Fast Spin Echo Imaging

Abstract

The images illustrated in Fig. 34.1 were acquired using (a) conventional and (b) fast spin echo techniques. The use of fast or turbo spin echo (FSE, TSE) imaging has become routine in MRI today. A spin echo sequence employs a 180° RF pulse (typically) to create the echo, which also corrects for dephasing effects from slight field inhomogeneities and chemical shift. In a conventional spin echo sequence, a phase encoding gradient of defined amplitude is applied prior to the collection of the echo during readout. The amplitude of the phase encoding gradient determines the line in k-space that will be filled as the echo is sampled. In a conventional spin echo sequence, one line of k-space is filled during each repetition (TR period) of the pulse sequence. In an FSE sequence, a series of 180° pulses produces a train of echoes during a single TR period, as illustrated in Fig. 34.2. The number of echoes produced in a single TR period is known as the echo train length (ETL). The phase encoding gradient amplitude will vary prior to each echo in the train so that each echo will fill a different line of k-space. In this way, multiple lines of k-space are filled during a single TR period. The number of lines filled in a single TR thus also corresponds to the ETL. As an example, using an ETL of 16, 16 lines of k-space will be filled during a single TR period. If a phase encoding matrix of 256 is selected, rather than requiring 256 repetitions of the pulse sequence to fill all the lines of k-space (assuming 1 for the number of signals averaged [NSA]), only 16 repetitions would be required (256/16 = 16). Increasing the ETL to 32 would require only 8 repetitions to fill all 256 lines of k-space. The use of FSE sequences has not only greatly reduced the time required to obtain MR images with a long TR, but also allows the use of longer TR times for improved tissue contrast.