Assessing motility of the colon may be extremely useful in understanding the symptoms of constipation^{1, 2} but to do so is challenging for invasive manometry as sensor placement requires an empty colon, which may alter its physiology. MRI work to date has concentrated on visualising large scale contractions of the colon wall^{3, 4}. Neither technique can monitor how this wall movement effects mixing of colonic contents, although mixing may affect stool consistency, gut transit with implications for symptoms, and the establishment of gut microbiota communities. Previous work utilised MR tagging to measure movement of the gastric⁵ or small bowel⁶ wall and contents. To use an MR tagging technique to observe mixing and flow in the ascending colon of 11 healthy and 11 constipated subjects following ingestion of a 500ml macrogol preparation.The study protocol was approved by the local research ethics committee. All participants gave written informed consent. 11 healthy volunteers, with no history of gastro-intestinal disease, (4 male, 7 female, aged 28±10 years with mean Body Mass Index (BMI) 25.0±4.6 kgm^-2) and 11 subjects with symptoms of constipation, as defined by Rome III criteria, (2 male, 9 female aged 36±12 years, with BMI 25.2±4.8 kgm^-2) formed the study groups. Subjects fasted from 2200 hours the previous evening and avoided alcohol and caffeine for 24 hours and strenuous exercise from the night before the experiment. Following the baseline, (fasted) MRI scan subjects ingested a 500ml dose of MOVIPREP® (Norgine Pharmaceuticals Ltd, Harefield, UK) a polyethylene glycol (Macrogol 3550) electrolyte solution within 30 minutes, followed by scans at 60 and 120 minutes. MRI scans were carried out using a 3T Philips MRI scanner (Philips, Best, The Netherlands). The subjects were positioned supine with a XL-torso receiver coil wrapped around the abdomen. The ascending colon was imaged using a tagged bTFE sequence with TR/TE 2.32/1.16 ms, FA 45 , with a single sagittal slice, thickness 15 mm, FOV 264 (AP), 330 (HF) with acquired resolution of 1.5x1.5 mm² and reconstructed to 0.98x0.98 mm². The horizontal tagging lines were spaced 9.8mm apart and a delay of 250 ms between application of the tag and the acquisition of the bTFE image sensitized the images to movement within the colonic chyme. 33 dynamic scans were acquired at 600 ms intervals within a 20 s breath-hold. If no motion of colonic contents occurred during the breath hold then all 33 tagged images would be identical. However movement of the contents resulted in changes in intensity from frame to frame derived from either gross movement of the tagged lines (lines visible but displaced) or ‘smearing’ of the tagged lines due to mixing as seen in Figure 1. The first 4 dynamic images were discarded due to global intensity changes as steady state was approached. Maps of the mean signal intensity (MI) and standard deviation (STDEV) of each pixel through the remaining 29 images were calculated using IDL® (Research Systems Inc, Boulder, CO) (Figure 2). A region was defined encompassing the AC, (Figure 2) and the average mean intensity (MI_R) and average STDEV within the region (STDEV_R) were calculated (using Analyze9 ^TM-Mayo Foundation Rochester, MN, USA). In order to normalise the variance to account for the increase in signal intensity in the water-loaded colon post-macrogol ingestion, the average coefficient of variation (%COV) for that defined region was defined as $$%COV= 100x STDEV_R/ MI_R $$ Statistical analysis was carried out using Prism 5 (GraphPad Software Inc.). Comparisons within group were performed using Wilcoxon’s matched-pairs signed rank test. Comparisons between groups were performed using Mann-Whitney rank sum test and differences were considered significant at p<0.05.Scans were unavailable for one healthy volunteer and two patients at t=60 mins and one healthy volunteer at t=120 mins for operational reasons. The raw images (Figure 1) and processed summary maps (Figure 2) showed movement down the centre of the colon with an unstirred layer at the edges. The %COV data obtained are plotted in Figure 3 and summarised in Table 1. An increase in %COV was seen within the HV group following ingestion of the test drink but not the constipation group. MRI tagging of the colonic contents is a feasible method of studying colonic motility and mixing. The method identified differences in intracolonic mixing between healthy and constipated subjects in response to a 500ml macrogol drink. The technique shows potential to generate novel insights into physiology and mechanisms of disease and insights into the environmental pressures on, and effects of, gut microbiota.