Nonlinear Curved Metasurfaces for Enhanced Thyroid Magnetic Resonance Imaging

Metasurfaces offer a promising approach to enhance the signal-to-noise ratio (SNR) in magnetic resonance imaging (MRI). Thyroid MRI suffers from insufficient SNR due to the anatomical irregularity of the neck and poor coil-to-tissue proximity, which fails to meet the demands of preoperative tumor diagnosis. This work introduces a nonlinear, curved metasurface tailored to address these limitations through a nonlinear mode control scheme based on up-frequency tuning. Unlike the previously reported down-frequency tuning scheme, here the metasurface's resonant frequency is selectively shifted to a frequency above the Larmor frequency during RF transmit via an integrated diode-inductor circuit, thereby minimizing B1+ perturbation and specific absorption rate increase. Concurrently, its conformal curved geometry focuses the enhanced field on the concave side, optimizing penetration for superficial targets. This adaptive mechanism minimally perturbs the transmit field while keeping the device fully active during signal reception. Experiments demonstrate that this thyroid imaging curved metasurface (TCM) provides an SNR enhancement by a factor of over six in the target thyroid region and an effective penetration depth of approximately 60 mm over the whole range of up-frequency inductances examined (27-100 nH), for which the two-fold gain is maintained to about 70 mm, validating its potential as a practical, wireless adjunct for high-quality thyroid MRI.