FLUID-ATTENUATED INVERSION RECOVERY
FLAIR is a T2-weighted sequence in which the CSF signal is suppressed. On traditional T2-weighted MRI, CSF appears bright, whereas on FLAIR this fluid appears dark. As a result, FLAIR is a very useful sequence to detect blood or gadolinium-based contrast agents in the CSF, and can, therefore, readily identify SAH and subdural hemorrhage. Early blood-brain barrier disruption in stroke appears as delayed gadolinium enhancement of hemispheric sulci on FLAIR sequences.Such enhancement—termed hyperintense acute reperfusion marker, or HARM—is associated with reperfusion, a higher risk of hemorrhagic transformation after thrombolytic therapy, and an increase in plasma levels of matrix metalloproteinase-9.
In general, acute ischemic stroke produces no signs on FLAIR in the first 6 h from onset, with areas of hyperintensity evolving thereafter. In patients with unwitnessed onset of stroke, including those whose deficits are present on awakening, a DWI lesion without a matching hyperintensity on FLAIR suggests that the stroke occurred <6 h previously. Studies to evaluate the use of MRI in this setting to select patients for treatment are ongoing.
In patients with acute ischemic stroke, portions of the intracranial arterial tree can appear hyperintense on FLAIR. This hyperintense vessel sign is indicative of slow blood flow distal to the site of acute arterial obstruction. This slowdown in blood flow is attributable to anterograde flow through an incomplete occlusion, or flow via leptomeningeal collaterals, and is associated with a large diffusion-perfusion mismatch, but is not predictive of response to thrombolytic therapy.
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