FDG dose reduction for PET/MR examinations in patients with Crohn's disease- how low can you go?

Introduction: PET/MR is a promising hybrid imaging modality for the evaluation of disease activity in Crohn's disease. It combines both PET metabolic information, using state-of-the-art high sensitivity digital PET detectors, and MRI high soft tissue contrast resolution, which can be useful in distinguishing fibrotic and inflammatory strictures to help guide treatment. The aim of this study is to determine if reduced-dose PET acquisitions, simulated by a short PET acquisition time, would provide similar diagnostic information to a 5-minute image acquisition.

Methods: Twenty patients with Crohn's disease had PET/MR exams (Signa PET/MR, GE Healthcare) of the abdomen to evaluate bowel disease. Exams were performed using 5mCi (+/- 10%) of FDG and standard PET/MR bowel technique. Simulated low-dose acquisitions of 1, 2, 3 and 4 minutes were reviewed in a random order and compared to a 5-minute image. Two nuclear medicine readers were blinded to the length of each image, and rated the image quality of the bowel and bone marrow using the 5-minute image as the standard reference, using the scale: 1=image quality equal to the 5-minute image; 2=slightly reduced image quality; 3=significantly reduced image quality; 4=non-diagnostic. A score of 1 & 2 indicated image quality was adequate for image interpretation. Results: Nearly all the 3 and 4 minute images were considered to have adequate quality for image interpretation relative to the standard reference 5-minute image (Bowel rated a 1 or 2: (Reader 1: 20/20; Reader 2: 20/20). Bone marrow: (Reader 1: 20/20; Reader 2: 19/20). Ratings for the image quality of the 2-minute images were much lower compared to the standard: Bowel score of 1 or 2: (Reader 1: 7/20; Reader 2: 7/20) Bowel score of 3: (Reader 1: 12/20; Reader 2: 13/20). Bone marrow score of 1 or 2: (Reader 1: 8/20; Reader 2: 8/20). Most ratings for the 1-minute images were considered non-diagnostic for bowel or bone marrow evaluation: Bowel score 4: (Reader 1: 20/20; Reader 2: 19/20). Bone marrow score 4: (Reader 1: 19/20; Reader 2: 18/20). There was a significant difference in the bowel image quality when the 3 and 4 minute acquisitions were compared to the 2-minute acquisitions (p<.0001, Fishers exact test). There was excellent inter-observer agreement between both readers across all categories.Conclusion: The evaluation of the PET/FDG image quality in patients with Crohn's disease performed on a state-of-the-art PET/MR scanner with high-sensitivity digital PET detectors suggests that image acquisition time can be reduced by 40% without a loss in diagnostic quality. More importantly, the dose of FDG can be reduced by 40% (to 3 mCi) thus significantly decreasing radiation exposure to this predominantly young patient population.