Figure 18a. Flat adenoma in the rectum. (a) Image from conventional colonoscopy shows a slightly discolored area (arrows), which is not raised from the background mucosa. Analysis of the biopsy specimen revealed a tubular adenoma. (b) Axial CT image of the same area shows no abnormality (arrow). Even in retrospect, the abnormality could not be seen with 2D or 3D imaging. Truly flat adenomas are nearly impossible to visualize at CT colonography.