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

TLDR

Reducing and continuously varying the refocusing flip angle in RARE sequences mitigates high RF power deposition and MTF distortion caused by relaxation. The study introduces a streamlined method to generate per‑prescription refocusing flip‑angle sequences that achieve high SNR and reduce blurring across diverse in‑vivo tissues. The authors define and experimentally validate a contrast‑equivalent echo time that accurately reflects T2 contrast despite mixing of stimulated and spin echoes. They demonstrate that adding magnetization‑preparation pulses, such as T2‑prep, can manipulate contrast to achieve clinically desirable head and body imaging. Published in Magn Reson Med 2006; © 2006 Wiley‑Liss, Inc.

Abstract

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 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.