In this study, we are developing new ultrasound and magnetic resonance imaging (MRI) methods to measure kidney and liver disease severity in autosomal recessive polycystic kidney disease (ARPKD). Participants in this study will include individuals with ARPKD, and a comparison group of healthy individuals without liver or kidney disease. Children and young adults up to 21 years old may take part in the study. Study procedures include an ultrasound and MRI scan of the abdomen (MRI is only done in participants able to lie still for up to an hour, generally at least 8 years old). Participants wtih ARPKD will also have a blood test and will be asked to complete 1 follow up visit per year for up to 3 years after baseline.