The occurrence of a low signal in regions of flow. For a spin echo sequence, this is caused in part by a lack of refocusing of blood, which is excited by the 90° pulse but not by the 180° pulse. For a gradient echo sequence, this is caused by the dephasing of blood signal.
(FLAIR) Fluid attenuation inversion recovery is a special inversion recovery sequence with long TI to remove the effects of fluid from the resulting images. The TI time of the FLAIRpulse sequence is adjusted to the relaxation time of the component that should be suppressed. For fluid suppression the inversion time (long TI) is set to the zero crossing point of fluid, resulting in the signal being 'erased'.
Lesions that are normally covered by bright fluid signals using conventional T2contrast are made visible by the dark fluid technique
FLAIR is an important technique for the differentiation of brain and spine lesions.