Medical Imaging Screening for Posterior-circulation Ischemic Stroke

* The ENDOSTROKE registry (Endovascular Stroke T reatment Registry), the use of MRI before EVT was associated with significantly better clinical outcomes as compared to CT. * The purpose of the study will sift patients with posterior circulation ischemia stroke who will get better outcome by Mechanical Thrombectomy by neuroimaging.Specifically, the following assumption will be evaluated: the modified pons-midbrain index in magnetic resonance imaging can predict the clinical outcome of patients with basilar artery thrombosis 90 days after surgery, and can guide whether patients will undergo mechanical thrombectomy later. * Pons-midbrain-medulla index (PMMI) : We divide pons-midbrain-medulla into the left and right sides by the median line of the brainstem,so we will evaluate ischemia of these six layers which are endowed different score according to the degree of ischemia. Instead of the previous pons-midbrain index (PMI), we select the image layer that shows the largest infarct size among the six parts.Then,we add up six scores that is patient's PMMI. Scoring criteria: At the layer of displaying the maximum infarct size, 2 points are scored if the infarct size exceeds 50% of the total area, 1 point is scored if the infarct size is less than 50% of the total area, and 0 points are scored if there is no infarct.Finally, PMMI values range from 0 to 12 points. * The investigators conduct a multicenter, prospective cohort study.Preoperative images of patients are evaluated by different groups of imaging physicians to describe the association between preoperative PMMI and clinical prognosis in patients with posterior circulation stroke and to verify its validity.