Use of MRI for assessment of CAD is a growing area that entails no radiation exposure to the patient. Advances in MRI have made it possible to accurately detect CAD, either as well or better than SPECT in unselected populations [6, 7]. However, adoption of MRI myocardial perfusion scans has been limited in part due to the challenges associated with the use of adenosine. Adenosine requires starting a second IV, and to use either a special expensive MRI-compatible infusion pump to deliver the drug, or long lengths of tubing to run to a pump outside the scanner room. Neither solution is ideal, and regadenoson would not require any such pumps or the starting of a second IV. Here the investigators propose to determine the sensitivity/specificity for dynamic contrast-enhanced myocardial perfusion MRI with the vasodilator regadenoson in a subpopulation of patients - those with atrial fibrillation.