Patient description: The patient is a 60-year-old woman admitted to the stroke department with a recent history of a second course of intravenous antibiotics for aspiration pneumonia. She is febrile and complained of abdominal pain and diarrhea (bowel movements eight times in 3 hours, large volumes of greenish, liquid stool each time). The patient's abdomen is generally tender and distend with hyperactive bowel sounds. She is febrile (38.4°C), tachycardic (113/min) and hypotensive (80/40 mmHg). The stool samples must were positive for Clostridium difficile toxin.

Clinical trial eligibility criteria: eligible ages (years): 18.0-85.0,  Acute Ischemic Stroke Patients must meet all of the criteria. 1. Diagnosis of acute ischemic stroke with onset less than 4.5 hours prior to the planned start of intravenous alteplase. Acute ischemic stroke is defined as a measurable neurological deficit of sudden onset, presumed secondary to focal cerebral ischemia. Stroke onset will be defined as the time the patient was last known to be without the new clinical deficit. Patients whose deficits have worsened in the last 4.5 hours are not eligible if their first symptoms started more than 4.5 hours before. If the stroke started during sleep, stroke onset will be recorded as the time the patient was last known to be at baseline. 2. Disabling neurological deficit attributable to acute ischemic stroke in the middle cerebral artery territory. 3. NIHSS less than or equal to 18 for left hemisphere strokes, NIHSS less than or equal to 16 for others. 4. Evidence of MCA occlusion (stem or branch) prior to drug administration by TCD, CTA or MRA. 5. Age 18-85 years, inclusive. 6. Able to sign informed consent. For MRI Arm only: 7. Screening MRI diagnostic of focal cerebral ischemia corresponding to the clinical deficits. The MRI evaluation must involve echo planar diffusion weighted imaging, MRA, and MRI perfusion. A normal appearing MRA with an appropriate perfusion deficit is eligible. An apparent stenosis or occlusion on MRA with normal appearing perfusion distally will not be eligible. Poor quality or uninterpretable MRA will not make patient ineligible. Patients who have a normal appearing DWI are eligible. 8. Evidence on PWI MRI or a perfusion defect corresponding to the acute stroke syndrome. The PWI will be assessed by relative mean transit time (MTT) images obtained prior to the start of rt-TPA therapy  Patients will be excluded from study participation for any of the following reasons: 1. Current participation in another study with an investigational drug or device within, prior participation in the present study, or planned participation in another therapeutic trial, prior to the final (day 30) assessment in this trial. 2. Absence of acoustic window to insonate the MCA on the involved side. 3. Time interval since stroke onset of less than 3 hours is impossible to determine with high degree of confidence. 4. Symptoms suggestive of subarachnoid hemorrhage, even if CT or MRI scan is negative for hemorrhage. 5. Evidence of acute myocardial infarction defined as having at least two of the following three features: 1) Chest pain suggestive of cardiac ischemia; 2) EKG findings of ST elevation of more greater than 0.2 mV in 2 contiguous leads, new onset left bundle branch block, ST segment depression, or T-wave inversion; 3) Elevated troponin I. 6. Acute Pericarditis. 7. Women known to be pregnant, lactating or having a positive or indeterminate pregnancy test. 8. Neurological deficit that has led to stupor or coma (NIHSS level of consciousness [item I a] score greater than or equal to 2). 9. High clinical suspicion of septic embolus. 10. Minor stroke with non-disabling deficit or rapidly improving neurological symptoms. 11. Baseline NIHSS greater than 18 for left hemisphere stroke or greater than 16 for others. 12. Evidence of acute or chronic ICH by head CT or MRI. 13. CT or MRI evidence of non-vascular cause for the neurological symptoms. 14. Signs of mass effect causing shift of midline structures on CT or MRI. 15. Persistent hypertension with systolic BP greater than 185 mmHg or diastolic BP greater than 110 mmHg (mean of 3 consecutive arm cuff readings over 20-30 minutes), not controlled by antihypertensive therapy or requiring nitroprusside for control. 