As for other full structural examinations of the colon it is mandatory to prepare the colon to prior to virtual colonoscopy. This is obvious as otherwise fecal residue hampers visualization of the mucosal surface of the colon.
There are two options:
This preparation combines a low residue or clear liquid diet with oral sodium phosphate (Phosphosoda 1 single dose of 45 ml) and two bisacodyl tablets. Despite an extensive cathartic cleansing frequently residual stool mimics a polypoid lesion (false positive) or inversely a polyp is interpreted as fecal residue (false negative). Furthermore residual fluid may cause drowned segments resulting in incomplete visualization of the colonic surface. This is less the case with oral sodium phosphate than with polyethylene glycol which is not considered a suitable cathartic for virtual colonoscopy anymore.
Fecal tagging is the peroral ingestion of contrast material (barium and/or iodinated contrast) to label or tag fecal residue remaining in the colon after the preparation. In fact as described a regular preparation without fecal tagging before virtual colonoscopy may cause an erroneous diagnosis. Fecal tagging can be performed in combination with a full cathartic cleansing. This has been done successfully by Pickhardt et al. In the largest trial on virtual colonoscopy ever performed, they obtained tremendous results of polyp detection in 1233 patients who were at average risk for colorectal cancer.
Not only fecal tagging reduces the lack of accuracy in polyp detection but also offers the occasion to reduce the preparation prior to virtual colonoscopy. In fact as the fecal residue is labeled in the colon, it is possible to leave more of it in the colon, thus to reduce the preparation. In a first study,we combined 750 ml of a 2.1 % w/v barium suspension with a reduced cleansing and obtained a sensitivity of % and 100 % for lesions 6-9 mm and > 1 cm respectively. In a recent study of 180 patients we obtained similar results (6-9 mm: %; > 1 cm: 100 %) using only a total of 60 ml of a 40 % w/v barium suspension to achieve efficient tagging.