A Numerical Alternative to MR Thermometry for Safety Validation of Multi-Channel RF Transmit Coils

This study proposes a numerical technique to estimate peak local specific absorption rate (SAR) uncertainty of multi-channel RF coils during the process of safety validation as an alternative to experimental temperature and electric-field measurements, and demonstrates its use to enable human studies at 10.5 T.

To ensure patient safety, SAR limits established under international guidelines must not be exceeded. Predicting SAR on state-of-the-art parallel transmit systems relies on electromagnetic simulations, which require extensive experimental validation. Despite a well-established validation workflow, SAR prediction errors are unavoidable and must be quantified as a safety margin. While MRT tests are commonly used for this purpose, their technical challenges necessitate an alternative. The proposed technique propagates the error between experimentally and numerically acquired $B_1^{+}$ distributions to the uncertainty in simulated peak local SAR using Monte-Carlo simulations without the need for MRT. This method was validated using a 16-channel transceiver 10.5 T torso coil, as well as an 8-channel 10.5 T head coil.

The proposed numerical technique proved more conservative than existing MRT-based SAR error quantification methods across all tested scenarios. Its application to validate three state-of-the-art head coils (16Tx/32Rx, 16Tx/80Rx, and 16Tx/128Rx) led to regulatory approval for human head imaging and high-quality diffusion and functional MRI results at 10.5 T.

The proposed technique requires only the experimental acquisition of $B_1^{+}$ maps for comparison with simulations, enabling the estimation of SAR prediction uncertainty. This technique was applied to three 16-channel transmit arrays, each used in conjunction with high-channel-count receive arrays for in vivo imaging.