Optimal Excitation of Single Mode Resonators: Demonstration with a 3t MRI Metasolenoid

Abstract

Wireless passive resonators have been developed to inductively couple to the birdcage body coil improving coil sensitivity and MRI image quality. Such wireless systems have been explored in the form of ceramic resonators with extremely high relative permittivity but also with metamaterial or metasurface devices in which a dense array of resonant structures can exhibit resonant behaviour at 1.5T or 3T proton Larmor frequency. The resonant amplification of the birdcage radiofrequency field is used to lower the input power during RF transmission and improve the RF sensitivity during signal reception. The main drawback is that such a gain is only obtained in a limited volume located within or close to the resonant structure. Nonetheless, these resonators have seen recent development and interest with applications to wrist or breast MRI mostly in 1.5T MRI scanners. Here we propose to design, build and study a passive metasolenoid resonator operating at 3T. To our knowledge, this structure has not yet been validated experimentally at such a high B0 field. The metasolenoid was characterized with 3T MRI measurements in a phantom with the aim of validating its impact on transmit efficiency and SNR with respect to the birdcage polarization excitation. We reported a very high transmit efficiency gain for circularly (3.9) and linearly polarized excitation (5.8). We confirmed that the metasolenoid specifically amplified the vertical component of the radiofrequency magnetic field produced by the birdcage. Consequently, according to analytical calculations, we demonstrated that when combined to a linearly polarized excitation, the metasolenoid had a transmit efficiency 26% higher than when combined to a circularly polarized excitation.

Keywords: mri, transmit efficiency, passive resonator, metasolenoid, metamaterials