Fast spin echo sequences with very long echo trains: Design of variable refocusing flip angle schedules and generation of clinical T 2 contrast

Reducing and continuously varying the flip angle of the refocusing RF pulses in a rapid acquisition with relaxation enhancement (RARE; fast/turbo spin echo) sequence is a useful means of addressing high RF power deposition and modulation transfer function (MTF) distortion due to relaxation. This work presents a streamlined technique to generate a sequence of refocusing flip angles on a per‐prescription basis that produces relatively high SNR and limits blurring in a wide range of materials encountered in vivo. Since the “effective TE” (traditionally defined as the time at which the center of k ‐space is sampled) no longer corresponds to the expected amount of spin‐echo T 2 contrast due to the mixing of stimulated and spin echoes, a “contrast‐equivalent” TE is defined and experimentally demonstrated that allows annotation of a more accurate effective TE that matches the contrast produced by 180° refocusing. Furthermore, contrast is shown to be manipulable by the addition of magnetization preparation pulse sequence segments, such as T 2 ‐prep, to produce clinically desirable contrast for routine head and body imaging. Magn Reson Med, 2006. © 2006 Wiley‐Liss, Inc.