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

Abstract

PurposeThis study proposes an alternative approach to MR thermometry (MRT) for the safety validation of multi-channel RF transmit coils and demonstrates its use to enable human studies at 10.5T. MethodsTo ensure patient safety, specific absorption rate (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+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 coil for imaging the human torso (henceforth referred to as a "body" coil) at 10.5T with two excitation scenarios, as well as an 8-channel 10.5T head coil with four excitation scenarios. ResultsThe 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 functional as well as diffusion MRI results at 10.5T. ConclusionA numerical alternative to MRT requires only the experimental acquisition of B+ maps for comparison with simulations, enabling the quantification of uncertainty in SAR prediction. This technique was applied to three 16-channel transmit arrays, each used in conjunction with high-channel-count receive arrays for in vivo imaging.