Fig. 3.

Chronic watershed infarcts resulting from nearly complete occlusion of the cervical internal carotid arteries at the carotid bifurcation. Axial FLAIR MRI from patient 3 demonstrates (a) right frontal cortical infarct and bilateral, left greater than right, chronic infarcts in the white matter of the centrum semiovale at the boundary of the anterior and middle cerebral artery territories. Volume rendered 3D TOF MRA of the intracranial vasculature (b, inferior projection; c, left lateral projection) shows complete absence of flow-related enhancement in the intracranial carotid arteries (asterisk). Note the large vertebrobasilar system and, in particular, the marked enlargement of the posterior communicating arteries that supply collateral flow to the anterior circulation (white arrows). An angiographic “string sign” is seen in the cervical ICA just above the carotid bifurcation (d, left anterior oblique projection of 2D time of flight MRA).