To determine whether the extent of the ischemic penumbra apparent on perfusion-diffusion MRI can be used to identify patients who would respond positively and safely to tissue plasminogen activator (tPA) beyond 3 hours post-stroke.

Inclusion Criteria:

• Patients who present:
• with acute hemispheric stroke within 3-6 hours of onset,
• have at least moderate limb weakness,
• a National Institute of Health Stroke Scale (NIHSS) score > 4,
• had a pre-stroke modified Rankin Scale (MRS) score of 0 - 2
• and who are able to undergo CT and MRI, are eligible for this study.

Exclusion Criteria:

• Females who are pregnant or breast-feeding,
• persons who have CT-verified hemorrhagic stroke, major ischemia ( > 33% of the middle cerebral artery (MCA) territory infarcted), subarachnoid hemorrhage, arteriovenous malformation, aneurysm, intracranial neoplasm that is terminal or poses a risk of hemorrhage ,
• are comatose or severely obtunded with fixed eye deviation and complete hemiplegia,
• have had another stroke within the past 6 weeks,
• have had a seizure prior to the administration of the study drug,
• have active peptic ulceration, bleeding diatheses, previous intracerebral hemorrhage,
• blood pressure > 185/110,
• major surgery or trauma within the past 30 days, or any other contraindications to tPA
• have a presumed septic embolus or a myocardial infarction within the past 30 days
• blood glucose values are < 2.8 or > 22.0 mmol/L,
• pacemakers, aneurysm clips, implanted devices, claustrophobia, or any other contraindications to MRI,
• decreased consciousness,
• rapid clinical improvement,
• confounding neurological condition (e.g. dementia),
• any other life-threatening illness, or who are participating in another clinical trial, will be excluded from this study.