Non Tagged Stool

In a minority of cases a small amount of stool remains non-tagged.

Non-Tagged Stool <6 mm i<

iiHB

Fig. 4.8. Patient with right sided sigmoid. There is tagged stool (arrowhead). There is also non-tagged "material" (arrow). This should be considered a lesion unless the contrary is proved. A correct diagnosis of an 8-mm sessile polyp was made

Non-tagged stool <6 mm is too small too cause any concern as it is generally accepted that polyps <6 mm do not need to be removed. Hence pseudopol-ypoid lesions <6 mm caused by non-tagged residual stool should not be taken into consideration. This non-tagged stool appears as pinpoint filling defects abutting the colonic wall or is frequently floating in barium pools without touching the colonic wall (Fig. 4.11).

Non-Tagged Stool >6 mm

In our experience non-tagged stool >6 mm is present in 3-5% of segments. Mostly it presents as one or two non-tagged stool balls completely surrounded by some barium or floating in barium pools (Fig. 4.12). This stool may also present with the typical imaging

Fig. 4.9.a Small amount of barium abutting a lesion (arrow- Fig. 4.10.a Supine view of the sigmoid in lung window set-head) in the sigmoid and hence improving polyp conspicu- tings (W/L 1500/-200) showing a stalked polyp abutting the ity. b Improved depiction of the lesion in soft tissue settings. colonic wall, surrounded by a barium layer (white arrow). b c The prone view shows a partially collapsed sigmoid. The Same image in soft tissue settings (W/L 400/100) showing barium again delineates the lesion improving visualisation improved conspicuity of the lesion. c The prone acquisition (white arrow) shows the corresponding segment is collapsed. The lesion is still visible as a negative filling defect as it is surrounded by tagged fluid (white arrowhead)

Fig. 4.9.a Small amount of barium abutting a lesion (arrow- Fig. 4.10.a Supine view of the sigmoid in lung window set-head) in the sigmoid and hence improving polyp conspicu- tings (W/L 1500/-200) showing a stalked polyp abutting the ity. b Improved depiction of the lesion in soft tissue settings. colonic wall, surrounded by a barium layer (white arrow). b c The prone view shows a partially collapsed sigmoid. The Same image in soft tissue settings (W/L 400/100) showing barium again delineates the lesion improving visualisation improved conspicuity of the lesion. c The prone acquisition (white arrow) shows the corresponding segment is collapsed. The lesion is still visible as a negative filling defect as it is surrounded by tagged fluid (white arrowhead)

Fluid Tagging

Fig. 4.12. Non-tagged stool >1 cm in the sigmoid (arrow). This non-tagged material shows the characteristics of non-tagged stool: completely surrounded by barium, hooked appearance, some minute air inclusions

Fig. 4.12. Non-tagged stool >1 cm in the sigmoid (arrow). This non-tagged material shows the characteristics of non-tagged stool: completely surrounded by barium, hooked appearance, some minute air inclusions findings of stool as seen with regular preparations without faecal tagging: 1) moving to the dependent part of the colon with dual positioning; 2) presenting with an air inclusion; 3) presenting with a hyper-dense peripheral ring and central hypodensity or air b inclusion; 4) having a hooked appearance; 5) having no attachment to the colonic wall.

Mostly this non-tagged stool does not cause any diagnostic problems. In a recent study of 180 patients (Lefere et al. 2005) using this preparation no false positives were caused by non-tagged stool (see Sect. 4.6, Results).

Tagged fluid typically is hyperdense or white. This enables visualisation of the colonic wall through the fluid on the 2D images and solves the issue of the drowned segment. In fact semicircular folds as well as tumoral lesions appear as negative filling defects in the fluid (Fig. 4.13). This avoids false negative findings. When tagged fluid is present in a collapsed segment, a lesion can sometimes be distinguished as a negative filling defect (Figs. 4.14 and 4.15). The semicircular folds show their typical appearance fading out in the colonic wall when scrolling through the axial slices (Fig. 4.15). Using the preparation as described above, the colon is quite dry. Fluid is detected in about 40% of segments. In most segments this fluid covers less than 25% of the colonic lumen on the axial slices. The density of the fluid is lower when compared with stool tagging and varies between 100 and 1000 H.U.

