Use of Cardiac-MRI to Predict Results for People With Severe Aortic Stenosis

Background: \- Aortic valve stenosis is a disease that makes a major heart valve get smaller. This reduces heart function and causes death. Severe aortic stenosis (AS) can be treated in a couple of ways, including replacing a heart valve. Objectives: Researchers want to study fibrosis in the heart. A sub-study will test whether heart function and blood supply improve after a valve replacement. Eligibility: \- Adults at least 18 years old with aortic stenosis. Design: * Participants will visit a clinic for 1 day for magnetic resonance imaging (MRI) of their heart. This uses magnets, radio waves, and computers to produce detailed pictures of the heart. * After this visit, participants will have their aortic valve procedure at the the Washington Hospital Center. A hospital team will contact participants for 1 year by phone or email. This follow-up will consist of 15 minutes of questions about the participant s health status. * Some participants will join a sub-study. * They will be given an additional medication to evaluate the blood supply of the heart. * They will visit a clinic for 1 day for an MRI of their heart, as part of the main study, prior to the aortic valve replacement. * After they have their valve replaced at the hospital, they will return to the clinic for another MRI. * They will have the same follow-up as in the main study.