The study is a prospective multi-center observational cohort study of patients with symptomatic angiographically confirmed vertebrobasilar atherostenosis (≥ 50%), or occlusion). Upon enrollment, patients will undergo hemodynamic assessment with noninvasive MR imaging (including QMRA and MR perfusion), the results of which will be kept blinded from treating physicians and the patients. Patients will be prospectively designated as demonstrating compromised or normal distal cerebral flow based upon an existing validated algorithm of individual posterior circulation vessel flow measurements. Baseline demographic, clinical and laboratory data will be gathered. Subsequently, patients will have monthly clinical follow-up and be re-imaged with QMRA at 6 month intervals for a minimum of 12 months. The primary endpoint will be stroke incidence in the vertebrobasilar territory at one year. Survival analysis methods, with censoring of patients not achieving endpoint at the end of the study period, will be used for comparison of patients with compromised versus normal blood flow.