PROJECT SUMMARY Dr. Pamela Weiss, with the support of this proposal, seeks to expand and improve her research program and her training, mentoring, and career development of greatly needed physician scientists pursuing patient- oriented research (POR) in pediatric rheumatology. Successful mentoring requires both an effective mentee and a skilled mentor, both of which are reflected in the training goals of this application and include building skills to 1) train junior faculty and peers to build and effectively manage their mentoring relationships, and 2) teach trainees how to effectively work with their mentoring team and navigate the research environment. The premise of this application is to leverage existing studies to improve the everyday clinical care of children with spondyloarthritis and to contribute to the design of studies of emerging targeted therapies. Three major obstacles block advancements in juvenile spondyloarthritis care. First, there is variability in the interpretation of diagnostic imaging studies because distinguishing immature structural bone from subchondral bone marrow edema at the sacroiliac joints in children is challenging. The apparent diffusion coefficient (ADC) from diffusion- weighted imaging (DWI) holds unique promise for the pediatric population as a tool to distinguish maturational changes from pathologic inflammation. Second, there are no pediatric diagnostic imaging criteria for sacroiliitis. Improved criteria is needed not only in the clinical setting to prevent overdiagnosis, but also in the research setting to accurately identify those who would benefit from clinical trials. Third, although there is a juvenile spondyloarthritis disease activity (JSpADA) index that is widely used in clinical research, the cut-off values for defining clinically important disease states are unknown. Since inactive disease remains unattainable for many with juvenile spondyloarthritis, there is a critical need to determine minimal clinically important improvement (MCII) and well-defined states of disease activity using the JSpADA index for use not only in clinical care but also studies of emerging therapies. The proposed research addresses these challenges and leverages existing studies conducted by the candidate at a world-renowned pediatric tertiary care center with expertise in spondyloarthritis and innovative imaging. Aim 1 will test ADC from DWI as a diagnostic biomarker for the presence of sacroiliitis in children. Aim 2 will determine candidate lesion-based criteria for defining a positive sacroiliac joint MRI. Aim 3 will determine the MCII and cut-off values for defining minimal disease activity and high disease activity for the JSpADA index. These aims will simultaneously provide significant added value to ongoing studies, aid the applicant to develop new research and mentoring skills, and offer a clinical research platform in which to mentor young physician scientists in POR and promote their transition to independence.