Fluid-attenuated inversion recovery
Fluid-attenuated inversion recovery is a magnetic resonance imaging sequence with an inversion recovery set to null fluids. For example, it can be used in brain imaging to suppress cerebrospinal fluid effects on the image, so as to bring out the periventricular hyperintense lesions, such as multiple sclerosis plaques. It was invented by Graeme Bydder, Joseph Hajnal, and Ian Young in the early 1990s. FLAIR can be used with both three-dimensional imaging or two dimensional imaging.
Technique
By carefully choosing the inversion time, the signal from any particular tissue can be nulled. The appropriate TI depends on the tissue via the formula:In other words, one should typically use a TI of around 70% of the T1 value. In the case of CSF suppression, the long TI is adjusted to a zero crossing point for water, so the signal of the CSF is theoretically "erased" from the image.
The standard T2 variant of FLAIR uses a long TE to develop T2-weighting and a long repetition time to increase signal to noise ratio, while the T1 variant of FLAIR uses a short echo time and short TR to develop T1-weighting. The TR also controls the SNR efficiency of the scan, which is optimal only for a particular T1.