Figure 3a. Pitfall of apparent mobility of a polyp. (a) Supine axial CT image shows a homogeneously attenuating pedunculated filling defect (arrow) in the sigmoid colon. Note that the filling defect is on the ventral, nondependent wall. (b) Prone axial CT image shows that the filling defect (arrow) is now on the dorsal wall. Again, the filling defect is on the nondependent wall. (c) Endoluminal CT image shows the filling defect (arrow), which measures 7 mm. (d) Image from conventional colonoscopy shows that the filling defect is a tubular adenoma (arrow). Apparent mobility of a filling defect, especially in the cecum, transverse colon, and sigmoid colon should be interpreted cautiously, since these areas of the colon may rotate when the patient is turned from supine to prone. If the colon is well prepared and the filling defect is homogeneously attenuating and polypoid, it should be considered a polyp.