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Objective:
To evaluate the added value of 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) to magnetic resonance (MR) enterography for detection of active ileocolonic Crohn’s disease.

Materials and Methods:
Under IRB approval and after informed consent, 23 adult subjects with known Crohn’s disease were prospectively enrolled to undergo PET/MR enterography on a simultaneous PET/MR scanner (Signa PET/MR, GE Healthcare), followed by colonoscopy. Imaging was performed one hour after intravenous administration of FDG. MR enterography was performed per standard clinical protocol, which included coronal dynamic pre- and postcontrast T1- and axial T2-weighted fat saturated imaging. Endoscopic evaluation of the colon and terminal ileum (or neo-terminal ileum) as well as tissue sampling (only performed if indicated per routine clinical practice) served as the reference standard. Blinded to endoscopy and histology, a dual board-certified radiologist/nuclear medicine physician with fellowship training in body MRI and nuclear medicine reviewed the images per following: MR enterography was reviewed first; the bowel was divided into three segments: 1) ascending colon/cecum/terminal ileum, 2) transverse colon, and 3) descending/sigmoid colon/rectum. Each segment was evaluated for presence of active disease on a 5-point Likert scale: 1 = definitely not, 2 = probably not, 3 = indeterminate, 4 = probably, and 5 = definitely. PET was then added to the evaluation, and a PET/MR score was provided on the same Likert scale. Bowel segments (3 per subject) were divided into two groups of active or inactive disease based on endoscopic and/or histologic evidence of active disease. MR and PET/MR scores, difference between the two scores, and uptake on PET (maximum standardized uptake value, SUVmax) were compared between the two groups using Wilcoxon-rank-sum test and receiver operating characteristic (ROC) curves.

Results:
A total of 29/68 segments in 18 subjects had active disease by the colonoscopy/histology reference standard (right hemicolectomy and right/transverse hemi colectomy in 2 patients lowered the total segment count). In 6/29 (20%) of segments with active disease, MR was scored as 1 or 2, but upgraded to 3 or higher by PET/MR. Diagnostic performance was higher for PET/MR than MR, respective areas under the ROC curve 0.892 and 0.803 (p = 0.02). No significant change was noted in the scores between MR and PET/MR in inactive segments. Conversely, scores of segments with active disease were higher by a median of 1 point on PET/MR (p = 0.001). Average SUVmax was significantly higher in segments with active disease than in those without, 9.2 versus 5.1 (p < 0.001).

Conclusion:
Compared with MR enterography, simultaneous PET/MR enterography is more sensitive and improves diagnostic confidence for detection of active inflammatory ileocolonic Crohn’s disease. This improved sensitivity can guide patient management and help assess treatment response using the quantitative nature of SUV in affected bowel segments.