An Adaptive Metasurface for Adjustable Magnetic Resonance Imaging Enhancement at 3 T and 7 T: A Feasibility Study

Ultra-high field (≥ 7 T) and 3 T MRI suffer from B₁ ⁺ inhomogeneity and signal loss. Conventional metasurfaces that mitigate B₁ ⁺ inhomogeneity and improve image SNR are "fixed" in nature and designed for a single Larmor frequency. This study demonstrates the feasibility of an adaptive metasurface utilizing voltage-controlled capacitors to dynamically adjust electromagnetic properties for adjustable image enhancement at both 3 T and 7 T ¹H Larmor frequencies.

A 3 × 3 unit-cell prototype was developed using voltage adjustable capacitors (33.0-16.5 pF) controlled by a 0-3 V DC source. Bench characterization and numerical simulations were performed to evaluate frequency characteristics and B₁ ⁺ distributions. Empirical data was acquired via GRE imaging and B₁ ⁺ mapping on 3 T and 7 T scanners. Multiple devices were also evaluated in tandem at 7 T to assess interference and spatial degrees of freedom.

Simulations and phantom imaging confirmed voltage-dependent behavior. At 7 T, decreasing capacitance to 16.5 pF (3 V) yielded a 4.30× local SNR increase and a 60.0% boost in transmit efficiency. At 3 T, maximum enhancement (1.65× local SNR) occurred at 33.0 pF (0 V), demonstrating the ability of the device to adjust the optimal impedance between 128 and 300 MHz. Multiple metasurfaces successfully operated simultaneously without mutual interference, enabling multiregion enhancement.

This work establishes the feasibility of an adaptive, low-power metasurface for remote subject-specific image enhancement. While the current enhancement was limited by the prototype's small size and capacitance range, the design provides a scalable foundation for future design iterations adaptable across different field strengths and potentially for multiple nuclei.