Patients who have Alagille Syndrome (AGS) also frequently have blockages (or "stenoses") of their pulmonary arteries. Little is known about the degree or variability of these stenoses, or the effect of this disease on the right ventricle (the chamber of the heart which pumps blood to the lungs). This study will first quantify and describe pulmonary artery stenosis in patients with Alagille Syndrome. The study will also assess the effect of these stenoses on the right ventricle. The investigators hope to learn the degree and characteristics of pulmonary artery stenosis in Alagille Syndrome. The investigators also hope to learn the effect of this pulmonary artery stenosis on the right ventricle in patients with Alagille Syndrome. This information is critical in the management of patients with Alagille syndrome, as there is currently no data to guide clinicians on the management of pulmonary artery stenosis. Furthermore, the information from this study may help physicians manage pulmonary artery stenosis in other patients as well.