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
$B_{1}^+$
.