Errors at CTC can be due to a multitude of factors. The most common causes of error are related to technique, experience/training, and lack of recognition of uncommon but important presentations of polyps. Technical inaccuracies are probably the most important cause of error, especially for larger lesions (1 cm or larger in diameter) [18]. Every technical step must be carefully followed in order to achieve a high-quality and accurate examination. The commonest problems are suboptimal dis-tention and inadequate preparation of the colon. Distention can be reliably achieved with the use of a mechanical insufflator, and a warning to and acceptance by patients of a brief episode of discomfort during image acquisition. The use of the spasmolytic butyl scopo-lamine has also been shown to be effective in improving distention. In patients with a colon segment that is poorly distended in both the prone and supine position, a de-cubitus view (usually right lateral decubitus) can be very helpful [19]. Inadequate preparation of the colon can be minimized by providing easily understood instructions, and if possible an educational encounter with the patient is recommended to ensure understanding about the timing and amounts of the various required preparatory agents.

Experience and training are the keys to high-quality interpretations [20]. Spinzi et al. has shown that interpreting at least 75 examinations is required for competent interpretation [21]. Our experience has shown that most individuals are not adequately trained after a weekend course of lectures, including a 40-50 case hands-on experience. In testing readers for the National CT Colonography Trial (ACRIN 6664), only one third of readers were deemed expert at the end of a weekend course. Most readers were able to perform at 60%-80% sensitivity for large lesions after this amount of training. Expertise requires additional experience, and review of positive cases with emphasis on detecting the most difficult lesions.

Familiarity with the appearance of the most difficult lesions [22], especially flat adenomas, is important in order to achieve the highest performance rating [23, 24]. The most common errors that the novice reader makes are misinterpreting a cancer as a segmental region of collapse. In addition, reporting polyps less than 5 mm in diameter is associated with an unacceptably high false positive rate. We encourage our staff not to report these biologically unimportant bumps of less than 5 mm. The expert reader most commonly makes mistakes by not recognizing a lesion that is seen in only a single position (either prone or supine). Experts also tend to discount lesions as stool in segments of the colon with suboptimal preparation. Both expert and novice readers find flat lesions difficult to detect. Lesions that have an irregular surface are also often falsely considered as stool. Recognition that some ade nomas, especially villous adenomas, have an irregular surface is important.

Tagging of stool should prevent many of these types of errors in the future. Tagging, however, can induce other errors. Tagging that is too dense can cause streak artifacts. In addition, barium tagging agents can adhere to the wall of the colon and even cover polyps. It is important to carefully inspect the wall beneath the barium (whether in pools or as a circumferential coating). In many instances a bone window setting is helpful to assess the colon wall adjacent to high attenuation contrast material. Pedunculated polyps can move within the colon lumen and resemble moving stool at first glance. It is important to search for the fixed attachment of the pedun-culated polyp stalk to the colon wall.

Large lesions, especially those within the right and transverse colon, can cause the colon to rotate as the patient is turned from supine to prone position [25]. The sheer weight of the lesion and a loose colon mesentery probably account for this occurrence. Therefore, if a large lesion is seen in the dependent location of the colon on both the supine and prone views, the filling defect is not necessarily stool. First, a search should ensue to confirm that the colon has rotated. This is straightforward in the right colon, as the ileocecal valve can be easily identified, and its position confirmed on both views. In other parts of the colon this can be difficult. Second, the cleanliness of the colon should be assessed. A filling defect in an otherwise clean colon should be regarded as a real lesion. Finally, if necessary, intravenous contrast material can be administered [26]. All polyps 5 mm or larger will enhance at least two-fold following intravenous contrast material. Untagged stool does not enhance. This can be very helpful in selected patients with a confusing filling defect. Intravenous contrast material can also be helpful in patients with a large amount of residual colonic fluid that cannot be moved between imaging positions if the patient did not receive a tagging agent. If stool and fluid tagging is utilized, IV contrast material could actually obscure a lesion.

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