(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.
(CASL) In order to take spins in a different magnetic state to that of the static tissue a train of RF pulses repeatedly saturates blood water spins flowing through the neck. The saturated (or 'labeled') spins flow into the brain and, assuming water is a freely diffusible tracer, exchange completely with brain tissue water, thus reducing the overall tissue magnetization. A steady state progresses where the regional magnetization in the brain is directly related to cerebral blood flow.