(CISS) This gradient echo sequence is a stimulated T2 echo. Two TrueFISPsequences are acquired with differing RF pulses and than combined for strong T2 Weighted high resolution 3D images.
These TrueFISPsequences are normally affected by dark phase dispersion bands, which are caused by patient induced local field inhomogeneities and made prominent by the relatively long TR used. The different excitation pulse regimes offset these bands in the 2 sequences. Combining the images results in a picture free of banding. The image combination is performed automatically after data collection, adding some time to the reconstruction process.
The advantage of the 3D CISS sequence is its combination of high signal levels and extremely high spatial resolution. Used for, e.g. inner ear, cranial nerves and cerebellum.
(DESS) This sequence was originally known as FADE. It combines both the gradient echoes acquired in FISP and PSIFsequences in separate acquisition periods during a single interpulse interval.
Phase encoding gradients are balanced to maintain the transverse steadystate signals.
The frequency encoding gradient is left on for the period of both the echoes, and is incompletely balanced to avoid dark banding artifacts otherwise associated with long TR fully balanced steadystatesequences.
The contrast of DESS is quite unique, true T2 or T1 contrast weighting is not possible. There is a strong fluid signal but fat is bright and other soft tissues appear similar to the short TR FISP image.
Used for, e.g. the joints, cartilage and the prostate.
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