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

Abstract

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 $B_{1}^+$ .