Division of Neurocritical Care, Department of Neurology, Massachusetts General Hospital, Harvard Medical School, 55 Fruit St, Boston, MA, 02114, USA. dychung@mgh.harvard.edu. Neurovascular Research Unit, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA, USA. dychung@mgh.harvard.edu. Departments of Neurology and Neurosurgery, Alpert Medical School, Rhode Island Hospital, Brown University, Providence, RI, USA. Department of Neurology, Perelman School of Medicine, Hospital of the University of Pennsylvania, University of Pennsylvania, Philadelphia, PA, USA. Division of Neurocritical Care, Department of Neurology, Massachusetts General Hospital, Harvard Medical School, 55 Fruit St, Boston, MA, 02114, USA. Neurovascular Research Unit, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA, USA. Biostatistics Center, Division of Clinical Research, Department of Medicine, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA. Department of Neurosurgery, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA. Department of Neurology, Detroit Medical Center, Wayne State University, Detroit, MI, USA. Department of Neurology, University of Texas Southwestern Medical Center, Dallas, TX, USA. Department of Neurological Sciences, Rush University Medical Center, Chicago, IL, USA.