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Background: CT perfusion (CTP) increases diagnostic confidence in ischaemic stroke. Despite bone-related artefacts, CTP has been shown to improve diagnostic sensitivity compared to non contrast-CT (NCCT) and Computed Tomography Angiography source images (CTA-SI) in posterior circulation strokes. The posterior circulation Acute Stroke Prognosis Early CT score (pcASPECTS) on CTA-SI predicts clinical outcome in patients with basilar artery occlusion (BAO). We assessed the prognostic value of pcASPECTS on CTP in BAO patients.Methods: A retrospective analysis of consecutive patients with BAO diagnosed on CT angiography was performed. pcASPECTS was assessed as evident hypoattenuation on CTA-SI, focally reduced cerebral blood flow (CBF) or cerebral blood volume (CBV), focally increased time to peak of the residue function (Tmax) or mean transit time (MTT). Two investigators independently reviewed the images blinded to clinical outcome. Reliability was assessed with intraclass correlation coefficient (ICC). Good outcome was defined as modified Rankin Scale≤3 at 3 months.Results: We included 43 BAO patients with whole-brain CTP images. In receiver-operating-characteristic (ROC) analysis, the area-under-curve (AUC) was 0.79 (95% CI 0.6-0.9) for pcASPECTS on CBF, 0.77 (95% CI 0.6-0.9) on Tmax, 0.73 (95% CI 0.6-0.9) on MTT, 0.72 (95% CI 0.6-0.9) on CBV, 0.67 (95% CI 0.5-0.8) on CTA-SI. In logistic regression adjusted for age and clinical severity, pcASPECTS<8 was associated with poor outcome on Tmax (OR 14.6, 95% CI 2.3-115; p=0.007) and CBF (OR 15.1, 95% CI 1.3-121; p=0.02). Although CTP AUC did not show higher accuracy in comparison with CTA-SI AUC (p=0.2), in logistic regression, CTA-SI pcASPECTS was not significantly associated with clinical outcome (OR 6.9, 95% CI 0.7-70; p=0.1, adjusted for age, NHSS). Higher pcASPECTS on CBF (OR 0.5 95%CI 0.2-0.9, p=0.04) and Tmax (OR 0.5 95% 0.3-0.9; p=0.02) were associated with lower mortality. Interrater reliability was good for CTP maps (ICC between 0.74 and 0.86 95% CI0.6-0.9 versus 0.66 for CTA-SI 95% CI 0.4-0.8).Conclusions: The CTP pcASPECTS may identify BAO patients at higher risk of disability and mortality.