Vertebrobasilar InsufficiencyStrokeBasilar ArteryBrain InfarctionPosterior Cerebral ArteryVertebral ArteryMagnetic Resonance AngiographyBrain IschemiaInfarction, Posterior Cerebral ArteryThrombolytic TherapyCerebral AngiographyTomography, X-Ray ComputedIntracranial AneurysmFibrinolytic AgentsBrain Stem InfarctionsCerebral ArteriesSubarachnoid HemorrhageArteriesCerebral InfarctionCerebrovascular CirculationIntracranial ArteriosclerosisAneurysm, RupturedProspective StudiesEmbolization, TherapeuticMoyamoya DiseaseVertebral Artery DissectionCerebrovascular DisordersCarotid Artery, InternalAnterior Cerebral ArteryIschemic Attack, TransientMagnetic Resonance ImagingUltrasonography, Doppler, TranscranialIntracranial EmbolismTreatment OutcomePulmonary ArteryCarotid ArteriesArterial Occlusive DiseasesAngiography, Digital SubtractionAngiographyFollow-Up StudiesRetrospective StudiesAcute DiseaseCerebral RevascularizationRisk FactorsAneurysm, DissectingConstriction, Pathologic