title: Ultrahigh field single-refocused diffusion weighted imaging using a matched-phase adiabatic spin echo (MASE).
abstract: OBJECTIVE
  To improve ultrahigh field diffusion-weighted imaging (DWI) in the presence of inhomogeneous transmit B1 field by designing a novel semi-adiabatic single-refocused DWI technique.
  
  
  METHODS
  A 180° slice-selective, adiabatic radiofrequency (RF) pulse of 4 ms duration was designed using the adiabatic Shinnar-Le Roux algorithm. A matched-phase slice-selective 90° RF pulse of 8 ms duration was designed to compensate the nonlinear phase of the adiabatic 180° RF pulse. The resulting RF pulse combination, matched-phase adiabatic spin echo (MASE), was integrated into a single-shot echo planar DWI sequence. The performance of this sequence was compared with single-refocused Stejskal-Tanner (ST), twice-refocused spin echo (TRSE) and twice-refocused adiabatic spin echo (TRASE) in simulations, phantoms, and healthy volunteers at 7 Tesla (T).
  
  
  RESULTS
  In regions with inhomogeneous B1 , MASE resulted in increased signal intensity compared with ST (up to 64%). Moderate increase in specific absorption rate (35-39%) was observed for adiabatic RF pulses. MASE resulted in higher signal homogeneity at 7T, leading to improved visualization of measures derived from diffusion-weighted images such as white matter tractography and track density images.
  
  
  CONCLUSIONS
  Efficient adiabatic SLR pulses can be adapted to single-refocused DWI, leading to substantially improved signal uniformity when compared with conventional acquisitions.
authors.affiliation: Translational and Molecular Imaging Institute, Icahn School of Medicine at Mount Sinai, New York, New York, USA.
authors.forename: Hadrien
authors.initials: H
authors.lastname: Dyvorne
authors.affiliation: Translational and Molecular Imaging Institute, Icahn School of Medicine at Mount Sinai, New York, New York, USA.
authors.forename: Rafael
authors.initials: R
authors.lastname: O'Halloran
authors.affiliation: Translational and Molecular Imaging Institute, Icahn School of Medicine at Mount Sinai, New York, New York, USA.
authors.forename: Priti
authors.initials: P
authors.lastname: Balchandani
mesh_terms: D000465:Algorithms; D001921:Brain / Q000000981:diagnostic imaging / Q000502:physiology; D002138:Calibration; D003198:Computer Simulation; D038524:Diffusion Magnetic Resonance Imaging / Q000379:methods*; D017352:Echo-Planar Imaging; D064368:Healthy Volunteers; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods; D019047:Phantoms, Imaging; D011846:Radio Waves; D059629:Signal-To-Noise Ratio; D066127:White Matter / Q000000981:diagnostic imaging / Q000502:physiology
keywords: 7T diffusion; B1; DTI; DWI; RF pulse; adiabatic; brain; diffusion; matched phase; radiofrequency; tractography; ultrahigh field