RF safety assessment of dental retainers for human head MRI array coils at 9.4 T

In this work we developed a numerical workflow for evaluating radiofrequency (RF) safety for ultra-high field (UHF, 7 T and above) MRI with a local transmit array coil in the presence of conductive objects such as dental retainers. The proposed workflow includes evaluation of the B₁⁺ distribution and peak Specific Absorption Rate (pSAR) for the standard circularly polarized modes, worst-case pSAR modes, and random RF modes. The workflow provides a safety margin, which limits the RF transmit power during MRI scanning. The proposed workflow is applicable to different array coils at different UHF strengths (e.g. 7 T, 9.4 T, 11.7 T, 14 T) as well as to other types of conductive objects (e.g., dental or neural implants). 16-element tight-fit transceiver and larger 16-element transmit-only head arrays were numerically evaluated at 9.4 T in the presence of the dental retainer. Updated safety margins were calculated for both arrays. In addition, we calculated Q-matrix and virtual observation point matrices, which facilitate the real-time SAR-monitoring and can be used for optimization of the B₁⁺ homogeneity and SAR.