Patients will be recruited by clinical rheumatologists working in a specialist adolescent and young adult inflammatory arthritis tertiary referral centre, on a person-to-person basis. Patients will be aged 12-24 and have a diagnosis of JIA according to established clinical criteria.8,9 Individuals with enthesitis-related arthropathy and those with other forms of JIA (extended oligorticular and polyarthritis (rheumatoid factor positive and negative)) will be divided into separate groups and asked to have an MRI scan. Control subjects will be recruited from the same clinic, and will consist of individuals with mechanical back pain (non-inflammatory - thought to arise from vertebrae, discs or ligaments) and normal inflammatory markers. Such patients still require an MRI scan as part of their routine care to confirm the mechanical cause for their back pain and definitively rule out inflammation as a cause, but serve as non-inflammatory controls. All patients will be asked to provide written consent at the time of enrolment.