A Transfer Function Measurement Setup With an Electro-Optic Sensor for MR Safety Assessment in Cascaded Media

This article introduces a transfer function (TF) measurement setup for cascaded media (CM) and demonstrates that the full immersion of an implant in a lossy medium might result in an underestimation of the RF-induced heating compared to partial immersion. A CMTF measurement setup was constructed using medium-specific local electric field sources and electro-optical sensor (EOS). TFs of bare and insulated wire samples were measured in the CM air/saline. Finite-difference time-domain (FDTD) simulations were implemented using a reciprocal approach with a monopole excitation source as well as a piecewise plane wave excitation method. As a theoretical guideline for understanding the CM behavior of the devices, an analytical model based on the buried wire models was derived. CMTFs agree with good precision between FDTD simulations, analytical modeling, and the EOS measurements. Measured TFs were calibrated and validated similar to ISO/TS 10974:Ed.2. Calculated tip SAR values under uniform incident field show an increase up to a factor of 5.7 for partially immersed wires compared to the full immersion. On average, FDTD-based TFs overestimate SAR by 25% compared to measurement-based TFs. As safety assessments for fully immersed implant leads or devices might not represent the worst-case scenario, single-medium assessments might underestimate the RF-induced heating.