Individualized and accurate SAR characterization method based on equivalent circuit model for MRI system

• Published in: Magnetic resonance in medicine Vol. 87; no. 6; pp. 2997 - 3010
• Main Authors: Jiang, Weiman, Yang, Fan, Wang, Kun
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.06.2022
• Subjects: Complexity; Electric power distribution; Energy methods; equivalent circuit model; Equivalent circuits; Frequency dependence; Frequency response functions; Heating; Humans; Magnetic resonance imaging; Magnetic Resonance Imaging - methods; Mathematical analysis; Patients; Phantoms, Imaging; pulse energy method; Radio frequency heating; Radio Waves; Reflectance; reflection coefficient; Resonant frequencies; RF heating; safety; SAR; pulse energy method; reflection coefficient; SAR; RF heating; safety; equivalent circuit model
• ISSN: 0740-3194; 1522-2594; 1522-2594
• DOI: 10.1002/mrm.29163

Purpose To protect patients from RF heating in MRI scan, this work proposes an accurate and patient‐specific whole‐body specific absorption rate (SAR) characterization method based on an equivalent circuit model. Compared to the standard pulse energy method defined in NEMA MS 8‐2016, this method avoids the complexity of integrating flux loops and has the potential to be easily implemented in MRI scanners. Theory and Methods In this study, we use an equivalent parallel circuit to model the power distribution on the transmit coil and subject. The coil and subject equivalent resistances are fitted by the frequency response functions of reflection coefficient and are thereafter used to calculate the power ratio between them. To assess the accuracy of this method, we measured the subject absorbed power of 2 phantoms and 5 volunteers and compared it with the standard pulse energy method with flux loops. Results The resistances, resonant frequencies, and quality factors of the transmit coil are fitted with the equivalent circuit model in both unloaded and loaded conditions. Whole‐body SAR of 5 volunteers is measured at 2 different landmarks. In addition, the relationship between SAR and the working frequencies of the transmit coil is measured and analyzed. The subject absorbed power measured by the proposed method demonstrates good accuracy (RMS error and maximum error of 3.77% and 9.47%, respectively) relative to the flux loop method. Conclusion The equivalent circuit model‐based method enables individualized, accurate, and simplified SAR characterization for clinical applications and research with moderate implementation complexity.