Identification of Factors which Influence Physiologic Colonic FDG Uptake on PET-CT Scans

Abstract

Purpose: Recent studies have demonstrated a role for PET-CT in assessing disease activity in Inflammatory Bowel Disease (IBD). However the use of PET-CT in this setting is limited by physiologic uptake of the labeled intravenous glucose marker (FDG) along the GI tract that can mimic pathological lesions. To date, the localization of this physiologic signal remains unclear. The goal of this study was to determine which conditions can alter physiologic colonic FDG uptake. Methods: A prospective cohort (N=13) of patients who demonstrate physiologic GI FDG uptake during a clinically-indicated PET-CT exam was enrolled, and underwent PET-CT imaging before and after a high-fat meal (to stimulate antegrade colonic motility). Change in anatomical location of FDG uptake was determined using the initial and delayed PET-CT studies. A retrospective cohort (N=1000) of patients who underwent PET-CT imaging had their FDG uptake (maximum standardized uptake value, SUVmax) correlated with use of medications, and post-surgical anatomy (ileostomy). Results: A high-fat meal induced antegrade colonic movement of FDG activity in 9/13 patients, 3/13 patients demonstrated no anatomical changes of the FDG activity and 1/13 patient demonstrated retrograde movement of the FDG activity. When the images of 25 patients with ileostomies were reviewed, 4 demonstrated FDG avidity within the ileostomy bag (Avg SUVmax 2.08+/-0.2). These data suggested the localization of the FDG uptake was luminal, requiring blood-lumen translocation of the labeled glucose (FDG). To examine this hypothesis, the colonic FDG uptake of diabetics taking oral hypoglycemics (OHGs) was compared to non-diabetics; FDG uptake was significantly higher in patients taking OHGs (Avg SUVmax 10.3) compared to patients not taking these medications (Avg SUVmax 6.7, p=0.0003). Conclusion: This study demonstrates antegrade colonic movement of FDG uptake after a high-fat meal and the presence of FDG signal within ileostomy contents; both suggesting luminal concentration of FDG. Since FDG is administered intravenously, this would require blood-lumen translocation of FDG. Diabetic patients exhibit higher colonic FDG signal than non-diabetics, possibly due to a higher blood-lumen glucose gradient, which would facilitate FDG translocation. This explanation may provide a mechanism to minimize “false positive” FDG uptake in the colon during PET-CT scans.