Comparing Enoxaparin to Fondaparinux to Prevent Venous Thromboembolism (VTE) in Bariatric Surgical Patients

Inclusion Criteria: 1. Men or women 18 years or older. 2. Body mass index (BMI) of 35kg/m2 or greater. 3. Undergoing laparoscopic bariatric surgery, i.e., laparoscopic sleeve gastrectomy, laparoscopic Roux-en Y gastric bypass or laparoscopic duodenal switch. Exclusion Criteria: 1. History of previous deep vein thrombosis. 2. History of previous pulmonary emboli. 3. History of documented clotting/coagulation disorder. 4. History of cancer. 5. Weight \< 50 kg or \> 200 kg or unable to fit in MRI scanner. 6. Presence of metallic foreign bodies 7. Recent history of smoking (within the last year). 8. History of venous stasis disease. 9. History of obesity hypoventilation syndrome. 10. Patients who are unable to lay flat for extended periods of time or are claustrophobic. 11. Patients who have a pacemaker, an implanted defibrillator or certain other implanted or electronic or metallic devices (implanted medical or metallic devices, shrapnel, or metal). 12. History of hypersensitivity reaction to anticoagulation products. 13. History of HIT (Heparin Induced Thrombocytopenia. 14. History of Renal Insufficiency (Creatinine Clearance \< 50). 15. Active clinically significant bleeding. 16. Acute bacterial endocarditis. 17. BMI \> 60. 18. Patients with metallic foreign body or implant (unable to have an MRV study).