What Is FLAIR on a Brain MRI?
FLAIR is a T2-like brain MRI sequence that turns down the bright signal from spinal fluid, making small lesions in the brain and around the ventricles much easier to see.
Plain-language definition
FLAIR stands for fluid-attenuated inversion recovery. It's a special kind of T2-weighted MRI sequence used almost exclusively in brain imaging. A normal T2 image lights fluid up brightly — which is usually a good thing, but the bright cerebrospinal fluid (CSF) inside the ventricles and around the brain can wash out small lesions sitting right next to it. FLAIR fixes that by applying an "inversion pulse" that knocks the CSF signal down to near-black while keeping other water-rich abnormalities bright.
The result is a brain image where ventricles look dark (like on T1) but where lesions in the brain tissue stay bright (like on T2). Anything sitting next to the ventricles — a multiple sclerosis plaque, a small-vessel ischemic change, a low-grade tumor — pops out clearly.
Why it matters
FLAIR is one of the most diagnostically valuable sequences in modern neuroradiology. It's the workhorse for evaluating:
- Multiple sclerosis. Periventricular and juxtacortical plaques stand out on FLAIR. Most published MS diagnostic criteria reference FLAIR findings.
- Small-vessel ischemic disease. The patchy white-matter changes commonly seen in patients with longstanding hypertension, diabetes, or aging are described on FLAIR images.
- Stroke (subacute and chronic). While the acute stroke is best seen on diffusion-weighted imaging, the older, healed stroke shows up clearly on FLAIR.
- Encephalitis, low-grade gliomas, and many other lesions where the abnormal tissue contains extra water but sits right beside the CSF.
When your report uses phrases like "FLAIR hyperintense foci in the periventricular white matter" or "no abnormal FLAIR signal," the radiologist is describing what they see on this specific sequence.
How FLAIR shows up in a Precise Imaging context
Every Precise brain MRI — and every spine MRI that includes brain views — has FLAIR as part of the protocol. It's a longer sequence (typically 3–5 minutes) than a basic T2, and the technologist will let you know during the scan when it's coming.
Because FLAIR is sensitive to motion, you may be reminded to stay especially still during that portion of the exam. There is no contrast required, no breath-holds, and no patient prep specific to FLAIR — the standard MRI safety screening covers everything.
Disclaimer
This explainer is general patient education. It is not a substitute for the personalized interpretation of your scan by your ordering provider and the reading radiologist. FLAIR findings must always be read in the context of your clinical history.
Frequently asked questions
Is FLAIR the same as T2?
They're related but distinct. FLAIR keeps T2's bright signal for most water-containing abnormalities while suppressing the bright signal from CSF. That makes lesions next to the ventricles much easier to see.
Why does my brain MRI report mention "FLAIR hyperintensities"?
That phrase means there are areas in the brain tissue that appear brighter than the surrounding normal brain on FLAIR images. The cause depends on age, distribution, and clinical context — common causes include small-vessel disease, MS, prior injury, or migraine-related changes. Your provider interprets the meaning for you.
Is FLAIR used outside the brain?
Rarely. FLAIR is designed around the magnetic properties of CSF. A specialized FLAIR variant is sometimes used in the spine and inner ear, but for most body imaging the standard T2 family of sequences is used.
Does FLAIR require contrast?
No. FLAIR is a non-contrast sequence. Post-contrast images use a different (T1-based) sequence.
Why is FLAIR so long?
FLAIR uses a long preparation pulse to suppress CSF signal. That preparation lengthens the sequence compared with basic T2. The extra image quality is worth the extra minute or two.