• Median change in NIH Stroke Scale score [ Time Frame: at day 90 ] [ Designated as safety issue: No ]
• Global test statistic [ Time Frame: at day 90 ] [ Designated as safety issue: No ]
• Hemorrhagic transformation [ Time Frame: at day 7 ] [ Designated as safety issue: No ]
• Serious adverse events [ Time Frame: at day 90 ] [ Designated as safety issue: No ]
• Day 90 mortality [ Time Frame: at day 90 ] [ Designated as safety issue: No ]

The purpose of this study is to compare the effectiveness of treating acute ischemic stroke with mechanical embolectomy using the Merci Retriever or the Penumbra System within 8 hours of symptom onset to standard medical treatment, and to identify people who might benefit from mechanical embolectomy by the appearance of stroke on multimodal computerized tomography (CT) or magnetic resonance (MR) imaging.

Stroke most commonly occurs when there is a blockage of blood flow to one of the arteries in the brain. The blockage is often caused by a clot. There is currently only one FDA-approved stroke treatment. However, only 2 percent of ischemic stroke patients receive this treatment because it must be given within 4.5 hours of the stroke onset. There is an overwhelming need for new treatments that extend the time window to treatment since most individuals with stroke arrive at the hospital after the 4.5-hour time window.

Two of the most promising new devices are the Merci Retriever, a tiny corkscrew device, and the Penumbra System, an aspiration device. Both are designed to remove clots from arteries and thereby restore blood flow to the brain.

The primary purpose of this study is to compare the effectiveness of treatment with the Merci Retriever or Penumbra System within 8 hours of symptom onset to standard medical treatment, and to identify people who might benefit from the devices by the appearance of the stroke on multimodal computerized tomography (CT) or magnetic resonance imaging (MRI). Previous testing has determined that the use of the Merci Retriever is successful in opening up blocked blood vessels in approximately ½ of the individuals in whom it is used, and the Penumbra System is successful in opening up blocked blood vessels in approximately 80% of the individuals in whom it is used.

A total of 120 participants from approximately 30 different medical centers will be enrolled into this study. Participants will be randomized to either receive treatment by mechanical embolectomy with the Merci Retriever or Penumbra System and standard medical care or treatment with standard medical care alone. Standard care for stroke patients may include intravenous fluid, careful regulation of blood pressure, blood-thinning medicine (such as heparin or warfarin), anti-platelet medicine (such as aspirin or clopidogrel), and rehabilitation therapies.

Participants undergoing the Merci Retriever or Penumbra System procedure will have a cerebral arteriogram with pictures taken with dye prior to the procedure to determine the location of the blockage, and following the procedure to determine if blood supply has been restored. The total mechanical embolectomy procedure with either device will take approximately 1 to 2 hours. Participants will have brief neurological exams several times during this time to monitor changes in their neurological condition.

Participants will have follow-up visits, including neurological exams, at 30 days and at 90 days.

This study is part of the Specialized Program of Translational Research in Acute Stroke (SPOTRIAS), which allows researchers to enhance and initiate translational research that ultimately will benefit stroke patients by treating more patients in less than 2 hours, and finding ways to treat additional patients later.

• Active Comparator: Participants will be randomized to receive mechanical embolectomy treatment either with the Merci Retriever or Penumbra System and standard medical care or treatment with standard medical care alone.
• No Intervention: standard medical care

Inclusion Criteria:

• New focal disabling neurologic deficit consistent with acute cerebral ischemia (NIHSS >/= 6)
• Age >/= 18 ≤ 85
• Clot retrieval procedure can be initiated within 8 hours from onset
• Large vessel proximal anterior circulation occlusion on MR or CT angiography (internal carotid, M1 or M2 MCA)
• Pretreatment MRI performed according to MR RESCUE protocol
• Signed informed consent obtained from the patient or patient's legally authorized representative
• Premorbid modified Rankin score of 0-1
• Allowed but not required: patients treated with IV tPA up to 4.5 hours from symptom onset with persistent target occlusion on post-treatment MR RESCUE MR or CT protocol performed at the completion of drug infusion (Note: Rapidly improving neurological signs prior to randomization is an exclusion)

Exclusion Criteria:

• NIHSS >/= 30
• Contraindication to MRI (pacemaker etc)
• Acute intracranial hemorrhage
• Coma
• Rapidly improving neurological signs prior to randomization
• Pre-existing medical, neurological or psychiatric disease that would confound the neurological, functional, or imaging evaluations
• Pregnancy
• Known allergy to iodine previously refractory to pretreatment medications
• Current participation in another experimental treatment protocol
• Contrast-Enhanced Neck MRA or CTA suggests proximal ICA occlusion, proximal carotid stenosis > 67%, or dissection
• INR > 3.0
• PTT > 3 x Normal
• Imaging data cannot be processed by the MR RESCUE computer
• Renal Failure (serum creatinine > 2.0 or Glomerular Filtration Rate [GFR] < 30)

MRI Exclusion Criteria:

• Contraindication to MRI (pacemaker, etc)

CT Exclusion Criteria:

• Contraindication to iodinated contrast**

  **Examples of possible iodinated contrast contraindications include:

• Hyperthyroidism
• History of severe allergic reaction to iodinated contrast material
• History of sever kidney disease as an adult, including tumor or transplant surgery, or family history of kidney failure
• Paraproteinemia syndromes or multiple myeloma
• Collagen vascular disease
• Severe cardiac insufficiency
• Severely compromised liver function
• Current therapy with metformin, aminoglycosides