Figure 1.

Schematic representation of different possible perfusion scenarios using the example of a patient with low vessel caliber in the Acom and the right Pcom. (A) Brain perfusion under normal conditions. (B) Occlusion of the left middle cerebral artery (MCA). As the occlusion site is distal to the circle of Willis (CoW), collateral flow could be expected only via usually not sufficient surface leptomeningeal collaterals through the watershed areas. Thus, the vascular core territory of the MCA will not be supplied adequately. (C) Occlusion of the left internal carotid artery (ICA). Collaterals via the Acom and left Pcom are sufficiently strong in this patient to maintain perfusion of all brain regions (‘good' collaterals, asymptomatic). (D) Occlusion of the right ICA. As the Acom and the right Pcom are not strong enough in this patient, in this situation the territories of the right ACA and MCA will not be supplied sufficiently (‘bad' collaterals, symptomatic).