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Introduction: We sought to validate ultrasound as a reliable means of assessing vessel stenosis of vertebral artery origins.Methods: We reviewed 1135 patient charts with ultrasound of the posterior circulation performed 2008-2015 in our Neurovascular Laboratory Imaging Institute. Inclusion criteria consisted of ultrasound and digital subtraction angiography (DSA) performed within three months as well as absence of prior stent placement, resulting in 218 vessels in 133 patients. Using DSA as the gold standard, we determined sensitivity and specificity of ultrasound in detecting occlusion at vertebral artery origin. All patients with non-occluded vertebral artery origins without stent were evaluated for degree of stenosis on DSA, and compared to mean flow velocity (MFV) on ultrasound.Results: Among 218 vertebral artery origins evaluated, ultrasound showed sensitivity of 85.7% (95% confidence interval (CI) 69.7-95.2%) for occlusion and specificity of 99. 5% (95%CI 96.9-99.9%). Among 126 arteries without occlusion, <50% stenosis had average MFV 39.5 cm/s (SD 19.9), 50-69% stenosis had average MFV 69.2 cm/s (SD 34.7), and severe 70-99% stenosis had average MFV 129 cm/s (SD 30.65), p<0.001 by Kruskal-Wallis test. For detecting ≥70% stenosis, c-statistic of ROC curve was 0.81, and MFV of 70 cm/s has 82% sensitivity and 88% specificity of ≥70% stenosis.Conclusion: Ultrasound has good sensitivity and excellent specificity for detecting vertebral origin occlusion. Flow velocity can be used to screen for severe stenosis of vertebral artery at origin.