Crohn's disease is a lifelong condition resulting in inflammation of the bowel. Treatment with powerful drugs aim to reduce inflammation by suppressing the immune system. It is important to regularly assess if the drugs are effective, so they can be stopped or changed if not. There are several ways doctors assess if the medication is effective as just relying on how the patient feels is not sufficient. Blood and stool tests looking for inflammation are useful but have limitations. Looking into the bowel using endoscopy is effective but very invasive. Medical imaging notably MR Enterography (MRE), a type of MRI scan, and intestinal ultrasound (IUS) visualise the bowel, and are safe, non-invasive and generally well tolerated tests frequently used in the NHS to assess how well treatment is working. There is however wide variation in which imaging test is selected without clear guidance. The investigators currently do not know if one is better in assessing response to treatment, and if they can be used interchangeably. The proposed study will investigate this question to guide the future care of Crohn's disease patients. The investigators will recruit patients having follow up imaging tests to assess their treatment and ask them to undergo both MRE and IUS. Radiologists will interpret the MRE and IUS, unaware of the findings of the other test, and state if the disease is improving or not. The doctors looking after the patient will then look at all available information e.g. symptoms, blood/stool tests, colonoscopy, and decide if the patient is the same, worse or better on the medication. This decision will be used to establish how accurate MRE and IUS are in assessing response, how often they agree and if they can be used interchangeably in the future, for example based on patient preference or availability at the hospital.