Collateral blood flow in a patient with a proximal basilar artery occlusion

Vertebral artery angiogram, lateral view. The proximal portion of the basilar artery is occluded. Blood flow is seen in the intracranial vertebral artery (A), which gives off the posterior inferior cerebellar artery (PICA) (B), which in turn anastamoses with the superior cerebellar artery (SCA) (C). From there, blood flows over the cerebellum to fill the rostral portion of the basilar artery (D).