Gallbladder and Small Intestinal Luminal Opacification by Vicarious Contrast Medium Excretion Can be Observed on Delayed Computed Tomography in Healthy Dogs

Vicarious excretion is a nonrenal pathway of excretion for intravenously injected iodinated contrast media, with a limited understanding of its influencing factors and imaging features. In this prospective pilot study, gallbladder opacification (GBO) and small intestinal luminal opacification (SILO) were assessed to identify vicarious excretion patterns following intravenous contrast media administration for CT in clinically healthy dogs. Eight beagles were studied using a crossover method, divided into fed and fasted groups. The fed group was fed at 5 and 13 h after the first CT scan, while the fasted group was fed only at 13 h. Noncontrast and postcontrast CT scans were performed at 90 s, 10 min, 1 h, 4 h, 12 h, and 24 h after iohexol injection. The GBO was subjectively scored from grade 0 to 5 based on the attenuation value and area of opacification. The SILO was evaluated subjectively based on contrast intensity (weak or marked) and distribution. The GBO was observed from 1 h after contrast injection. Significant differences were noted in median GBO scores within the groups at 4, 12, and 24 h on Friedman's test followed by the post hoc Wilcoxon signed-rank test. The scores were significantly higher in the fasted group at 12 h on the Wilcoxon signed-rank test. The SILO occurred 10 min after contrast administration, with various distributions. In conclusion, GBO and SILO can be observed during delayed CT phases, and fasting increases the intensity and duration of GBO in clinically healthy dogs. These findings should not be interpreted as pathological changes.