Evaluation and Correction of $B_{1}^+$-Based Brain Subject-Specific SAR Maps Using Electrical Properties Tomography

The specific absorption rate (SAR) estimates the amount of power absorbed by the tissue and is determined by the electrical conductivity and the E-field. Conductivity can be estimated using Electric Properties Tomography (EPT) but only the E-field component associated with $B_{1}^+$ can be deduced from $B_{1}$ -mapping. Herein, a correction factor was calculated to compensate for the differences between the actual SAR and the one obtained with $B_{1}^+$ . Numerical simulations were performed for 27 head models at $128 \,\mathrm{M}\mathrm{Hz}$ . Ground-truth local-SAR and 10g-SAR (SAR _GT ) were computed using the exact electrical conductivity and the E-field. Estimated local-SAR and 10g-SAR (SAR _EST ) were computed using the electrical conductivity obtained with a convection-reaction EPT and the E-field obtained from $B_{1}^+$ . Correction factors (CFs) were estimated for gray matter, white matter, and cerebrospinal fluid (CSF). A comparison was performed for different levels of signal-to-noise ratios (SNR). Local-SAR/10g-SAR CF was 3.08 $\pm$ 0/06 / 2.11 $\pm$ 0.04 for gray matter, 1.79 $\pm$ 0/05 / 2.06 $\pm$ 0.04 for white matter, and 2.59 $\pm$ 0/05 / 1.95 $\pm$ 0.03 for CSF. SAR _EST without CF were underestimated (ratio across [ $\infty$ - 25] SNRs: 0.52 $\pm$ 0.02 for local-SAR; 0.55 $\pm$ 0.01 for 10g-SAR). After correction, SAR _EST was equivalent to SAR _GT (ratio across [ $\infty$ - 25] SNRs: 0.97 $\pm$ 0.02 for local-SAR; 1.06 $\pm$ 0.01 for 10g-SAR). SAR maps based on $B_{1}^+$ can be corrected with a correction factor to compensate for potential differences between the actual SAR and the SAR calculated with the E-field derived from $B_{1}^+$ .