Rivaroxaban Versus Warfarin in the Evaluation of Progression of Coronary Calcium

Key Inclusion Criteria: 1. Eligible patients with atrial fibrillation at enrollment or two or more episodes of atrial fibrillation or flutter, as documented by electrocardiography, at least 2 weeks apart in the 12 months before enrollment 2. Age 18-84 3. On Warfarin for 6 months prior to enrollment at a stable dose. 4. Willingness to participate in the study and ability to sign informed consent 5. Minimum CAC score of 10 Key Exclusion Criteria: 1. Atrial fibrillation due to a reversible cause, moderate or severe mitral stenosis, or conditions other than atrial fibrillation that require anticoagulation (e.g., a prosthetic heart valve) 2. Prior apixaban, dabigatran, rivaroxaban use. 3. A need for aspirin at a dose of \>165 mg a day or for both aspirin and clopidogrel, 4. Renal insufficiency (serum creatinine level of 12.5 mg per deciliter or calculated creatinine clearance of \<50 ml per minute). 5. Serious bleeding event in the previous 6 months or a high risk of bleeding (eg, active peptic ulcer disease, a platelet count of \<100,000/mm3 or hemoglobin level of \<10 g/dL, stroke within the previous 10 days, documented hemorrhagic tendencies, or blood dyscrasias) 6. Weight in excess of 325 pounds 7. Resting hypotension (systolic blood pressure of \<90mmHg) or resting hypertension (systolic blood pressure of \>170mmHg or diastolic blood pressure of \>110 mmHg). 8. History of active malignancy requiring concurrent chemotherapy. 9. Any unstable medical, psychiatric, or substance abuse disorder that in the opinion of the principal investigator is likely to affect the subject's ability to complete the study. 10. Known allergy to iodinated contrast material 11. Pregnancy or breast feeding