The time to initiate intravenous thrombolysis for acute ischemic stroke is generally limited to within 4.5 hours after the onset of symptoms. Some trials have suggested that the treatment window may be extended in patients with ischemic but not yet infarcted brain tissue on imaging. New research findings are summarized in a short video:
https://www.nejm.org/do/10.1056/NEJMdo005512/full/
You can see the full original article at: 
Thrombolysis Guided by Perfusion Imaging up to 9 Hours after Onset of Stroke
