Fig. 3

a Three-dimensional view of the rectum showing a small polypoid defect surrounded by some fluid (white arrowhead). b The corresponding axial image in the supine position shows a negative filling defect floating in a contrast material pool (white arrowhead). White arrow hyperdense tip of the rectal catheter. c In the prone position the filling defect has moved to the anterior part of the rectum (white arrowhead). Both positional shift and floating of the defect in contrast material allow for a correct diagnosis of non-tagged stool