PROJECT SUMMARY . Acute ischemic stroke is the leading cause of disability in the United States and the second-leading cause of death worldwide. AIS that involves a major cervical or cerebral artery is termed a large vessel occlusion, and recent landmark randomized studies found that endovascular thrombectomy is an effective treatment for ischemic stroke caused by large vessel occlusion of the internal carotid, middle, or anterior cerebral arteries (anterior circulation). However, up to 15% of large vessel occlusions occur in the vertebral or basilar arteries, and these posterior circulation stroke patients were largely excluded from modern endovascular thrombectomy trials. Clinical outcomes in patients with vertebral or basilar artery occlusions are often poor with severe disability or death occurring in 30-54% and 36-86% of patients, respectively. There are no prospective or randomized data designed to determine which imaging strategies should be used to guide thrombectomy treatment decisions in this understudied population. PRECISE (PeRfusion imaging to identify postErior CIrculation candidateS for thrombEctomy) is a prospective cohort study of patients with acute ischemic stroke due to occlusion of the vertebral or basilar artery within 24- hours of symptom onset. Patients will undergo CT or MRI cerebral perfusion imaging prior to endovascular thrombectomy treatment. The results of this study will determine if cerebral perfusion imaging can identify a subset of patients who are most likely to have a favorable outcome after thrombectomy treatment. PRECISE has the potential to improve the imaging evaluation of patients with acute ischemic stroke of the posterior circulation, to provide valuable prognostic information regarding thrombectomy efficacy in these patients, and to define sub-groups of patients who might benefit from future neuroprotective strategies.