01/02/2026
In the world of MRI, FLAIR (Fluid Attenuated Inversion Recovery) is often considered the "bread and butter" of neuroimaging. As a tech, if I see a brain order, I’m almost certainly running a FLAIR.
Here is a technical and clinical breakdown of why this sequence is so vital.
1. The Core Mechanism: "Nulling" the Water
The "magic" of FLAIR is in its name: Fluid Attenuation. In a standard T2-weighted image, CSF (cerebrospinal fluid) is bright white. Because many brain pathologies (like strokes or MS plaques) are also bright on T2, they can "hide" next to the bright CSF in the ventricles or sulci.
How it works: We apply an initial 180
∘
inversion pulse. We then wait for a specific amount of time—called the Inversion Time (TI)—until the signal from the moving water (CSF) reaches the "null point" (zero signal).
The Result: The CSF turns pitch black, while the rest of the T2-weighted tissue properties remain bright. This makes abnormalities "pop" against the dark background.
2. Clinical Applications
FLAIR is the most sensitive sequence for detecting subtle lesions near the fluid-filled spaces of the brain.
Multiple Sclerosis (MS): It is the gold standard for seeing Dawson’s Fingers (periventricular lesions). Without FLAIR, these white spots would blend into the white CSF of the ventricles.
Subarachnoid Hemorrhage (SAH): FLAIR is incredibly sensitive to blood in the sulci. If there is acute bleeding, the normally black CSF spaces will appear bright or "dirty."
Infarction (Stroke): While DWI is faster, FLAIR shows us when a stroke has "matured." If a stroke is visible on DWI but not on FLAIR, it’s often within the thrombolysis window (less than 4.5 hours old).
Cortical Dysplasia: In epilepsy cases, FLAIR helps us find subtle thickening or blurring of the grey-white matter interface.
3. Advantages vs. Limitations
Advantages
High Contrast: Exceptional contrast-to-noise ratio for edema and inflammation.
Lesion Conspicuity: You don't have to squint to find a lesion; the dark CSF background does the work for you.
Versatility: It can be acquired in 2D or 3D (which allows for reconstructed views in any plane).
Limitations
Flow Artifacts: Sometimes, slowly moving CSF (especially in the posterior fossa) doesn't null perfectly, creating "fake" bright spots that can look like disease.
Scan Time: Because of the long Inversion Time (TI), FLAIR is one of the longer sequences in a protocol (usually 4–6 minutes).
Supplemental Oxygen: If a patient is on high-flow oxygen, it can sometimes change the signal in the sulci, mimicking a hemorrhage.
4. Quick Tech Specs
If you were looking at the scanner console, a typical 3T Brain FLAIR looks like this:
TR (Repetition Time): Very Long (~9,000 ms)
TE (Echo Time): Long (~120 ms)
TI (Inversion Time): ~2,500 ms (This is what kills the CSF signal)
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