Subject- and resource-specific monitoring and proactive management of parallel radiofrequency transmission

• Published in: Magnetic resonance in medicine Vol. 76; no. 1; pp. 20 - 31
• Main Authors: Deniz, Cem M., Alon, Leeor, Brown, Ryan, Zhu, Yudong
• Format: Journal Article
• Language: English
• Published: United States Blackwell Publishing Ltd 01.07.2016; Wiley Subscription Services, Inc
• Subjects: Computer Simulation; Computer-Aided Design; Electromagnetic Fields; high field MRI; Humans; Magnetic Resonance Imaging - instrumentation; Magnetic Resonance Imaging - methods; Models, Theoretical; parallel transmission; Phantoms, Imaging; pulse design; Radiation Exposure - analysis; Radiation Exposure - prevention & control; Radiation Monitoring - instrumentation; Radiation Monitoring - methods; Radiation Protection - methods; Radio Waves; Reproducibility of Results; RF safety; Sensitivity and Specificity; specific absorption rate; high field MRI; specific absorption rate; pulse design; parallel transmission; RF safety
• ISSN: 0740-3194; 1522-2594; 1522-2594
• DOI: 10.1002/mrm.25828

Purpose Develop a practical comprehensive package for proactive management of parallel radiofrequency (RF) transmission. Methods With a constrained optimization framework and predictive models from a prescan based multichannel calibration, we presented a method supporting design and optimization of parallel RF excitation pulses that accurately obey the forward/reflected peak and average power limits of the RF power amplifiers in parallel transmit imaging experiments and Bloch simulations. Moreover, local SAR limits were incorporated into the parallel RF excitation pulses using electromagnetic field simulations. Virtual transmit coils concept for minimization of reflected power (effecting subject‐specific matching) was additionally demonstrated by leveraging experimentally calibrated power models. Results Incorporation of experimentally calibrated power prediction models resulted in accurate compliance with prescribed hardware and global specific absorption rate (SAR) limits. Incorporation of spatial average 10 g SAR models, facilitated by simplifying numerical approximations, provided assurance of patient safety. RF pulses designed with various constraints demonstrated excellent excitation fidelity—the normalized root‐mean‐square error of the simulated excitation profiles was 2.6% for the fully constrained pulses, comparable to that of the unconstrained pulses. An RF shimming example showed a reduction of the reflected‐to‐forward power ratio to 1.7% from a conventional approach's 8.1%. Conclusion Using the presented RF pulse design method, effective proactive management of the multifaceted power and SAR limits was demonstrated in experimental and simulation studies. Magn Reson Med 76:20–31, 2016. © 2015 Wiley Periodicals, Inc.