 This study is a prospective, randomized study of patients with single ventricle heart disease who are to undergo superior cavo-pulmonary anastomosis, or Glenn operation. Such patients have historically undergone cardiac catheterization to ensure suitability for the procedure. Cardiac magnetic resonance imaging cardiac MRI is a newer technology that provides excellent anatomic and functional imaging of the heart. This study is designed to demonstrate our hypothesis that cardiac magnetic resonance imaging will provide comparable information to catheterization, with less side effects. 