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Real‐time assessment of potential peak local specific absorption rate value without phase monitoring: Trigonometric maximization method for worst‐case local specific absorption rate determination
Author(s) -
Meliadò Ettore Flavio,
Sbrizzi Alessandro,
Berg Cornelis A. T.,
Luijten Peter R.,
Raaijmakers Alexander J. E.
Publication year - 2021
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.28635
Subject(s) - specific absorption rate , phase (matter) , voxel , maximization , computer science , trigonometry , signal (programming language) , algorithm , mathematics , mathematical optimization , physics , artificial intelligence , telecommunications , mathematical analysis , quantum mechanics , antenna (radio) , programming language
Purpose Multi‐transmit MRI systems are typically equipped with dedicated hardware to sample the reflected/lost power in the transmit channels. After extensive calibration, the amplitude and phase of the signal at the feed of each array element can be accurately determined. However, determining the phase is more difficult and monitoring errors can lead to a hazardous peak local specific absorption rate (pSAR 10g ) underestimation. For this purpose, methods were published for online maximum potential pSAR 10g estimation without relying on phase monitoring, but these methods produce considerable overestimation. We present a trigonometric maximization method to determine the actual worst‐case pSAR 10g without any overestimation. Theory and Method The proposed method takes advantage of the sinusoidal relation between the SAR 10g in each voxel and the phases of input signals, to return the maximum achievable SAR 10g in a few iterations. The method is applied to determine the worst‐case pSAR 10g for three multi‐transmit array configurations at 7T: (1) body array with eight fractionated dipoles; (2) head array with eight fractionated dipoles; (3) head array with eight rectangular loops. The obtained worst‐case pSAR 10g values are compared with the pSAR 10g values determined with a commonly used method and with a more efficient method based on reference‐phases. Results For each voxel, the maximum achievable SAR 10g is determined in less than 0.1 ms. Compared to the reference‐phases‐based method, the proposed method reduces the mean overestimation of the actual pSAR 10g up to 52%, while never underestimating the true pSAR 10g . Conclusion The proposed method can widely improve the performance of parallel transmission MRI systems without phase monitoring.