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Background: Collateral status has been shown to be a potent determinant of long-term clinical outcomes in subjects with intracranial atherosclerosis, yet the grading of collaterals can be challenging. We developed a simple collateral metric of spatial and temporal hemodynamic changes at the anterior borderzone in MCA stenoses.Methods: Conventional angiography acquired at baseline in SAMMPRIS was analyzed in the subset of MCA stenoses. Two readers independently measured the anterior watershed angle (AWSA) or borderzone shift on AP views. The angle and the relative timing of arterial flow of the MCA and ACA were compared with the previously recorded collateral composite of TICI antegrade flow combined with compensatory ASITN grade.Results: 176/195 (90%) subjects with MCA stenoses in SAMMPRIS had baseline angiography with AP projections adequate for both the spatial and temporal characterization of the anterior borderzone, with previously defined collateral status in 165/195 (85%). AWSA ranged from 16-65° (mean RR°, SD TT). Arterial opacification at this borderzone revealed synchronous ACA and MCA filling in 116 or 59% of cases, delayed ACA collaterals in 54/176 (31%) and early collaterals in 6/176 (3%). Inter-rater reliability was excellent (IRR=0.87). AWSA > 30° was associated with impaired MCA TICI flow, yet the relative arrival of arterial collaterals varies extensively. AWSA was closely related (r=-.72, p<0.001) to the previously established TICI score of antegrade flow reduction in the MCA. The combined spatial and temporal data of AWSA and associated arterial filling exhibited strong association (AUC of 96% for nonlinear regression) with SAMMPRIS collateral status (impaired, normal, robust collaterals).Conclusions: A novel metric incorporating both the degree of borderzone shift and arterial collateral filling may be easily and reliably quantified to determine collateral status, a strong predictor of outcome in intracranial atherosclerosis. Validation of this simple marker and correlation with noninvasive imaging features are proceeding.