Application of voxel-wise SAR efficiency to evaluate suitability of novel materials in MRI with parallel transmit

Objective. Novel dielectric materials and metamaterials have been proposed for passive RF shimming in MRI. However, state-of-the-art systems rely on multi-channel parallel transmission (pTx) techniques to enable advanced excitation strategies. Currently, there is no method to prospectively assess whether a novel material can be practically integrated into pTx systems, given the large degrees of freedom allowed by transmit shim vectors. In this work, we investigate the applicability of voxel-wise specific absorption rate (SAR) efficiency as a quantitative metric to evaluate the compatibility of novel materials with pTx systems.Approach. Electromagnetic simulations were performed for a range of previously published materials integrated into new pTx configurations and evaluated using virtual body models at different field strengths (1.5, 3 and 7 T). In addition, we applied and reviewed a post-processing method for virtual observation points compression and subsequent voxel-wise optimization of the shim vectors to find the maximum SAR efficiency(B1+SARlocal,max).Main results. The results demonstrate the versatility of the proposed metric for evaluating materials performance across different pTx arrays. In particular, we show that certain dielectric configurations can enhance efficiency in targeted anatomical regions more effectively than increasing the number of transmit channels. For brain imaging, 4-channel systems in addition with a dielectric helmet achieved SAR efficiency comparable to a 16-channel system, allowing for a reduction of system complexity and cost. Moreover, in systems with a high number of channels, the incorporation of such materials can further amplify the performance gains.Significance. This study advances the evaluation of novel materials, including dielectric materials and metasurfaces in pTx systems by demonstrating the applicability of a standardized, quantitative method for evaluation. This approach enables a more reliable assessment of their practical compatibility and facilitates translation towards clinical implementation.