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FIG 2. Anteroposterior (A and C) and lateral (B and C) images from a left internal carotid artery angiogram obtained during the early (A and B) and delayed angiographic phases (C and D) in a patient with acute ischemic stroke due to occlusion at the M1 segment (arrow). Note the retrograde opacification of the MCA branches via pial collateral vessels extending from the ACA (arrowheads). Because there is reverse opacification of the MCA extending to the distal M1 segment, a pial collateral formation score of 1 was assigned.
