(FSE) In the pulse sequence timing diagram, a fast spin echo sequence with an echo train length of 3 is illustrated. This sequence is characterized by a series of rapidly applied 180° rephasing pulses and multiple echoes, changing the phase encoding gradient for each echo.
The echo time TE may vary from echo to echo in the echo train. The echoes in the center of the K-space (in the case of linear k-space acquisition) mainly produce the type of image contrast, whereas the periphery of K-space determines the spatial resolution. For example, in the middle of K-space the late echoes of T2 weighted images are encoded. T1 or PD contrast is produced from the early echoes.
The benefit of this technique is that the scan duration with, e.g. a turbo spin echo turbo factor / echo train length of 9, is one ninth of the time. In T1 weighted and proton density weighted sequences, there is a limit to how large the ETL can be (e.g. a usual ETL for T1 weighted images is between 3 and 7). The use of large echo train lengths with short TE results in blurring and loss of contrast. For this reason, T2 weighted imaging profits most from this technique.
In T2 weighted FSE images, both water and fat are hyperintense. This is because the succession of 180° RF pulses reduces the spin spin interactions in fat and increases its T2 decay time. Fast spin echo (FSE) sequences have replaced conventional T2 weighted spin echo sequences for most clinical applications. Fast spin echo allows reduced acquisition times and enables T2 weighted breath hold imaging, e.g. for applications in the upper abdomen.
In case of the acquisition of 2 echoes this type of a sequence is named double fast spin echo / dual echo sequence, the first echo is usually density and the second echo is T2 weighted image. Fast spin echo images are more T2 weighted, which makes it difficult to obtain true proton density weighted images. For dual echo imaging with density weighting, the TR should be kept between 2000 - 2400 msec with a short ETL (e.g., 4).
Other terms for this technique are:
Turbo Spin Echo
Rapid Imaging Spin Echo,
Rapid Spin Echo,
Rapid Acquisition Spin Echo,
Rapid Acquisition with Refocused Echoes

(CPS) Sequence of a 90° RF pulse followed by repeated 180° RF pulses to produce a train of spin echoes; is useful for measuring T2.

(CPMG) This type of spin echo pulse sequence consisting of a 90° radio frequency pulse followed by an echo train induced by successive 180° pulses and is useful for measuring T2 weighted images. It is a modification of the Carr-Purcell RF pulse sequence, with 90° phase shift in the rotating frame of reference between the 90° pulse and the subsequent 180° pulses in order to reduce accumulating effects of imperfections in the 180° pulses. Suppression of effects of pulse error accumulation can alternatively be achieved by switching phases of the 180° pulses by 180°.

(DFSE) Simultaneously acquired T2 and density weighted TE in FSE echo images.

(DTSE / DE TSE) Simultaneously acquired T2 and density weighted echoes in a TSE sequence.

See also Fast Spin Echo.

(DEFAISE) Simultaneously acquired T2 and density weighted echoes in a FSE sequence.

See Fast Spin Echo.

(HASTE) A pulse sequence with data acquisition after an initial preparation pulse for contrast enhancement with the use of a very long echo train (Single shot TSE), whereat each echo is individually phase encoded. This technique is a heavily T2 weighted, high speed sequence with partial Fourier technique, a great sensitivity for fluid detection and a fast acquisition time of about 1 sec per slice. This advantage makes it possible for using breath-hold with excellent motionless MRI, e.g. used for liver and lung imaging.

See also Segmented HASTE.

(MESS) See Multiple Echo Imaging, Single Shot Technique and Ultrafast Gradient Echo Sequence.

(RARE) If the image lines from multiple echoes are used for the same image, this results in the RARE pulse sequence. The sequence is similar to fast spin echo.

See Fast Spin Echo.

(TSE) A pulse sequence characterized by a series of rapidly applied 180° rephasing pulses and multiple echoes.

See Fast spin echo.

(UTSE) The ultrashort turbo spin echo (TSE / FSE) sequence is a technique with extremely short echo spacing, resulting in shorter scan times. This is an advantage in areas where motion is a problem, for example dynamic or abdominal imaging. The shorter scan time and echo spacing are achieved by using a higher TSE factor and an increased data sampling rate.
Disadvantages are the decrease in SNR (caused through the increase of the bandwidth) and artifacts if minimum echo spacing is used (incomplete dephasing of the 180° pulse FID).

See also Fast Spin Echo.