Stool Residual

Gourtsoyiannis et al. 2004). This method can be used to detect colonic polyps and differentiate them from residual stool. Polyps present with variable degrees of enhancement, while residual stool do not exhibit any enhancement at all (Papanikolaou et al. 2003 Lauenstein et al. 2001). 

Initially MRC should be read in the multiplanar reformation mode, scrolling through the contrast-enhanced 3D data set in all three orthogonal planes. Whenever a mass protruding from the colonic wall is detected, the identical part of the colon should be analysed on the pre-contrast scan. Increase in signal intensity within the mass can aid in differentiating between residual stool particles and colorectal lesions while colorectal lesions always show strong enhancement, residual stools never show any contrast uptake (Fig. 17.8b,c). 

Fig. 4.1. The residual stool in the ascending and descending colon is tagged and appears white on the two-dimensional images, facilitating diagnosis...

The mobile nature of these segments mimicks positional change of lesions, possibly causing erroneous diagnosis of mobile residual stool (Fig. 8.5) (Park et al. 2005). 

Fig. 8.5a,b. False negative diagnosis polyps simulating fecal residue in mobile segments. Differential diagnosis of mobile stool or small sessile lesions in a mobile transverse colon a supine scan shows two lesions in the transverse colon, (arrows) b prone scan shows the lesions in the transverse colon in an apparent different position (arrows). Conventional colonoscopy revealed the presence of two small sessile polyps. Lesson Polyps, located in mobile colonic segments such as the transverse colon can cause erroneous diagnosis of mobile residual stool... 

Small sessile polyps frequently represent hyperplastic polyps. Hyperplastic lesions tend to flatten out in well distended segments, explaining the fact that those lesions might only be visible in somewhat underdistended segments. In that way, those lesions are frequently only recognised on either prone or supine position, and can therefore be mistaken as residual stool. 

Hyperplastic lesions however are not to be considered precancerous, and should therefore be considered as "leave-alone lesions. Misinterpreting those lesions as residual stool is therefore rather beneficial for the patients, avoiding unnecessary conventional colonoscopy (Pickhardt et al. 2004a) (Fig. 8.17). 

Sessile cancers may also be mistaken for residual stool because of marked surface irregularity, usually attributed to residual stool (Gluecker et al. 2004). 

Pedunculated polyps however change in position between prone and supine images, and may moreover include gas between the stalk and the bowel wall, mimicking residual stool (Fig. 8.19). 

Mischaracterisation as fecal residues is caused by the fact that there are three observations that are made to distinguish stool from polyps presence of gas, morphology (polyps and small cancers have rounded and lobulated smooth borders), and the mobility. In particular, the mobility of the lesions is used in favour of residual stool, analogue to the findings on double contrast barium enema (Laks et al. 2004). 

The effect of preparation is important, not only in terms of cleansing of the colon of residual stool, but also of residual fluid. Most CTC reports have been done on consenting research patients who are already scheduled to undergo a non-research conventional colonoscopy. The preparation is often chosen by the gastroenterologist. A colonic lavage... 

Fig. 13.3a,b. Inverted diverticulum pseudopolypoid shape on virtual endoscopic images. On 2D images, these lesions contain some air, residual stool, or fat attenuation because of a central umbilication in the inverted part of the diverticulum or due to an inclusion of perisigmoidal fat. (Used with permission of Lefere et al. 2003)... 

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