Allograft Dysfunction in Heart Transplant

Inclusion Criteria: * Age greater than or equal to 18 years old. * At least three months status post heart transplantation. Exclusion Criteria: * Biopsy proven acute rejection episode in the past 3 months. * Patients with symptoms or signs of acute myocardial ischemia or recent acute coronary syndrome in the past 3 months. * Uncontrolled obstructive ventilatory disease including asthma and COPD. * History of generalized tonic-clonic seizures. * Second or third degree AV nodal block. * Sinus node dysfunction. * Contraindications to MRI including cardiovascular implantable electronic devices. * Renal dysfunction with an estimated GFR less than 30 mL/min/1.73m2. * Prior adverse reaction to either regadenoson or gadolinium contrast. Prior adverse reaction to adenosine or dipyridamole will be assessed on a case-by-case basis. * Systolic blood pressure greater than 180 or less than 85 mmHg. * Diastolic blood pressure greater than 120 or less than 40 mmHg. * Resting heart rate greater than 120 or less than 45 beats per minute. - Severe claustrophobia.