Colonoscopy is considered crucial for the diagnosis and quantification of ulcerative colitis (UC). However, there are several drawbacks related to the invasiveness, procedure-related discomfort, risk of bowel perforation (especially in the period of acute inflammation), and relatively poor patient acceptance. Most patients regard the necessary bowel cleansing as burdensome. Feasible, accurate and well accepted non-invasive diagnostic techniques are needed for the determination of inflammatory activity and optimal tailoring of therapy. Hybrid PET/MRI represents an innovative combination of two established, non-invasive diagnostic tools: Magnetic resonance imaging (MRI), allowing for anatomic-functional imaging of the abdomen at high soft tissue contrast and positron emission tomography (PET) utilizing 18F-fluorodeoxyglucose (FDG) a non-invasive tool to monitor glucose metabolism and allowing a detection and quantification of inflammatory processes. Since MRI has limited sensitivity in UC and may be hampered by retained stool, a combination with another imaging modality is very appealing. PET, on the other side provides functional information, yet with limited anatomical landmarks and is relatively unsusceptible to artifacts associated to retained stool. In combination, these modalities might provide a valid alternative for the non-invasive assessment of the inflammatory activity in UC patients without the need for bowel purgation. It will therefore have to be investigated whether fecal material does impede the diagnostic quality of the combination of FDG-PET and MRI. For this purpose, the investigators will include 50 patients with confirmed ulcerative colitis. Dependent on clinical activity of the inflammation, patients will be randomized to undergo PET/MRI enterography either with or without prior bowel purgation followed by a colonoscopy. Inflammatory activity in 7 bowel segments will be analyzed based on PET/MRI with and without bowel purgation with the results of colonoscopy as standard of reference.

Patient acceptance of PET/MRI with and without bowel purgation as well as colonoscopy will be compared. PET/MRI with and without bowel cleansing will be compared with regard to diagnostic accuracy as well as for its patients' acceptance in comparison to colonoscopy.

The investigators hypothesize that PET/MRI will eventually be highly accurate to detect and monitor inflammatory activity in patients with ulcerative colitis. Additional information about extra-intestinal findings might also change the therapeutic concept. PET/MRI might serve as a non-invasive diagnostic option in patients with UC to quantify inflammatory activity especially when bowel cleansing or colonoscopy is not applicable.

Inclusion Criteria:

• patients with confirmed ulcerative colitis verified by the defining symptoms (rectal bleeding, diarrhea), endoscopy and histopathology
• clinically indicated colonoscopy and 18F-FDG PET as either initial assessment or follow- up examination
• patient age ≥ 18 years

Exclusion Criteria:

• Patients aged < 18years
• Patients with MRI contraindications, e.g. presence of cardiac pacemaker, implanted cardioverter-defibrillator, neurostimulation systems or with claustrophobia.
• acute renal failure, severe chronic renal failure (calculated glomerular filtration rate [GFR] < 30 ml/min)
• allergy to i.v. gadolinium based contrast agents