Currently, the only approved therapy for acute stroke patients is a medication called tissue plasminogen activator (tPA). This medication is given as an intravenous infusion and can dissolve blood clots, thereby restoring blood flow to the brain. Early restoration of blood flow can prevent the permanent damage to the brain which typically occurs after a stroke. As a result, patients who achieve early restoration of blood flow have less disability than stroke patients in whom blood flow is not restored. Unfortunately, only a very small fraction of stroke patients is treated with tPA and benefits from tPA. Nationwide only 3 percent of stroke patients receive this therapy. The short treatment time-window is one of the main reasons that patients are not eligible for this treatment. Previously, tPA was only recommended in the 0 - 3 hour time window after stroke onset, but recent studies have shown efficacy our to 4 ½ hours. AHA guidelines now recommend treatment with iv tPA up to 4 ½ hrs. However, the number of stroke patients who will benefit from treatment remains small despite expansion of the time-window from 3 to 4 ½ hrs. This is the result of two main limitations of tPA. First, the majority of stroke patients present beyond the 4 ½ hour time-window and will therefore remain ineligible for treatment. Second, stroke patients who receive tPA do not always benefit because the treatment does not restore blood flow in all patients. Our research has shown that depending on the location of the blood clot, blood flow is restored in only 20 to 50% of stroke patients treated with tPA.