Quantification of (18)FDG in the Normal Colon—A First Step in Investigating Whether Its Presence Is a Marker of a Physiological Process

The visibility of the colon in positron emission tomography (PET) scans of patients without gastrointestinal disease indicating the presence of (18)F Fluorodeoxyglucose ((18)FDG) is well recognised, but unquantified and unexplained. In this paper a qualitative scoring system was applied to PET scans from 30 randomly selected patients without gastrointestinal disease to detect the presence of (18)FDG in 4 different sections of the colon and then both the total pixel value and the pixel value per unit length of each section of the colon were determined to quantify the amount of (18)FDG from a randomly selected subset of 10 of these patients. Analysis of the qualitative scores using a non-parametric ANOVA showed that all sections of the colon contained (18)FDG but there were differences in the amount of (18)FDG present between sections (p<0.05). Wilcoxon matched-pair signed-rank tests between pairs of segments showed statistically significant differences between all pairs (p<0.05) with the exception of the caecum and ascending colon and the descending colon. The same non-parametric statistical analysis of the quantitative measures showed no difference in the total amount of (18)FDG between sections (p>0.05), but a difference in the amount/unit length between sections (p<0.01) with only the caecum and ascending colon and the descending colon having a statistically significant difference (p<0.05). These results are consistent since the eye is drawn to focal localisation of the (18)FDG when qualitatively scoring the scans. The presence of (18)FDG in the colon is counterintuitive since it must be passing from the blood to the lumen through the colonic wall. There is no active mechanism to achieve this and therefore we hypothesise that the transport is a passive process driven by the concentration gradient of (18)FDG across the colonic wall. This hypothesis is consistent with the results obtained from the qualitative and quantitative measures analysed.