Ultrahigh field single-refocused diffusion weighted imaging using a matched-phase adiabatic spin echo (MASE)

PurposeTo improve ultrahigh field diffusion-weighted imaging (DWI) in the presence of inhomogeneous transmit B-1 field by designing a novel semi-adiabatic single-refocused DWI technique. MethodsA 180 degrees 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 degrees RF pulse of 8 ms duration was designed to compensate the nonlinear phase of the adiabatic 180 degrees 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). ResultsIn regions with inhomogeneous B-1, 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. ConclusionEfficient adiabatic SLR pulses can be adapted to single-refocused DWI, leading to substantially improved signal uniformity when compared with conventional acquisitions. Magn Reson Med 75:1949-1957, 2016. (c) 2015 Wiley Periodicals, Inc.