MRI analysis of fecal volume and dryness: Validation study using an experimental oxycodone-induced constipation model.

Background Opioids are known to affect gastrointestinal motility, but their effect on fluid absorption and secretion is poorly understood in humans. Purpose To investigate the effect of oxycodone on colonic fecal volume and stool dryness by using a novel MRI-based technique. Study Type Prospective, randomized, double-blinded, crossover study. Subjects Twenty-five healthy male volunteers (median age: 24 years [range: 21-56]; mean body mass index [BMI]: 23.9 kg/m(2) [range: 22.9-25.0]) without known gastrointestinal disease. Field Strength/Sequence T-2-weighted and two-point Dixon MRI scans of the abdomen at 1.5 T. Assessment Subjects were treated for 5 days with prolonged-release oxycodone or a placebo. Imaging was performed on the first and last study day in each period. Images of the colon were analyzed with semiautomatic k-means-based segmentation software. Regional colonic fecal volumes were quantified excluding gas volume and colon wall. Two-point Dixon and T-2-weighted MRI signal intensity were assessed as a proxy of colonic stool dryness. Data were obtained in a previously reported study. Statistical Tests Intraclass correlation coefficients were used to test the reliability of measurements between days, while repeated measures mixed models were applied to test treatment effects. Results After oxycodone treatment, total colonic fecal volume was significantly increased compared with placebo (mean change 100 mL vs. -13 mL; P = 0.001), with the largest increase (24%) observed in the ascending colon/cecum (P = 0.001). Dixon signal increased (less water in colon content) after oxycodone treatment compared with placebo (mean 0.09 vs. -0.02; P < 0.001). T-2-weighted signal decreased (less water in colon content) after oxycodone treatment compared with placebo (mean -0.03 vs. 0.03; P = 0.002). Data Conclusion The 5-day oxycodone treatment increased colonic fecal volume and increased stool dryness compared with placebo. This imaging-based method for noninvasive analysis of colon content has the potential to characterize gastrointestinal symptoms in general, such as in constipation. Technical Efficacy Stage: 1 J. Magn. Reson. Imaging 2019;50:733-745.