Tagged Stool

With faecal tagging the residual stool appears as a bright hyperdense or white spot or mass in the colonic lumen with or without air inclusion (Fig. 4.7). This bright stool almost lights up when scrolling through the 2D images simplifying interpretation as there is no likelihood of mistaking it for a lesion. In that way the time consuming comparison between supine and prone position to detect an eventual change in location of the residual stool becomes superfluous. This shortens the reading time considerably, avoids false positives and improves polyp conspicuity because of the improved contrast difference between the non-tagged lesion and the tagged stool. When reading the tagged data sets, looking for non-tagged "material" is imperative. This non-tagged material is highly suspicious and should be

Tagged Colon Polyps

Fig. 4.4.a Stalked polyp in the sigmoid (white arrow) with some tagged fluid (black arrowheads). b Corresponding endoluminal view showing the head of the polyp (white arrow)

Fig. 4.4.a Stalked polyp in the sigmoid (white arrow) with some tagged fluid (black arrowheads). b Corresponding endoluminal view showing the head of the polyp (white arrow)

Fig. 4.5.a Coronal reformatted image of the transverse colon. Yellow open triangle shows the virtual camera looking in the transverse colon near the hepatic flexure. b Corresponding 3D view shows pseudopolypoid lesions caused by the tagged residue (arrowheads)

Fig. 4.5.a Coronal reformatted image of the transverse colon. Yellow open triangle shows the virtual camera looking in the transverse colon near the hepatic flexure. b Corresponding 3D view shows pseudopolypoid lesions caused by the tagged residue (arrowheads)

Transverse Colon Yellow

Fig. 4.6a-c. Axial image of the ascending colon displayed in different W/ L settings and showing tagged residue with a very high density of 2717 H.U.: a soft tissue settings (W/L 400/10). The semicircular fold (arrowhead) is only faintly seen; b lung window settings (W/L 1500/-200). Slightly improved visualisation of the semicircular fold; c bone window settings (W/L 3500/400) with very good visualisation of the semicircular fold allowing detailed inspection

Fig. 4.6a-c. Axial image of the ascending colon displayed in different W/ L settings and showing tagged residue with a very high density of 2717 H.U.: a soft tissue settings (W/L 400/10). The semicircular fold (arrowhead) is only faintly seen; b lung window settings (W/L 1500/-200). Slightly improved visualisation of the semicircular fold; c bone window settings (W/L 3500/400) with very good visualisation of the semicircular fold allowing detailed inspection a b a c

Colonoscopy Positioning

Fig. 4.7a,b. Examples of stool tagging (arrow and arrowheads). Confusion with a true lesion is excluded. No comparison between supine and prone images is necessary to exclude a lesion considered a lesion unless it has the specific characteristics of residual stool (Fig. 4.8). Even in the case of an obvious change in position between supine and prone position, a polyp has to be excluded. In fact polyps with a long stalk may show a considerable change in location with dual positioning. A similar lesion was mistaken as stool and caused a false negative (one of the two missed lesions >1 cm) in the landmark study of Fenlon et al. (1999). The density of the tagged stool varies between 100 and 3000. H.U. It is striking that there is a wide intra- and inter-patient variability. There is no apparent reason to explain this variability.

As with optical colonoscopy, tagged stool frequently abuts a lesion. This makes polyps more conspicuous for detection (Figs. 4.8, 4.9, and 4.10). In these cases polyp conspicuity is improved using soft tissue settings.

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