title: Robust time-shifted spoke pulse design in the presence of large B0 variations with simultaneous reduction of through-plane dephasing, B1+ effects, and the specific absorption rate using parallel transmission.
abstract: To design parallel transmission spokes pulses with time-shifted profiles for joint mitigation of intensity variations due to B1+ effects, signal loss due to through-plane dephasing, and the specific absorption rate (SAR) at 7T.
  
  
  
  We derived a slice-averaged small tip angle (SA-STA) approximation of the magnetization signal at echo time that depends on the B1+ transmit profiles, the through-slice B0 gradient and the amplitude and time-shifts of the spoke waveforms. We minimize a magnitude least-squares objective based on this signal equation using a fast interior-point approach with analytical expressions of the Jacobian and Hessian.
  
  
  
  Our algorithm runs in less than three minutes for the design of two-spoke pulses subject to hundreds of local SAR constraints. On a B0/B1+ head phantom, joint optimization of the channel-dependent time-shifts and spoke amplitudes allowed signal recovery in high-B0 regions at no increase of SAR. Although the method creates uniform magnetization profiles (ie, uniform intensity), the flip angle varies across the image, which makes it ill-suited to T1-weighted applications.
  
  
  
  The SA-STA approach presented in this study is best suited to T2*-weighted applications with long echo times that require signal recovery around high B0 regions. Magn Reson Med 76:540-554, 2016. © 2015 Wiley Periodicals, Inc.
authors.affiliation: Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, USA.
authors.forename: Bastien
authors.initials: B
authors.lastname: Guérin
authors.affiliation: Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, USA.|Physics Department, Harvard University, Cambridge, Massachusetts, USA.
authors.forename: Jason P
authors.initials: JP
authors.lastname: Stockmann
authors.affiliation: John A. Burns School of Medicine, University of Hawaii, Honolulu, Hawaii, USA.
authors.forename: Mehran
authors.initials: M
authors.lastname: Baboli
authors.affiliation: Medical Image Analysis and Biometry Laboratory, University Rey Juan Carlos, Mostoles Spain.|Madrid-MIT M+ Vision Consortium, Madrid, Spain.
authors.forename: Angel
authors.initials: A
authors.lastname: Torrado-Carvajal
authors.affiliation: John A. Burns School of Medicine, University of Hawaii, Honolulu, Hawaii, USA.
authors.forename: Andrew V
authors.initials: AV
authors.lastname: Stenger
authors.affiliation: Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, USA.|Harvard-MIT Division of Health Sciences Technology, Cambridge, Massachusetts, USA.
authors.forename: Lawrence L
authors.initials: LL
authors.lastname: Wald
mesh_terms: D065638:Absorption, Radiation; D000465:Algorithms; D016477:Artifacts; D001921:Brain / Q000033:anatomy & histology*; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted
keywords: B0 artefact; B1+ homogeneity; parallel transmission; specific absorption rate; spokes