1 Oxford Centre for Clinical Magnetic Resonance Research, Division of Cardiovascular Medicine, Radcliffe Department of Medicine, University of Oxford, Oxford, United Kingdom; Oxford Heart Centre, John Radcliffe Hospital, Oxford, United Kingdom; Division of Cardiovascular Medicine, Radcliffe Department of Medicine, University of Oxford, Oxford, United Kingdom; Brigham and Women's Hospital, Radiology, Cardiovascular Imaging, Boston, Massachusetts.