Additional Study Information: Growth of the skeleton happens at the growth plate, a disc of cartilage at the ends of all long bones. The purpose of this research study is to find out if a technique using investigational MRI called diffusion tensor imaging of the growth plate cartilage of the knee (MRI-DTI), can tell us early whether children and adolescents who are receiving growth hormones are responding to therapy. Participants will come for MRI before initiation of growth hormone therapy and then at 4, 8, and 12 months after initiation of growth hormone therapy. In each visit, participants will complete a short questionnaire. Participants will receive reimbursement for each visit.