Platelet Testing to Predict Stroke Risk

Inclusion Criteria: * Stroke is defined as symptoms lasting \>24 hours and associated with imaging evidence of acute ischemia in the distribution of the stenotic vessel on head CT or brain MRI. Minor stroke is defined as NIHSS\<6, as used in prior studies. * Eligible TIA, defined as transient neurological symptoms lasting \<24 hours, need to be: a) accompanied by DWI abnormalities in the distribution of the stenotic artery; or b) multiple (\>1), stereotyped events associated with unequivocal ischemic symptoms (i.e. weakness, aphasia, diplopia), and attributed to the symptomatic artery. The intent of these restrictive inclusion criteria for TIA is to exclude potential stroke mimics. * ICAD should involve the intracranial carotid, middle cerebral, intracranial vertebral or basilar arteries. Isolated anterior and posterior cerebral artery stenosis is not included as it is uncommon in these locations and non-invasive criteria for high-grade ICAD are not well established for these vessels. * Stenosis 50-99% will be quantified by CTA. The criteria for 50-99% are: measured stenosis by WASID criteria (percent stenosis = (1-\[diameter stenosis/diameter normal\]) x 100%. * Age ³30; those 30-49 years of age must also have the presence of established atherosclerotic disease in another vascular bed (coronary, extracranial carotid, peripheral) or the presence of 2 or more risk factors (hypertension, diabetes mellitus, hyperlipidemia, tobacco abuse within the last 2 years). The rationale for this criterion is to exclude non-atherosclerotic vasculopathies. * Provide informed consent for participation in the study. Exclusion Criteria: * Other determined etiology or established cause of the acute stroke or TIA: atrial fibrillation, mitral stenosis, mechanical valve, intracardiac thrombus or vegetation, dilated cardiomyopathy or ejection fraction \<30%, proximal extracranial carotid or vertebral stenosis \>50%. * Contraindications to MRI, including MR-incompatible metallic implants (i.e. certain artificial cardiac valves, penile implants, other prosthesis), implanted electronic devices (i.e. pacemaker/defibrillator, neurostimulators, cochlear implants), other potentially mobile ferromagnetic material (i.e. shrapnel, magnetic aneurysm clips), pregnancy (women in fertile age should have a negative pregnancy test), lactation, morbid obesity, and severe claustrophobia.