Hyperacute Arterial Stroke: MR Flair

History

Acute change in neurologic status; rule out stroke.

Exam

Head MR FLAIR Sequence
Arterial stroke is not a single entity, but rather starts as an ictal event generating symptoms and then evolves. However, the ischemic process may have occurred earlier, but was clinically silent, and only becomes symptomatic following clot lysis with downstream secondary embolization or reperfusion hemorrhage. Likewise, collateralization begins immediately. If sufficient the ischemic event ends up as a TIA; if insufficient it ends up a stroke. However, the depth and duration of this event(s) determines whether there is a temporary neurologic deficit, a completed stroke (adds glutamate cascade with vasogenic edema), or a lesser ischemic injury (no glutamate cascade leaving only cytogenic edema). Thus, acute clinical stroke is actually an unstable dynamic process. The CTA venocapillary pool CT density provides information about depth of the ischemic injury at the time of the exam (i.e. is the venocapillary pool normal, or less than normal, or absent), but does not reflect the stroke-age.

Purpose

MR FLAIR can provide information about stroke-age and depth and duration of the oligemic insult, but cannot distinguish these two effects. FLAIR becomes positive 2-3 hours after the stroke ictus from cytogenic edema (with minimal conspicuity). If the stroke is completed it becomes more positive (obvious conspicuity after 4-6 hours) with the onset of the proinflammatory response producing vasogenic edema. In essence, the conspicuity of FLAIR can reflect either the time of onset, and the amount of ischemic injury.

If there is no positive FLAIR the onset is either very early or the ischemic injury is minimal. If minimally positive the onset is either in the hyperacute timeframe or the depth of the stroke is less severe. If clearly positive, the stroke is passed the stroke therapy window and is likely likely to have moderate or greater ischemic injury.
FLAIR positivity proceeds over time peaking in the acute phase (3 hours to 3 days). Thus, FLAIR adds useful information about the stroke timeline and/or the stroke depth and duration only during the initial period of stroke stabilization or if there has been a new event with progression of clinical findings.

Findings on FLAIR, as used in this discussion, is graded as if on a stroke-age timeline (not positive=very early, somewhat positive=early, clearly positive=outside the treatment window). However, the FLAIR conspicuity could just as well be based on the stroke depth and duration (minimal, moderate, or advanced). Nevertheless, the less obvious the FLAIR the earlier the ischemic event for determining stroke therapy or the lesser the stroke injury (which is always good).

It is up to the imager to use FLAIR conspicuity language to best fit the clinical context and the timing of the MR exam relative to the stroke ictus and the interval from the CTA.

Prior Study

CT Head

CTA Final Impression

MR Diffusion

Findings

Evaluate for evidence of hyperacute post ischemic cytogenic edema on FLAIR sequence

There is no apparent abnormal FLAIR in any arterial zone (implying stroke timeframe is very recent < 2-3 hours, and is within the treatment window). [Yes/No]

There is minimally abnormal FLAIR in any site of positive diffusion imaging (implying stroke timeframe is very recent 3-6 hours). [Yes/No]

There is clearly abnormal FLAIR within a recognizable arterial zone (implying stroke timeframe is beyond 6-8 hours, as seen on the prior callout video for Finding 2). [Yes/No]

There is positive FLAIR in an arterial wall or lumen indicating the presence of residual acute thrombus. [Yes/No]

There is sufficient brain swelling or hemorrhage to produce evidence of brain shift and/or herniation. [Yes/No]

Other

Other findings are present more consistent with a diagnosis other than stroke. [Yes/No]

Impression

Hyperacute Stroke - Arterial, Diagnostic: MR Flair

There is no evidence of acute stroke.
There is evidence of acute stroke.
There is an abnormality, but it is related to a process other than stroke.