Patients with ischemic stroke in the posterior circulation continue to have high rates of mortality and disability, even with aggressive treatment. We wanted to evaluate preoperative imaging to screen patients with a good prognosis from mechanical embolization. We assess the degree of ischemia by defining the pons-midbrain-medulla index (PMMI) and correlate the preoperative PMMI with the clinical prognosis of postoperative patients to verify the validity of PMMI in predicting the clinical prognosis of patients with embolization.

• 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.

Inclusion Criteria:

1. Patient admitted from Jun. 1st 2019 to Jan. 1st 2029;
2. Patients with acute ischemic stroke recently；
3. Symptoms persisted at admission, and MRI (including DWI) confirmed acute ischemic vertebrobasilar artery occlusion in the posterior circulation:（single right or left intracranial branch of vertebral artery occlusion ; single basilar occlusion ; both two vertebral arterys and basilar artery）；

Exclusion Criteria:

1.Patiens can not meet the MRI examination conditions(with absolute and relative contraindications).

All acute ischemic strvoke patients with vertebrobasilar artery occlusion.