Tagging of feces, especially fluid, by an oral contrast agent such as a barium suspension or water-soluble iodinated contrast material, is a promising method for differentiating residual feces from polyps and thus improving the accuracy in the detection of polyps (Bielen et al. 2003; Lefere et al. 2002, 2004a, b, ; PickhARDt et al. 2003; ThOMEER et al. 2003).
Digital bowel cleansing is an emerging technology for removing the tagged stool and fluid, and thus it is useful for reducing bowel cleansing while maintaining the accuracy of human readers in detecting polyps (Pickhardt and Choi 2003; Zalis and Hahn 2001; Zalis et al. 2003, 2004b).
Tagging of feces introduces an additional challenge to CAD because reduced bowel cleansing tends to introduce a large amount of fecal residue, some of which may be tagged well, whereas some may not be tagged completely. Such a mixture of tagged and untagged stool can be a cause of false positives in CAD (Yoshida et al. 2004a). Figure 11.10 shows examples of stool (arrows) adhering to the colonic wall that were not tagged by the barium-based tagging regimen and thus were erroneously detected as polyps by CAD.
Moreover, digital cleansing may introduce artifacts because of the partial volume effect and a suboptimal mucosal reconstruction method, especially at the interface of air and tagged fluid along the colonic wall or at the interface of air, fluid, and a fold (Zalis et al. 2004a). Digital cleansing may also create 3D artifacts that simulate polyps because incomplete cleansing due to suboptimal opacifica-tion of luminal fluid can result in artifacts that may have the appearance of polyposis (Pickhardt and Choi 2003), which can be a cause of false positives in CAD. Current investigations of CAD for fecal-tagging cases with cathartic preparation (Summers et al. 2005) and with reduced or minimum preparation (Yoshida et al. 2004a; Z alis et al. 2004a) are encouraging in that CAD showed the potential to detect polyps not only in the dry region of the colon, but also submerged in the tagged fluid. The left image in Figure 11.11 shows an example of a polyp (black arrow) that was submerged in the tagged fluid; this polyp was correctly detected and segmented by CAD, as indicated by the black region in the right image.
Although encouraging, the results of these studies are limited and are not conclusive; therefore, CAD for fecal-tagging CTC remains a subject for future research.
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