FLAIR sequence

FLAIR stands for Fluid-Attenuated Inversion Recovery. It is a structural MRI sequence designed to suppress the signal from cerebrospinal fluid while keeping other tissues bright. The result is a scan where the ventricles and other CSF spaces appear dark, and pathology in the surrounding tissue stands out.

It has become the standard view for assessing white matter abnormalities in clinical and research neuroimaging.

Why it matters

On a conventional T2-weighted scan, both CSF and many lesions appear bright, which can make small lesions near the ventricles hard to see. FLAIR fixes this by selectively suppressing the CSF signal through a sequence of radiofrequency pulses.

The result is that lesions, especially those near fluid spaces, become much more conspicuous.

What it shows

FLAIR is the workhorse sequence for visualising:

• White matter hyperintensities of small-vessel disease
• Multiple sclerosis plaques
• Subacute stroke
• Periventricular oedema
• Encephalitis and infectious lesions
• Cortical malformations

It is part of any standard neurological MRI protocol, paired with T1-weighted and diffusion imaging.

How it compares

A simple mnemonic for the three core structural sequences:

• T1: anatomy. White matter is bright, grey matter is darker, CSF is dark.
• T2: fluid is bright. Useful for oedema and many lesions.
• FLAIR: like T2, but with CSF suppressed. Lesions near the ventricles become visible.

In automated WMH segmentation pipelines, FLAIR is the primary input.