Specific Imaging Findings

In the hyperacute stage (< 6 h), non-contrast CT is negative in anywhere from 20 to 70% of cases, the earlier obtained after ictus the more likely it is to appear normal. Early changes of cerebral infarction on CT include loss of the gray–white matter (GM–WM) differentiation (loss of the insular ribbon, deep gray matter definition, and focal cortico-subcortical differentiation), effacement of the cortical sulci, and a hyperdense vessel (usually middle cerebral artery) suggestive of acute thrombus. Viewing with stroke windows (such as W 30, L 30) often improves the conspicuity of the findings, especially the loss of the GM–WM differentiation. Diffusion MRI is by far the most sensitive technique for detection of hyperacute infarcts and becomes positive within 30 min after vessel occlusion. Infarcted tissue shows hyperintensity on DWI and low signal on ADC maps, consistent with reduced diffusion. Abnormalities on T2WI and FLAIR images usually become evident 3–6 h after onset, as increased signal intensity and mild swelling of the infarcted tissue. A hyperintense vessel may be seen on FLAIR, corresponding to CT hyperdensity. Contrast-enhanced T1WI may demonstrate arterial enhancement secondary to slow flow, collateral flow, or hyperperfusion following early recanalization. Parenchymal, frequently “gyriform” enhancement may occasionally appear early, suggesting higher hemorrhage risk. CTA and MRA demonstrate arterial occlusions and critical stenoses as well as the status of collateral vessels.