Fig. 4.11.a Supine view of the rectosigmoid showing tiny non-tagged residue in the rectum (white arrowhead) and sigmoid (open black arrowhead). This stool is too small to cause any diagnostic problem. b Ultra low dose scan (64-slice) showing tagged fluid level with floating non-tagged 6-mm residue (arrow). There is no contact with the colonic wall, so no confusion with a polyp is possible

Fig. 4.11.a Supine view of the rectosigmoid showing tiny non-tagged residue in the rectum (white arrowhead) and sigmoid (open black arrowhead). This stool is too small to cause any diagnostic problem. b Ultra low dose scan (64-slice) showing tagged fluid level with floating non-tagged 6-mm residue (arrow). There is no contact with the colonic wall, so no confusion with a polyp is possible

Fluid Tagging

Fig. 4.13a,b. Fluid tagging in: a soft tissue; b bone window settings. The bone window setting enables visualisation of the colonic wall and semicircular folds (arrowhead) through the dense fluid a c

Fig. 4.14.a Supine image of the ascending colon obtained at ultra low dose (64-slice) showing sessile 8-mm polyp (black arrow). b Prone view shows the lesion covered by fluid. Because the fluid is tagged the lesion appears as a negative filling defect (arrow). c The sagittal reformatted image confirms this finding (arrow). This image shows different densities of tagged fluids in the same patient (white arrowheads)

Fig. 4.14.a Supine image of the ascending colon obtained at ultra low dose (64-slice) showing sessile 8-mm polyp (black arrow). b Prone view shows the lesion covered by fluid. Because the fluid is tagged the lesion appears as a negative filling defect (arrow). c The sagittal reformatted image confirms this finding (arrow). This image shows different densities of tagged fluids in the same patient (white arrowheads)

Fig. 4.15.a Ultra low dose scan (64-slice). Supine view of the rectum showing an 8-mm sessile polyp on the anterior border above a small level of tagged fluid, besides the first valve of Houston (black arrows). The valve of Houston is visible in the fluid as a linear filling defect. Small non-tagged residue in the fluid (open black arrowhead). c Corresponding endoluminal view. Despite the ultra low dose there are no streak artefacts. The polyp is easy to detect (black arrow) besides the first valve of Houston

(black arrowheads)

Fig. 4.15.a Ultra low dose scan (64-slice). Supine view of the rectum showing an 8-mm sessile polyp on the anterior border above a small level of tagged fluid, besides the first valve of Houston (black arrows). The valve of Houston is visible in the fluid as a linear filling defect. Small non-tagged residue in the fluid (open black arrowhead). c Corresponding endoluminal view. Despite the ultra low dose there are no streak artefacts. The polyp is easy to detect (black arrow) besides the first valve of Houston

(black arrowheads)

The density of this fluid also shows intra- and interpatient variability (Fig. 4.14).

To assess the efficacy of fluid tagging with barium as the sole tagging agent we reviewed 200 patients. The residual fluid was evaluated on the axial slices according to its proportion to the maximal anteroposterior diameter of the segment of the colon where it was detected. In this way four different groups were generated: 0%, <25%, 25-50%, >50%. There was residual fluid in 43.9% of segments. In 14.8% of segments there was non-tagged fluid. However this fluid covered less than 25% of the colonic lumen on the axial slices in 12% of segments being 79.5% of segments with non-tagged fluid. Entire visualisation of the colonic wall was obtained in all patients as the non-tagged fluid nicely redistributed with dual positioning (i.e. supine-prone scanning) (Fig. 4.16).

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