entry slice phenomenon
Entry slice phenomenon occurs when unsaturated spins in blood first enter into a slice or slices. It is characterized by the bright signal in a blood vessel (artery or vein) at the first slice that the vessel enters. Usually, the signal is seen on more than one slice, fading with distance. This mechanism is used in a positive fashion to generate flight MR angiograms.
This artefact has been confused with thrombosis with disastrous results. The characteristic location and if necessary, the use of gradient echo flow techniques can be used to differentiate entry slice artefacts from occlusions.
Spatial saturation bands place before the first slice and after the last can be used to eliminate this artefact.