How do ischemic strokes appear on imaging?
On computed tomography (CT), most ischemic strokes eventually become visible as hypodensities of the brain parenchyma, but CT is largely normal for at least 6 hours.
The earliest CT signs are loss of the cortical ribbon and gyral edema. On magnetic resonance imaging (MRI), strokes will appear hyperintense on T2-weighted sequences.
Diffusion-weighted imaging (DWI) detects cytotoxic edema that accompanies ischemic cell death and can appear very shortly after onset of ischemia. True diffusion positivity will appear dark on the apparent diffusion coefficient (ADC) map sequence.
Onset of DWI positivity may be delayed up to 48 to 72 hours, particularly in the brain stem, and typically lasts 10 to 14 days.
The ADC map normalizes more quickly, often 4 to 7 days after onset.
