Fast spin echo sequences with very long echo trains: Design of variable refocusing flip angle schedules and generation of clinical <i>T</i>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 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 degrees refocusing. Furthermore, contrast is shown to be manipulable by the addition of magnetization preparation pulse sequence segments, such as T2-prep, to produce clinically desirable contrast for routine head and body imaging.