Electronic cleansing (EC) is an emerging method for segmentation of fecal materials, which are tagged by an X-ray-opaque oral contrast agent in CT colonography (CTC) images, effectively removing them for digital cleansing of the colon. Due to the partial volume effect, voxels at the boundary between the lumen and tagged materials, called the L-T boundary, not only have CT values that are close to those of soft-tissue structures, but also have gradient values that are similar to a thin soft-tissue structure that is sandwiched between the tagged region and the lumen, which we call a tagging-tissue-air layer. Degradation of thin colonic wall and folds, as well as creation of pseudo soft-tissue structures at the periphery of tagged regions, are main artifacts in existing EC approaches, which tend to use a gradient-based method to delineate tagged regions. In this study, we developed a novel delineation method of tagged regions by applying local iso-surface roughness. The local iso-surface roughness is defined by the sum of differences between the local curvedness at adjacent scales over all scales. In our approach, the roughness values around the periphery of the tagged regions are integrated into the speed function of a level-set segmentation method for delineation of the tagged regions. As a result, L-T boundaries are subtracted along with tagged regions, whereas the thin soft-tissue structures within the tagging-tissue-air layers are preserved. Application of our computer-aided detection (CAD) scheme showed that the use of the new EC method substantially reduced the number of false-positive detections compared with that of our previous gradient-based method.