Figure 12a. Residual fluid obscuring a polyp in the descending colon in a 67-year-old man. Colonoscopic examination could not be completed because of a suspected stenosis. (a) Prone CT scan shows a fluid level (arrow) in the ventral dependent portion of the descending colon. (b) On a supine CT scan, the fluid level has shifted to the dorsal dependent portion of the colon, revealing a 9-mm polyp (arrow). The polyp was not seen on the prone CT scan because the lesion was submerged in residual fluid.