Figure 4b. Eight-millimeter round filling defect in the rectum. (a) Axial CT image shows a well-circumscribed, homogeneously attenuating, round filling defect (arrow) on an interhaustral fold. Arrowhead = rectal catheter. (b) Endoluminal CT image shows the round polypoid morphology of the lesion (arrow). Arrowhead = rectal catheter. (c) Image from conventional colonoscopy shows the identical morphology (arrow). Histologic analysis revealed a tubular adenoma.