4. Evidence on PWI MRI of a perfusion defect corresponding to the acute stroke syndrome of at least 2cm in diameter in both long and short axis in any slice. The PWI will be assessed by relative mean transit time (MTT) images. The MRI evaluation must involve echo planar diffusion weighted imaging, MRA, and MRI perfusion. A normal appearing MRA with an appropriate perfusion defect 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 DWI are eligible.