Patients with the clinical diagnosis of large or small duct PSC made at any time prior to enrollment are screened for eligibility to participate in this prospective cohort study. The site PI will determine eligibility following review of MRCP or ERCP images with the site radiologist to confirm presence of an abnormal cholangiogram at the time of diagnosis of large duct PSC. Liver histopathology obtained at the time of diagnosis of small duct PSC will be reviewed with the site pathologist prior to enrollment. Individuals must meet all of the Inclusion criteria in order to be eligible to participate in the study: 1. Aged 2 through 25 years at time of screening. 2. Diagnosis of large duct PSC based on review of cholangiogram by MRC, ERC, or intraoperative cholangiogram (IOC) by the site radiologist and interpreted to be consistent with PSC, based on one or more of the following: - Focal structuring of the bile duct(s) - Dominant stricture of the common bile duct - Saccular dilatation of bile duct(s) - Beaded appearance of bile duct(s) - Pruning appearance of the distal bile duct branches AND/OR 3. Diagnosis of small duct PSC based on review of liver histopathology by the site pathologist and interpreted to be compatible with PSC: - Probable small duct PSC: biopsy with ≥3 of 5 criteria: periductal edema, concentric inflammation, bile duct injury, ductular reaction, and neutrophils in bile ducts (cholangitis) OR... - Definitive small duct PSC: Periductal fibrosis/ "onion skinning" around interlobular bile ducts or smaller profiles 4. Stated willingness to comply with all study procedures and availability for the duration of the study. 5. Able to provide informed consent/assent Participants for the imaging study are eligible if they are: 1. Aged 8 through 25 years at the time of screening 2. No absolute contraindication to MRI 3. No skin condition that could be aggravated by MREL 4. Meet all other eligibility criteria of the PSC Observational Study 5. For whom none of the

apply Exclusion Criteria: An individual who meets any of the following criteria at baseline will be excluded from participation in this study. 1. History of liver transplantation 2. History bone marrow transplantation 3. History of primary or acquired immunodeficiency predisposing to secondary sclerosing cholangitis, for instance: hyper-IgM syndrome, severe combined immunodeficiency (SCID) syndrome, common variable immunodeficiency (CVID) syndrome, cartilage hair hypoplasia syndrome, or HIV/AIDS 4. History of histiocytosis, including Langerhans cell histiocytosis (LCH), or hemophagocytic lymphohistiocytosis (HLH) 5. History of ischemic cholangitis 6. History of portal vein thrombosis with biliopathy, veno-occlusive disease, or abdominal radiation vasculopathy 7. History of recurrent pyogenic cholangitis 8. History of biliary tract surgery for cholecystolithiasis prior to cholangiogram/liver biopsy evaluated to determine enrollment 9. History of biliary tract surgery for choledochal cyst 10. History of hepatocellular carcinoma, or hepatoblastoma 11. History of surgical biliary trauma 12. History of congenital cytomegalovirus (CMV) hepatitis 13. History of Sickle Cell Disease 14. History of cystic fibrosis, biliary atresia, Caroli disease/congenital hepatic fibrosis, or progressive familial intrahepatic cholestasis type 3/MDR3 disease 15. History of cardiac hepatopathy. 16. History of metabolic disorders, including Wilson's disease, glycogen storage disorder, Alpha-1 Antitrypsin deficiency 17. Diagnosis of systemic lupus erythematosus (SLE) 18. Concurrent pregnancy at the time of enrollment -