Inclusion Criteria- Age 60 or greater at time of enrollment
- Written informed consent from patient or legal representative
- Diagnosis of stroke or TIA made by a stroke specialist within 90 days and fulfilling the following criteria:
- A TIA must involve a focal speech/language, motor or visual deficit (transient monocular blindness, amaurosis fugax) referable to the distribution of a carotid artery and lasting less than 24 hours.
- A stroke consisting of deficits as noted above with duration greater than 24 hours and/or confirmed on cerebral imaging. Post event Modified Rankin Score of 2 or less.
- Stroke meets the Trial of ORG 10172 in Acute Stroke Treatment (TOAST) criteria for large artery atherosclerosis
- Carotid Doppler, CTA or MRA confirming the presence of bilateral atherosclerotic disease resulting in carotid stenosis of any degree. Stenosis will be measured following the method used in NASCET for CTA and MRA. Carotid Doppler measurements will follow the criteria defined by the Society for Ultrasound consensus conference.
- 12 lead ECG or Holter monitor confirming the absence of atrial fibrillation.
Exclusion Criteria- TIA or stroke in the vertebrobasilar system
- Index event was primary hemorrhage
- History of intermittent atrial fibrillation
- Cardiac source of embolus suspected as cause of index event (artificial valve, segmental or global LV dysfunction, congenital cardiac defect)
- Diagnosis of vasculitis, dissection, or non-atherosclerotic carotid disease (Ehlers-Danlos, Marfans)
- Sinovenous thrombosis, endocarditis or hypercoagulable state
- Pacemaker, ICD or other contraindications to MRI
- Diminished Kidney Function
- Contraindication to radiation exposure (eg: pregnancy)
- Severe Claustrophobia