Patients will be divided according to diagnostic group - group A will consist of patients with ERA (and also controls who are subsequently diagnosed with mechanical back pain), while group B will consist of patients with ERA, extended oligo- and polyarthritis. As detailed below, the scan protocol will be different for these two groups. JIA patients in both groups will be scanned at enrolment and will have repeat scans every 6-12 months, depending on clinical need, for up to five years in total. Thereafter, scans will be performed as defined by the requirements of their clinical care up to year 10 of study. The control group in group A will only be scanned once. They will be asked to provide written consent prior to enrolling. The imaging component of the study will run for up to five years in total, and approximately 200 patients will be recruited in total.