Inclusion Criteria:

   - Age greater than or equal to 18 years old.

   - At least three months status post heart transplantation.

   - Heart-transplant patients with normal graft function (left ventricular ejection
   fraction equal to or greater than 55%) and no prior history of clinically significant
   acute rejection episodes that required modification of the immunosuppressive regimen
   or cardiac allograft vasculopathy.

   - Heart-transplant patients with nonspecific allograft dysfunction (left ventricular
   ejection fraction equal to or less than 50% AND decrease from post-transplant baseline
   ejection fraction by an absolute difference of 10% or greater, no formal diagnosis of
   allograft vasculopathy by coronary angiogram or coronary vascular ultrasonography, and
   no history of prior acute rejection episodes known to have decreased left ventricular
   ejection fraction to or less than 50%).

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.

   - Second or third degree AV nodal block.

   - Sinus node dysfunction.

   - Contraindications to MRI including pacemakers or implantable
   cardioverter-defibrillators.

   - 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 will be assessed on a case-by-case basis.

   - Any invasive procedure, including endomyocardial biopsy and left coronary angiogram,
   performed within one week.

   - 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.