(DIR or DIRT1) Double inversion recovery T1 measurement is a T1 weighted black blood MRA sequence in which the signal from blood is suppressed. The inversion time to suppress blood is described as the duration between the initial inversion pulse and time point that the longitudinal magnetization of blood reaches the zero point. The readout starts at the blood suppression inversion time (BSP TI) and blood in the imaging slice gives no signal. This inversion time is around 650 ms with a 60 beat per minute heart rate at 1.5 T.
The TI can be decreased by using a wider receive bandwidth, shorter echo train length and/or narrow trigger window. Wide bandwidth also decreases the blurring caused by long echo trains at the expense of signal to noise ratio. In case of in plane or slow flow the suppression of the signal from blood may be incomplete. With increased TE or change of the image plane the blood suppression can be improved.
Double inversion recovery is a breath hold technique with one image per acquisition used in cardiovascular imaging. The patient is instructed to hold the breath in expiration (if not possible also inspiration can be taken), so that the end diastolic volume in the cardiac chambers would be the same during entire scanning. DIR provides fine details of the boundary between the lumen and the wall of the cardiac chambers and main vascular and heart structures, pericardium, and mediastinal tissues.

(FAIR) In this sequence 2 inversion recovery images are acquired, one with a nonselective and the other with a slice selective inversion pulse. The z-magnetization in the first sequence is independent of flow. Inflowing spins give z-magnetization from second pulse. A major signal loss in FAIR is the T1 relaxation of tagged blood in transit to the imaging slice. Sharper edges of the inversion pulse give narrow spacing between the inversion edge and the 1st slice because reduced transit time gives lower T1 relaxation induced signal loss. The difference of the images in a consequence contains information proportional to flow (blood partition coefficient). Standard adiabatic inversion RF pulse does not have good slice-profile, because of power/SAR limitation. A c-shaped frequency offset corrected inversion (FOCI) RF pulse can help to increase the signal.
Perfusion imaging, e.g. myocardial, using tissue water as endogenous contrast is suggested.

(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.

(IR) Inversion recovery is an MRI technique, which can be incorporated into MR imaging, wherein the nuclear magnetization is inverted at a time on the order of T1 before the regular imaging pulse-gradient sequences. The resulting partial relaxation of the spins in the different structures being imaged can be used to produce an image that depends strongly on T1. This may bring out differences in the appearance of structures with different T1 relaxation times. Note that this does not directly produce an image of T1. T1 in a given region can be calculated from the change in the MR signal from the region due to the inversion pulse compared to the signal with no inversion pulse or an inversion pulse with a different inversion time. This sequence involves successive 180° and 90° pulses. The inversion recovery sequence is specified in terms of three parameters, inversion time (TI), repetition time (TR) and echo time (TE).

See also Inversion Recovery Sequence and FLAIR.

(IRSE) Form of inversion recovery imaging in which the signal is detected as a spin echo. For TE short compared to the T2 relaxation time, there will be only a small effect of T2 differences on image intensities; for longer TE's, the effect of T2 may be significant.