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 $\text{B}-{0}=4.7$ T MRI are simulated including realistic losses. These methods enable robust design of thin MM liners for arbitrary bore sizes and B0 field strengths.