Evaluation and Correction of B^+-Based Brain Subject-Specific SAR Maps Using Electrical Properties Tomography

• Published in: IEEE journal of electromagnetics, RF and microwaves in medicine and biology Vol. 7; no. 2; pp. 168 - 175
• Main Authors: Martinez, Jessica A., Arduino, Alessandro, Bottauscio, Oriano, Zilberti, Luca
• Format: Journal Article
• Language: English
• Published: IEEE 01.06.2023
• Subjects: <named-content xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:ali="http://www.niso.org/schemas/ali/1.0/" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" content-type="math" xlink:type="simple"> <inline-formula> <tex-math notation="LaTeX"> B_{1}</tex-math> </inline-formula> </named-content>-mapping; Ampère's law; Conductivity; electrical conductivity mapping; electrical-properties tomography; Grey matter; Head; head-SAR maps; Magnetic resonance imaging; Signal to noise ratio; specific absorption rate; White matter
• ISSN: 2469-7249; 2469-7257
• DOI: 10.1109/JERM.2023.3236153

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 <inline-formula><tex-math notation="LaTeX">B_{1}^+</tex-math></inline-formula> can be deduced from <inline-formula><tex-math notation="LaTeX">B_{1}</tex-math></inline-formula>-mapping. Herein, a correction factor was calculated to compensate for the differences between the actual SAR and the one obtained with <inline-formula><tex-math notation="LaTeX">B_{1}^+</tex-math></inline-formula>. Numerical simulations were performed for 27 head models at <inline-formula><tex-math notation="LaTeX">128 \,\mathrm{M}\mathrm{Hz}</tex-math></inline-formula>. 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 <inline-formula><tex-math notation="LaTeX">B_{1}^+</tex-math></inline-formula>. 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 <inline-formula><tex-math notation="LaTeX">\pm</tex-math></inline-formula> 0/06 / 2.11 <inline-formula><tex-math notation="LaTeX">\pm</tex-math></inline-formula> 0.04 for gray matter, 1.79 <inline-formula><tex-math notation="LaTeX">\pm</tex-math></inline-formula> 0/05 / 2.06 <inline-formula><tex-math notation="LaTeX">\pm</tex-math></inline-formula> 0.04 for white matter, and 2.59 <inline-formula><tex-math notation="LaTeX">\pm</tex-math></inline-formula> 0/05 / 1.95 <inline-formula><tex-math notation="LaTeX">\pm</tex-math></inline-formula> 0.03 for CSF. SAR EST without CF were underestimated (ratio across [<inline-formula><tex-math notation="LaTeX">\infty</tex-math></inline-formula> - 25] SNRs: 0.52 <inline-formula><tex-math notation="LaTeX">\pm</tex-math></inline-formula> 0.02 for local-SAR; 0.55 <inline-formula><tex-math notation="LaTeX">\pm</tex-math></inline-formula> 0.01 for 10g-SAR). After correction, SAR EST was equivalent to SAR GT (ratio across [<inline-formula><tex-math notation="LaTeX">\infty</tex-math></inline-formula> - 25] SNRs: 0.97 <inline-formula><tex-math notation="LaTeX">\pm</tex-math></inline-formula> 0.02 for local-SAR; 1.06 <inline-formula><tex-math notation="LaTeX">\pm</tex-math></inline-formula> 0.01 for 10g-SAR). SAR maps based on <inline-formula><tex-math notation="LaTeX">B_{1}^+</tex-math></inline-formula> 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 <inline-formula><tex-math notation="LaTeX">B_{1}^+</tex-math></inline-formula>.