Fig. 11.5. Improvement of detection performance of human reader by use of CAD in the detection of polyps those encountered by radiologists (Yoshida et al. 2002a,b). Most of the CAD techniques depend on a shape analysis that assumes that polyps appear to have a cap-like shape, i.e., they appear as polypoid lesions. Therefore, polyps that do not protrude sufficiently into the lumen (e.g., diminutive polyps and flat lesions), whose shape deviates significantly from polypoid (e.g., infiltrating carcinoma), those that lose a portion due to the partial volume effect, those that are located in a collapsed region of the colon, or those that are submerged in fluid, may be missed by CAD. Improvement of the CAD techniques for reliable detection of these types of polyps remains for future investigation.
Representative examples of CAD false negatives are shown in Figure 11.6. Figure 11.6a shows a magnified view of a 6-mm polyp at the proximal transverse colon (white arrow), and Fig. 11.6b shows its 3D endoscopic view (white arrow). This polyp was located in a narrow valley where two folds merge, and thus the shape of the polyp was distorted. Moreover, a motion artifact made the polyp appear blurred, and thus it was a false-negative polyp. The neighboring polyp (black arrow), located below the convergence of the two folds, was detected by CAD because it was less distorted than the above polyp. Figure 11.6c shows a 7-mm polyp in the sigmoid colon, and Fig. 11.6d shows an 8-mm polyp in the sigmoid colon. These polyps appear smaller than expected from their size, mainly because they lost a portion due to the partial volume effect, and thus these polyps were false-negative polyps in CAD.
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