Brain ischemia is a condition in which there is insufficient blood flow to the brain to meet metabolic demand. This leads to poor oxygen supply or cerebral hypoxia and thus to the death of brain tissue or cerebral infarction / ischemic stroke.
An acute stroke starts suddenly and typically worsens rapidly. Most strokes are acute strokes. Sometimes, a stroke may be preceded by a transient ischemic attacks (TIA), which is a temporary stroke that reverses before it causes any lasting effects.
The goal of imaging in a patient with acute stroke is:
- Exclude hemorrhage
- Differentiate between irreversibly affected brain tissue and reversibly impaired tissue (dead tissue versus tissue at risk)
- Identify stenosis or occlusion of major extra- and intracranial arteries
CT has the advantage of being available 24 hours a day and is the gold standard for hemorrhage.
Hemorrhage on MR images can be quite confusing.
On CT 60% of infarcts are seen within 3-6 hrs and virtually all are seen in 24 hours.
The overall sensitivity of CT to diagnose stroke is 64% and the specificity is 85%.
On PD/T2WI and FLAIR infarction is seen as high SI.
These sequences detect 80% of infarctions before 24 hours. They may be negative up to 2-4 hours post-ictus!