Acute thromboembolic infarction in a 57-year-old: Axial non-contrast CT scan demonstrates left hyperdense middle cerebral artery (MCA) sign (blue arrow), with increased density in the M1 segment of the left MCA, consistent with acute thromboembolic occlusion. There is subtle loss of gray-white matter differentiation, hypodensity, and engorgement along the left insular ribbon and temporal operculum (loss of insular ribbon sign), reflecting acute infarction (green arrow).