Trial of Apixaban vs Warfarin in Reducing Rate of Cognitive Decline, Silent Cerebral Infarcts and Cerebral Microbleeds in Patients With Atrial Fibrillation

Inclusion Criteria: * Non-valvular Atrial Fibrillation * CHA2DS2-VASc Score \> or = to 2 * Never been treated with Apixaban (Eliquis) or prior treatment of \< 1 month * Candidate for oral anticoagulation as assessed by a treating physician Exclusion Criteria: * Valvular Atrial Fibrillation (Rheumatic valve disease,Moderate or greater mitral stenosis,Mechanical cardiac valve) * Active Bleeding * Prior treatment with Apixaban \>1 month * Recent stroke within 7 days * Dementia * Implanted devices not compatible with MRI/any cardiac implanted device * Claustrophobia * Active alcohol/drug abuse * Life expectancy \< 1 year * Taking asprin with \>100mg doses * Known hypersensitivity to warfarin or Apixaban * Severe renal insufficiency * Prior severe bleeding (Intracranial bleeding (subdural, subarachnoid, intraparenchymal bleeding),GI bleed requiring transfusion,Bleeding from other sites requiring transfusion) * Psychosocial reasons that make study participation impractical * Currently enrolled in another IND or IDE trial that has not completed the primary endpoint or that clinically interferes with the current study endpoints * Co-morbid condition(s) that could limit the subject's ability to participate in the trial or to comply with follow-up requirements, or that could impact the scientific integrity of the study. * Prisoners or subjects who are involuntarily incarcerated * Subjects who are compulsorily detained for treatment of either psychiatric or physical illness * Current or expected systemic treatment with strong dual inhibitors of CYP3A4 and P-gp (Any one of rifampin, carbamazepine, phenytoin, St. John's Wort, ketoconazole, itraconazole, ritonavir, clarithromycin) * Need for dual anti-platelet therapy with aspirin and another agent such as thienopyridine