TwinTree Insert

08-02 The RARE Pulse Sequence

he RARE sequence (Rapid Acquisition with Relaxation Enhancement; also cal­led Rapid Spin Echo, RSE; Fast Spin Echo, FSE; or Turbo Spin Echo, TSE) was intro­duced by Jürgen Hennig in 1986 [⇒ Hennig 1986]. It is based on the mul­ti­ple echo sequence.

Over the years, RARE has mostly replaced the conventional mul­ti­ple spin-echo pul­se se­quence, which was the most common se­quence used in clinical imaging. Blur­ring in fine detail, however, will hinder a complete re­pla­ce­ment. The classical spin-echo se­quence remains the most solid, reliable, and repro­ducible pul­se se­quen­ce in MR imaging.

Rather than using the same amount of phase-encoding for each echo and each echo as one line for an image associated with a particular TE, in RARE se­quen­ces dif­fe­rent amounts of phase-encoding can be applied to each echo. This en­ables them to be used as different lines in a single image (Figures 08-03 and 08-04).

Figure 08-03:
The RARE pulse sequence. A train of echoes is created and each echo is individually phase-encoded. Usually eight to sixteen echoes are used. There are several different variations with dif­fe­rent styles of gradient switching.

Figure 08-04:
A comparison of (a) a multiple spin-echo and (b) a RARE sequence. Every echo in the SE sequence is used to create an individual image (one echo = one line per image), whereas in a RARE sequence se­ve­ral echoes contribute lines to a single image of the raw data matrix (k-space), as in this example, or to two images.

For example, a multiple echo sequence with 8 echoes, im­plemented as a RARE se­quen­ce can con­tribute 8 phase-encoding lines to a single image, or 4 phase-en­cod­ing lines to two im­ages. This results in reductions of 8 or 4, re­spectively, in the num­ber of excitations re­quired to collect the full data set, while re­taining most of the clinically useful contrast of the spin-echo sequence.

Considering a RARE sequence in which 8 echoes are used to provide 8 lines for a 128×128 image, each line has a different echo time and hence T2 weighting. This is an undesirable constraint of the RARE se­quence, but it can be overcome by altering the assignation of the particular echoes to respective lines in k-space. The probably most effective version of the RARE sequence uses the first half of the echo train to pro­vide the lines for a ρ-image and the second half for a T2-weighted image.

In this way, one retains the clinically useful double-echo sequence, while reducin­g the scan time by a factor of between 2 and 8, depending on the number of echoes used [⇒ Melki 1991].