SAR validation of RF coils and implants in a B0-free RF safety laboratory

As MRI continues to advance toward higher field strengths, RF safety assessment has become more complex. Accurate estimation of specific absorption rate (SAR) is essential for evaluating both hardware and implant safety, yet thermometry and simulation-only approaches face limitations. In addition, these evaluations are typically performed within the MR scanner, where failures or excessive loading can damage the transmit chain and result in costly repairs and scanner downtime. To overcome these challenges, alternative experimental approaches that enable direct and controlled SAR validation outside the MR environment are needed.

This study demonstrates direct SAR validation of RF hardware and implants in a dedicated B0-free RF safety laboratory. The work aims to establish direct SAR measurement as a practical and accurate alternative for quantitative safety verification while expanding the range of tools available for MR hardware evaluation beyond the constraints of the scanner environment.

Hardware assessments were conducted in the safety lab equipped with a 16-channel broadband RF amplifier system (45-450 MHz), a 100 dB shielded Faraday cage, automated 3D and 1D motion systems, and a high-precision SAR probe. For coil validation, direct SAR mapping was performed on a custom-built parallel transmit (pTx) coil and a commercial single-channel RF coil at 447 and 297 MHz, respectively, and results were compared with electromagnetic simulations. Transfer function (TF) validation for a directional DBS electrode at 128 MHz was performed in a rectangular electric-field generator, whose field distribution was first validated using the SAR probe. The same probe was then used to compare measured SAR with TF-based SAR predictions, providing a direct quantitative validation of the TF method.

Measured SAR distributions showed strong quantitative agreement with simulations across all experiments. For the 8-channel pTx head coil at 447 MHz, the normalized root-mean-square error (NRMSE) across four excitation patterns was below 13%. For the commercial single-channel head coil at 297 MHz, plane-wise comparisons across axial, coronal, and sagittal slices yielded an average NRMSE value of 7.18%, demonstrating spatial consistency between measured and simulated fields. The electric-field generator used for DBS transfer function (TF) validation exhibited 3.68% NRMSE relative to simulation, confirming accurate field reproduction. SAR measurements using the same probe further showed close agreement with TF-based SAR predictions across all seven lead trajectories, confirming both the accuracy and broadband applicability of the direct SAR method.

Direct SAR measurement provides a reliable and quantitative approach for validating RF coil and implant safety, matching the accuracy of conventional scanner-based thermometry and simulation methods. Conducting these measurements in a B0-free RF safety laboratory further simplifies and accelerates the process by eliminating magnetic field constraints and the waiting periods required for the setup to return to thermal equilibrium in thermometric techniques. This configuration provides an efficient and controlled platform for systematic SAR validation across a wide range of MR frequencies.