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 FLAIR pulse 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 T2 contrast are made visible by the dark fluid technique FLAIR is an important technique for the differentiation of brain and spine lesions.

See also Inversion Recovery.

Fluoroscopic triggering is a method (see automatic bolus detection) used to time the start of a contrast enhanced dynamic or MRA sequence. After the bolus injection of a contrast medium, the bolus can be tracked with a real time sequence. The operator starts the sequence, when the contrast is visual identified on the monitor.

Invisible lines of force that extend around a magnetic material. The greatest density is at the two poles of the magnet.

The number of lines of force per unit area of a magnetic material.