Inclusion Criteria:  Patients age ≥ 12 years with anatomic and/or functional univentricular heart receive Fontan procedure followed at our institute (including the patients with heart failure or pregnancy). Patients without taking organic nitrates and not allergic to Sildenafil.  Exclusion Criteria:  The patients cannot safely receive MRI examination, including patients with severe renal dysfunction, or after pacemaker or implantable cardioverter defibrillator implantation. Patients having known liver disease (such as viral hepatitis, alcoholic liver disease, etc.) other than FALD. Patients are under sildenafil or any pulmonary vasodilator therapy for lowering pulmonary resistance, or have been using any of these medications in recent 3 months.