Assess the Penumbra System in the Treatment of Acute Stroke

The purpose of this trial is to assess the safety and effectiveness of the Penumbra System as an adjunctive treatment to intravenous (IV) recombinant human tissue plasminogen activator (rtPA)in patients with acute ischemic stroke from large vessel occlusion in the brain. IV rtPA is the only drug approved for the treatment of acute ischemic stroke but it does not work very well in cases where the stroke is caused by a large vessel occlusion. The hypothesis being tested is to determine if the addition of a treatment by a mechanical thrombectomy device like the Penumbra System can improve the clinical outcome of the patient over just using IV rtPA alone.