Fast spin echo sequences with very long echo trains: Design of variable refocusing flip angle schedules and generation of clinical T2 contrast

Abstract

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 T2 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 manipulate by the addition of magnetization preparation pulse sequence segments, such as T2-prep, to produce clinically desirable contrast for routine head and body imaging. © 2003 Wiley-Liss, Inc.