Evaluation and Correction of B1+-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 mod-els at 128 MHz. 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 com-puted 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 cere-brospinal fluid (CSF). A comparison was performed for different levels of signal-to-noise ratios (SNR). Local-SAR/10g-SAR CF was 3.08 +/- 0/06 / 2.11 +/- 0.04 for gray matter, 1.79 +/- 0/05 / 2.06 +/- 0.04 for white matter, and 2.59 +/- 0/05 / 1.95 +/- 0.03 for CSF. SAR(EST) without CF were underestimated (ratio across [infinity -25] SNRs: 0.52 +/- 0.02 for local-SAR; 0.55 +/- 0.01 for 10g-SAR). After cor-rection, SAREST was equivalent to SAR(GT) (ratio across [infinity -25] SNRs: 0.97 +/- 0.02 for local-SAR; 1.06 +/- 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(+).