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Local SAR, global SAR, and power‐constrained large‐flip‐angle pulses with optimal control and virtual observation points
Author(s) -
Vinding Mads S.,
Guérin Bastien,
Vosegaard Thomas,
Nielsen Niels Chr.
Publication year - 2017
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.26086
Subject(s) - specific absorption rate , flip angle , torso , acceleration , computer science , pulse (music) , simulated annealing , physics , algorithm , control theory (sociology) , telecommunications , artificial intelligence , medicine , magnetic resonance imaging , radiology , control (management) , anatomy , classical mechanics , detector , antenna (radio)
Purpose To present a constrained optimal‐control (OC) framework for designing large‐flip‐angle parallel‐transmit (pTx) pulses satisfying hardware peak‐power as well as regulatory local and global specific‐absorption‐rate (SAR) limits. The application is 2D and 3D spatial‐selective 90° and 180° pulses. Theory and Methods The OC gradient‐ascent‐pulse‐engineering method with exact gradients and the limited‐memory Broyden‐Fletcher‐Goldfarb‐Shanno method is proposed. Local SAR is constrained by the virtual‐observation‐points method. Two numerical models facilitated the optimizations, a torso at 3 T and a head at 7 T, both in eight‐channel pTx coils and acceleration‐factors up to 4. Results The proposed approach yielded excellent flip‐angle distributions. Enforcing the local‐SAR constraint, as opposed to peak power alone, reduced the local SAR 7 and 5‐fold with the 2D torso excitation and inversion pulse, respectively. The root‐mean‐square errors of the magnetization profiles increased less than 5% with the acceleration factor of 4. Conclusion A local and global SAR, and peak‐power constrained OC large‐flip‐angle pTx pulse design was presented, and numerically validated for 2D and 3D spatial‐selective 90° and 180° pulses at 3 T and 7 T. Magn Reson Med 77:374–384, 2017. © 2015 Wiley Periodicals, Inc.