Reduction of radiofrequency induced implant heating via flexible metasurface shielding at 7 T

Abstract

Passive implanted devices are commonly contraindicated at ultra-high field MRI due to the risk of radiofrequency heating. Mitigation of this risk has come in many forms, such as modifying implant materials or creating novel radiofrequency coils. These methods require substantial involvement from manufacturers and may not benefit patients with existing implants. In this study, a tailored metasurface design is demonstrated to improve implant safety at 7 T by shielding the local B₁⁺ field. A prototype metasurface was designed and implemented with a unit cell size of 15 mm using discrete capacitors of 30 pF values. Phantom and human body model simulations were used to validate differences in the SAR distribution with and without the metasurface. Fiber optic temperature probes were used to measure temperature increase across two representative orthopedic screws placed inside a tissue mimicking phantom during a high-SAR sequence. Phantom and in-vivo imaging were performed to assess the metasurface effect on image quality. With the metasurface, an average maximum temperature decrease of 0.50 °C or 34.9 % near the implant was observed. RF field simulations yielded similar decreases in SAR for the phantom (40.7 %) and substantial decreases for the in-vivo leg model (97 %). Phantom image SNR showed a global 8.5 % decrease with the metasurface while in-vivo images showed a 4.8 % decrease in SNR, with the region in its immediate vicinity experiencing substantial signal drop. These results demonstrate the feasibility of a metasurface designed to substantially reduce local RF induced heating with only minor degradation of image quality. Future work will focus on refinement of the metasurface design and further in-vivo testing.