Fast spin-echo MR imaging of the brain and spine: current concepts.

Fast spin-echo (FSE) sequences are recently developed modifications of the rapid acquisition relaxation enhanced (RARE) sequences initially described by Henning et al. [1-4]. The basic pulse sequence underlying all RARE/FSE methods is the Carr-Purcell-Meiboom-Gill (CPMG) echo train. This sequence was originally introduced over 30 years ago in order to make transverse relaxation time (T2) measurements that would be largely free from errors introduced by imperfect refocusing pulses [5], the latter being accomplished by making a 90#{176} phase shift between the excitation pulse and ensuing refocusing pulses. In RARE/FSE sequences, each echo of the CPMG train is phase encoded with a distinct phase-encode gradient, read out in the presence of a frequency-encoding gradient, and phase “unwound” after readout [1]. It has now been shown that with a judicious selection of the number of echoes per train and tailored phase-encode reordering algorithms, FSE sequences can be fashioned to yield high-quality Ti -weighted, proton density-weighted, and T2-weighted images far more rapidly than is possible with conventional spin-echo (CSE) sequences [6-i 0]. Furthermore, FSE images retain true spin-echo T2 contrast features rather than the T2* contrast features associated with gradient-echo methods. A number of dinical studies are currently under way, and several have been completed comparing FSE and CSE sequences. The results have shown a promising correlation between the two sequences [9, 10], with the implication that the slower CSE sequences will ultimately be replaced by selected FSE sequences. Improvements in image quality and FSE sequence innovations have been so rapid, however, that many of the techniques implemented and images obtained for comparative purposes have become almost outdated before the completion of controlled clinical trials. This article describes current concepts concerning the optimization of FSE pulse sequences in the brain and spine and comments on the strengths and weaknesses of FSE. Stateof-the-art and potential future applications of this pulse sequence are discussed from a practical perspective.