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

Abstract

The theoretical foundations of a metamaterial (MM) liner for the MRI bore that facilitates the propagation of reduced-cutoff cylindrical waveguide modes are presented in the Part 1 companion paper to this work. Here, in Part 2, the practical design and modelling of the novel MM liner is applied to a body MRI radio-frequency (RF) transmitter for 4.7T. An equivalent network mesh model is developed to reduce the distributed structure of the MM to one that uses lumped discrete tuning elements. The close match between full-wave simulation and the network model is demonstrated by comparison of the longitudinal propagation dispersion curves and input impedance. The application of the methods developed for analysis of the MM liner behavior are demonstrated in the design of a practical MM liner as an effective MRI RF transmitter. The liner's simulated performance was evaluated using three key metrics of MR radio-frequency (RF) coil performance: transmit efficiency, the variation of the transmit field (a measure of homogeneity), and the 10g averaged local specific absorption rate (SAR). These metrics are compared to those of the commonly used birdcage (BC) coil. The main advantage of the MM liner is the 29.6% reduction in maximum SAR normalized to transmit field relative to the BC coil, with comparable transmit efficiency and homogeneity. Thus, the liner can potentially replace the BC coil as an RF transmitter and provide an enhanced safety margin, or allow the use of faster, more SAR-aggressive imaging sequences.

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

“…While the models are valuable design tools, full-wave simulation (Section IV) is still required to determine the MRI performance metrics as well as to verify the final design. Comparison of the MM liner approach to conventional methods of MRI excitation by a birdcage coil and evaluating the impact of realistic body loading is the focus of the Part 2 companion paper [50].…”

“…7 (c-d) is due to this interaction, particularly with the ENNZ case where the C φ capacitance is small and therefore closer in magnitude to the z-directed stray capacitance. If the capacitance between rings is increased (e.g., reducing spacing or intentionally adding electrical connections) it would be necessary to include a more complicated EMM model, or to account for coupling between rings and longitudinal currents with a full network model of the MM liner as presented in the Part 2 companion paper [50]. An effective medium model is not included in the Part 2 companion paper, and the dispersion is derived differently.…”

“…The objectives of this study, along with the companion paper [50], are to 1) outline a comprehensive method that can be used to design the MM liner with EMM for any field strength (1.5T, 3T, 4.7T, 7T etc. ); 2) demonstrate whether the MM liner can provide a homogeneous transmit field suitable for MRI; and 3) enable thoracic and abdominal imaging studies 4.7T with a practical whole-body transmit resonator design.…”