High‐resolution gradient‐recalled echo imaging at 9.4T using 16‐channel parallel transmit simultaneous multislice spokes excitations with slice‐by‐slice flip angle homogenization

• Published in: Magnetic resonance in medicine Vol. 78; no. 3; pp. 1050 - 1058
• Main Authors: Tse, Desmond H. Y., Wiggins, Christopher J., Poser, Benedikt A.
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.09.2017; John Wiley and Sons Inc
• Subjects: Adult; Algorithms; Brain - diagnostic imaging; Excitation; Full Papers—Imaging Methodology; High resolution; high‐resolution GRE SMS‐pTX; Homogeneity; Homogenization; Humans; Image Processing, Computer-Assisted - methods; Image resolution; Imaging; Inhomogeneity; Least-Squares Analysis; Magnetic resonance; Magnetic Resonance Imaging - methods; magnitude least‐squares; Male; Medicine; Noise; Optimization; parallel transmission; Radio frequency; Reduction; Resonance; Scaling; Scaling factors; simultaneous multislice; Spokes; spokes pulses; ultrahigh‐field MR; Waveforms; magnitude least-squares; high-resolution GRE SMS-pTX; spokes pulses; parallel transmission; simultaneous multislice; ultrahigh-field MR
• ISSN: 0740-3194; 1522-2594
• DOI: 10.1002/mrm.26501

Purpose In order to fully benefit from the improved signal‐to‐noise and contrast‐to‐noise ratios at 9.4T, the challenges of B1+ inhomogeneity and the long acquisition time of high‐resolution 2D gradient‐recalled echo (GRE) imaging were addressed. Theory and Methods Flip angle homogenized excitations were achieved by parallel transmission (pTx) of 3‐spoke pulses, designed by magnitude least‐squares optimization in a slice‐by‐slice fashion; the acquisition time reduction was achieved by simultaneous multislice (SMS) pulses. The slice‐specific spokes complex radiofrequency scaling factors were applied to sinc waveforms on a per‐channel basis and combined with the other pulses in an SMS slice group to form the final SMS‐pTX pulse. Optimal spokes locations were derived from simulations. Results Flip angle maps from presaturation TurboFLASH showed improvement of flip angle homogenization with 3‐spoke pulses over CP‐mode excitation (normalized root‐mean‐square error [NRMSE] 0.357) as well as comparable excitation homogeneity across the single‐band (NRMSE 0.119), SMS‐2 (NRMSE 0.137), and SMS‐3 (NRMSE 0.132) 3‐spoke pulses. The application of the 3‐spoke SMS‐3 pulses in a 48‐slice GRE protocol, which has an in‐plane resolution of 0.28 × 0.28 mm, resulted in a 50% reduction of scan duration (total acquisition time 6:52 min including reference scans). Conclusion Time‐efficient flip angle homogenized high‐resolution GRE imaging at 9.4T was accomplished by using slice‐specific SMS‐pTx spokes excitations. Magn Reson Med 78:1050–1058, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine.