FIG 1. Scoring of the anatomic extent of pial collateral blood flow from the ACA territory to the MCA territory during occlusion of the M1 segment. Scoring corresponds to the angiographically visible retrograde reconstitution of the MCA segments on the delayed venous phase. Each color is depicts the furthest extent of retrograde opacification depicted on anteroposterior cerebral angiograms for each pial collateral score.