Aged Aged, 80 and over Brain Ischemia--diagnostic imaging Cerebral Infarction--diagnostic imaging Cerebrovascular Circulation--physiology Contrast Media Endovascular Procedures Female Fibrinolytic Agents--therapeutic use Humans Magnetic Resonance Imaging Male Middle Aged Outcome Assessment, Health Care Prognosis Prospective Studies Stroke--diagnostic imaging Tissue Plasminogen Activator--therapeutic use Tomography, X-Ray Computed--methods