Domino volumetric metamaterial resonator for very-low-field MRI

Very-low-field magnetic resonance imaging (VLF-MRI) plays a significant role in medical imaging diagnosis due to its low cost and light weight. High-quality MR images are essential for accurate medical diagnosis. It is urgent to explore a low-cost, simple and convenient approach to boost the signal-to-noise ratio (SNR) of VLF-MRI system to make medical diagnosis more accurate.

The aim of this study was to evaluate the effect of the proposed metamaterial resonator on enhancing the performance of the 70 mT VLF-MRI system. The domino volumetric metamaterial resonator (DVMR), which consisted of an array of rectangular plan spiral resonator cell, was designed to work at the Larmor frequency of the 70 mT VLF-MRI system. When placed around the realistic multi- tissue voxel human wrist phantom, the DVMR improved the SNR in the region of interest (ROI).

The equivalent circuits of DVMR cell and radio frequency (RF) coil were analyzed by the coupling mode theory and circuit model theory. The non-dominated sorting genetic algorithm II (NSGA-II) was used to optimize the structure of the DVMR cell. The DVMR was composed of multiple coaxial DVMR cells stacked. The 10 g-averaged specific absorption rate (SAR_av.10g), the $|B_1^{-}|$ field enhancement factor, and the SNR enhancement factor were analyzed. The enhancement effect of the DVMR on the performance of the VLF-MRI system was assessed by comparing the cases with and without the DVMR. The proposed DVMR was compared with the identically-sized solenoid coil. To further verify the performance of the proposed DVMR, the preliminary experiments are performed.

Due to the introduction of the proposed DVMR, the SNR enhancement factor for the cuboid phantom reached up to 5.06 by comparing with using the RF coil alone. For the radial direction of the realistic human wrist phantom, the SNR using the DVMR was higher than that using the identically- sized solenoid coil, with the maximum enhancement of 1.15 times. Simultaneously, the maximum of $\mathrm{SA}{R}_{\mathrm{av}.10g}$ was significantly below the safety threshold of 2.0 W/kg. The experimental results show that the RF magnetic field is significantly enhanced in the presence of DVMR.

The proposed DVMR is the low-cost and convenient passive resonator, which could significantly improve the SNR in the ROI. It has significant potential to effectively enhance the performance of VLF-MRI systems.