16. Anticipated need for major surgery within 72 hours after start of study drugs, e.g., carotid endarterectomy, hip fracture repair. 17. Any intracranial surgery, intraspinal surgery, or serious head trauma (any head injury that required hospitalization) within the past 3 months. 18. Stroke within the past 3 months. 19. History of ICH at any time in the past. 20. Major trauma at the time of stroke, e.g., hip fracture. 21. Blood glucose greater than 200 mg/dl. 22. Presence or history of intracranial neoplasm (except small meninigiomas) or arteriovenous malformation. 23. Intracranial aneurysm, unless surgically or endovascularly treated more than 3 months before. 24. Seizure at the onset of stroke. 25. Active internal bleeding. 26. Major hemorrhage (requiring transfusion, surgery or hospitalization) in the past 21 days. 27. Major surgery, serious trauma, lumbar puncture, arterial puncture at a non-compressible site, or biopsy of a parenchymal organ in last 14 days. Major surgical procedures but are not limited to the following: major thoracic or abdominopelvic surgery, neurosurgery, major limb surgery, carotid endarterectomy or other vascular surgery, and organ transplantation. For non-listed procedures, the operating surgeon should be consulted to assess the risk. 28. Presumed or documented history of vasculitis. 29. Known systemic bleeding or platelet disorder, e.g., von Willebrand's disease, hemophilia, ITP, TTP, others. 30. Platelet counts less than 100,000 cells/micro L. 31. Congenital or acquired coagulopathy (e.g., secondary to anticoagulants) causing either of the following: 1. Activated partial thromboplastin time (aPTT) prolongation greater than 2 seconds above the upper limit of normal for local laboratory, except if due to isolated factor XII deficiency. 2. INR greater than or equal to 1.4. Patients receiving warfarin prior to entry are eligible provided INR is less than 1.4 and warfarin can be safely discontinued for at least 48 hours. 32. Life expectancy less than 3 months. 33. Other serious illness, e.g., severe hepatic, cardiac, or renal failure; acute myocardial infarction; or complex disease that may confound treatment assessment. 34. Severe renal failure: Serum creatinine greater than 4.0 mg/dL or dependency on renal dialysis. 35. AST or ALT greater than 3 times the upper limit of normal for the local laboratory. 36. Treatment of the qualifying stroke with any thrombolytic, anti-thrombotic or GPIIbIIIa inhibitor outside of this protocol. 37. Any administration of a thrombolytic drug in the prior 7 days. 38. Treatment of the qualifying stroke with intravenous heparin unless aPTT prolongation is no greater than 2 seconds above the upper limit of normal for local laboratory prior to study drug initiation. 39. Treatment of the qualifying stroke with a low molecular weight heparin or heparinoid. 40. Known hypersensitivity to TPA. 41. Anticoagulation (evidenced by abnormal INR, aPTT, or platelet count) caused by herbal therapy. FOR non-MRI arm only (#42-43): 42. Ischemic changes on screening CT of greater than approximately one third of the territory of the middle cerebral artery territory by qualitative assessment. 43. Patients who were excluded by screening MRI, except for exclusions #45 (contraindication to MRI) and #46 (PWI was not obtained or is uninterpretable) and #50 (MRI not obtainable because it would have put the patient out of the 3 hour time window for alteplase). FOR MRI arm only (#44-51): 44. Contraindication to MRI scan. 45. PWI not obtained or uninterpretable. 46. No MTT defect corresponding to acute stroke deficit. 47. DWI abnormality larger than approximately one third of the territory of the middle cerebral artery territory by qualitative assessment. 48. Satellite DWI hyperintensity with corresponding hyperintensity on T2 weighted image or FLAIR in a vascular territory different than the index stroke (this is evidence of a new ischemic lesion greater than 3 hours in duration). 49. Evidence of multiple microbleeds on gradient echo MRI (GRE). 50. MRI not obtained because it would have put the patient out of the 3 hour time window for alteplase