Tunable Metasurfaces for Switchable Magnetic Field Enhancement Regions in 1.5 T MRI

Metasurfaces have been reported to boost the signal-to-noise ratio (SNR) of magnetic resonance imaging (MRI) through their magnetic field enhancement capabilities. The varying region-of-interest (ROI) sizes in clinical imaging limit metasurfaces from realizing their potential, since the field enhancement regions of the most metasurfaces are fixed after fabrication. In this paper, a tunable metasurface (TMS) is proposed to significantly boost SNR of 1.5 T MRI in regions with different sizes. The TMS is composed of unit cells that contain a planar metal spiral loaded with a variable capacitor and placed on a square dielectric substrate. The feasibility of switching between a wide enhancement region (WER) mode and a narrow enhancement region (NER) mode by adjusting the capacitance is validated through simulations and experiments. The enhancement of the magnetic field by the WER mode of TMS boosts the SNR of the human brain voxel model by a maximum of 14.01 times, reaching a higher value of 22.81 times with the NER mode. This work offers an effective approach that can change the region sizes of field enhancement flexibly, enabling metasurfaces to adapt for various MRI scenarios such as disease detection in the large region or therapeutic monitoring in the specific small region.