Fluid-attenuated inversion recovery

Postcontrast FLAIR of a case of meningitis. It shows hyperintensity of meninges at the tentorium and in the parietal region, with evidence of dilated ventricles.

Fluid-attenuated inversion recovery (FLAIR) 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 (CSF) effects on the image, so as to bring out the periventricular hyperintense lesions, such as multiple sclerosis (MS) 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 (3D FLAIR) or two dimensional imaging (2D FLAIR).

Technique

By carefully choosing the inversion time (TI), the signal from any particular tissue can be nulled. The appropriate TI depends on the tissue via the formula:

${\textstyle {\textrm {TI}}=\ln(2)\cdot T_{1}}$

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 (inversion time) is adjusted to a zero crossing point for water (none of its signal is visible), 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 (TR) to increase signal to noise ratio (SNR), while the T1 variant of FLAIR uses a short echo time (TE) 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.

Clinical applications

The FLAIR sequence analysis has been especially useful in the evaluation and study of CNS disorders, involving:

• Lacunar infarction
• Multiple sclerosis (MS) plaques
• Subarachnoid haemorrhage
• Head trauma
• Meningitis and other leptomeningeal diseases

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  Axial fluid-attenuated inversion recovery MRI image demonstrating tumor-related infiltration involving lenticular nuclei (Arrow).
• 
  Axial fluid-attenuated inversion recovery MRI image demonstrating tumor-related infiltration involving both temporal lobes (Short arrow), and the substantia nigra (Long arrow).

See also

• Relaxation (NMR)

Further reading

• McRobbie DW, Moore EA, Graves MJ, Prince MR (April 2017). MRI from Picture to Proton. Cambridge University Press. pp. 40–42.
• Hashemi RH, Bradley Jr WG, Lisanti CJ (eds.). MRI: The Basics (2nd ed.). Philadelphia. p. 272.