CAD False Positives

False positives may lead to unnecessary further workups such as polypectomy by colonoscopy; therefore, knowledge about the pattern of CAD false positives is important for dismissing them. Studies showed that most of the false positives detected by CAD tend to exhibit polyp-like shapes, and the major causes of CAD false positives are the following (Yoshida et al. 2002a,b). Approximately half (45%) of the false positives are caused by folds or flexural pseudotumors. They consist of sharp folds at the sigmoid colon, folds prominent on the colonic wall,

Fig. 11.6a-d. Example of CAD false negatives. (Reprint, with permission, from Yoshida and Daohman 2005)

two converging folds, ends of folds in the tortuous colon, and folds in the not-well-distended colon. One-fifth (20%) are caused by solid stool, which is often a major source of error for radiologists as well. Approximately 15% are caused by residual materials inside the small bowel and stomach, and 10% are caused by the ileocecal valve. Among other causes of false positives are rectal tubes, elevation of the anorectal junction by the rectal tube, and motion artifacts, each amounting to less than 3%.

Representative examples of CAD false positives are shown in Figure 11.7. Figure 11.7a shows a prominent fold (arrow). The tip of the fold appeared to be a cap-like structure, and thus it was incorrectly identified by CAD as a polyp. Figure 11.7b shows a piece of solid stool (arrow). This polyp-mimicking stool has a cap-like appearance and a solid internal texture pattern, and thus it was detected incorrectly as a polyp. Figure 11.7c shows an ileocecal valve (arrow). The tip of the ileocecal valve often has the cap-like appearance of a polyp and thus can be a cause of false positives in CAD. Figure 11.7d shows the residual materials (arrow) inside the small bowel and stomach. Although a majority of the small bowel and stomach is removed in the colon extraction step, a small piece of them may be extracted along with the colon, and thus residual materials in the small bowel and stomach can cause false-positive detections.

Studies show that radiologists can dismiss the majority of these false positives relatively easily based on their characteristic locations and appearance (Dachman et al. 2002). For example, false positives due to ileocecal valves and the rectal tube can easily be dismissed based on their anatomic location and shape; a semi-automated recognition of ileocecal valves may make this already easy task even easier (Summers et al. 2004). Solid stool can be distinguished from polyps by visual correspondence analysis between prone and supine views; this relatively elaborate process can be facilitated by a computer aid (see Sect. 11.5.2).

However, there are types of false positives, such as solid stool that mimics the shape of polyps and adheres to the colonic wall, which are difficult to differentiate from polyps even for an experienced

Fig. 11.7a-d. Example of CAD false positives. (Reprint, with permission, from Yoshida and Dachman 2005)

Fig. 11.7a-d. Example of CAD false positives. (Reprint, with permission, from Yoshida and Dachman 2005)

c radiologist. Moreover, the pattern of the false positives may differ as new CAD techniques are developed. More research is required for establishing how radiologists can remove these false positives to make a correct final diagnosis reliably.

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