title: Optimized amplitude modulated multiband RF pulse design.
abstract: OBJECTIVE
  Multiband pulses are characterized by highly temporally modulated waveforms. Rapid phase or frequency modulation can be extremely demanding on the performance of radiofrequency (RF) pulse generation, which can lead to errors that can be avoided if pulses are restricted to amplitude modulation (AM) only. In this work, three existing multiband pulse design techniques are modified to produce AM waveforms.
  
  
  METHODS
  Multiband refocusing pulses were designed using phase-optimization, time-shifting, and root-flipping. Each technique was constrained to produce AM pulses by exploiting conjugate symmetry in their respective frequency domain representations. Pulses were designed using the AM and unconstrained techniques for a range of multiband factors (ie, number of simultaneously excited slices), time-bandwidth products, and slice separations. Performance was compared by examining the resulting effective pulse durations. Phantom and in vivo experiments were conducted for validation.
  
  
  RESULTS
  Acquired data confirmed that AM pulses can produce precise results when unconstrained designs may produce artifacts. The average duration of AM pulses is longer than the unconstrained versions. Averaged across a range of parameters, the duration cost for AM pulses was 26, 38, and 20% for phase-optimizing, time-shifting and root-flipping, respectively.
  
  
  CONCLUSIONS
  Amplitude modulation multiband pulses can be produced for a relatively small increase in pulse duration. Magn Reson Med 78:2185-2193, 2017. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
authors.affiliation: Division of Imaging Sciences and Biomedical Engineering, King's College London, London, United Kingdom.
authors.forename: Samy
authors.initials: S
authors.lastname: Abo Seada
authors.affiliation: Division of Imaging Sciences and Biomedical Engineering, King's College London, London, United Kingdom.
authors.forename: Anthony N
authors.initials: AN
authors.lastname: Price
authors.affiliation: Division of Imaging Sciences and Biomedical Engineering, King's College London, London, United Kingdom.
authors.forename: Joseph V
authors.initials: JV
authors.lastname: Hajnal
authors.affiliation: Division of Imaging Sciences and Biomedical Engineering, King's College London, London, United Kingdom.
authors.forename: Shaihan J
authors.initials: SJ
authors.lastname: Malik
mesh_terms: D000465:Algorithms; D016477:Artifacts; D003198:Computer Simulation; D038524:Diffusion Magnetic Resonance Imaging; D005583:Fourier Analysis; D006801:Humans; D007091:Image Processing, Computer-Assisted; D015233:Models, Statistical; D019047:Phantoms, Imaging; D011846:Radio Waves; D013053:Spectrophotometry
keywords: RF pulse design; excitation stability; interslice artifact; multiband pulse design; root flipping; simultaneous multislice