Numerical Simulation-Based RF Safety Assessment Using Different Normalization Methods in 7T MRI

Accurate computation of the specific absorption rate (SAR) is crucial to prevent tissue overheating and ensure compliance with radiofrequency (RF) safety regulatory standards in a magnetic resonance imaging (MRI). However, different RF transmitting magnetic (B1+)-field normalization methods produce varying results, therefore, understanding their impact is vital for comparing studies, optimizing MRI protocols, and adapting to new technologies. Proper normalization also helps tailor safety assessments to individual patients with anatomical variation, ultimately improving MRI safety. We analyzed SAR values using electromagnetic simulations with four human head models (two adults and two children) within a 7T MRI birdcage coil. To account for size variation in patients, two scaled versions of each model were included, resulting in 12 total head models. We compared and analyzed RF safety by applying five B1+-field normalization methods: setting the input power to 1 W and using the B1+-field mean values from a single voxel, a specific volume, the entire slice, and the whole head model within the B1+-field. Our findings indicate that child models exhibited higher B1+-field mean values and normalized SAR1W values (hdSAR and psSAR10g) compared to adult models, primarily due to their smaller size and mass, despite identical input power. While hdSAR decreased with increasing model size, psSAR10g showed no clear linear trend. Additionally, comparisons between SARNF and SAR1W normalization revealed significant discrepancies, emphasizing the importance of considering model size and mass in RF exposure evaluations. This study emphasizes the importance of recognizing differences in SAR values according to normalization methods to ensure accurate quantitative analysis and MRI safety.