotropic EM properties of the MM liner structure are calculated by a combined circuit-and effective-medium model, and the analytical dispersion characteristics are compared to full-wave eigenmode simulations. The results of the effective medium model representation of the MM liner agree fundamentally with simulation. The model may thus assist the design and tuning of the liner to achieve a standing-wave resonance with field distribution and mode spacing suitable for MRI RF excitation. The transmission properties and EM fields of a full-scale MM lined bore for B 0 = 4.7 T MRI are simulated including realistic losses. These methods enable robust design of thin MM liners for arbitrary bore sizes and B 0 field strengths.

Metamaterial Liner for MRI Excitation—Part 2: Design and Performance at 4.7T

“…In this study, we have developed a robust methodology for designing and evaluating the MM liner as a whole-body transmit MRI coil. This follows the introduction of the theoretical underpinnings of the MM liner in the Part 1 companion paper [2]. Our main findings are as follows.…”

“…In MM liner designs such as that presented in Ref. [23], or the MM liner presented in Part 1 [2], there may be no conductive traces between rings, and the coupling between rings will be only due to C zd and the mutual inductance between the azimuthally directed currents on the rings.…”

“…The transverse magnetic field for the 1 st -order transverse electric (TE) mode of a circular waveguide is largely homogeneous, while producing electric fields that are also spread out more uniformly than conventional MRI coils, and therefore may produce lower local SAR for the same B 1 . In the Part 1 companion paper to this work [2] the design of a metamaterial (MM) liner…”

“…A fully anisotropic MM liner was produced VOLUME 10, 2022 using periodically reactively loaded and axially stacked rings, and was found to also improve Tx performance [23]. In the Part 1 companion paper to this work [2], an accurate effective medium model was presented for the MM liner and a method of design for a MM lined MRI bore was introduced. This work (Part 2) explores the practical design and implementation of the MM liner as an MRI coil.…”