The investigators have a great deal of experience with stress and rest myocardial perfusion MRI. In a preliminary study imaging three patients with AF that then went to X-ray angiography (cath), two of the three subjects had significant stenoses by cath, and one did not. This agreed with the regadenoson stress perfusion MRI findings. These MRI acquisitions were performed on a Siemens Verio MRI scanner. The Verio operates at twice the magnetic field strength (3 Tesla, or 3T) of most MRI scanners, which operate at 1.5T. The higher magnetic field offers images with significantly less noise (almost twice the signal-to-noise ratio).