Figure 2a. Mobility as an indicator of residual fecal material. (a) Supine axial CT image shows a 15-mm homogeneously attenuating filling defect (arrow) on the dorsal aspect of the rectum. (b) Prone axial CT image shows that the filling defect (arrow) is now on the ventral aspect of the rectum, a finding indicative of mobility. In general, mobility indicates residual fecal material.