The purpose of this study is to test the utility of PET/MRI in diagnosis and management of Inflammatory bowel disease (IBD) in children and adolescents, hereby||To test if PET/MRI scan is an accurate method to diagnose and differentiate Crohn's disease and Ulcerous Colitis in children and adolescents suspected of IBD.|To evaluate whether PET/MRI scan in children and adolescents with Crohn's disease is an accurate method to diagnose relapses and to monitor the effect of biological treatment with monoclonal antibodies directed towards Tumor Necrosis Factor-alpha.