Colorectal lesions: evaluation with CT colography.

Computed tomographic (CT) colography is a promising technique for differentiating malignant or premalignant colorectal disease from benign lesions. In this technique, helical CT data are used to produce reformated two-dimensional (2D) CT images and simulated endoscopic images of the colon. Adenomatous polyps 0.7 cm in diameter or larger are easily detected on CT colographic images: A pedunculated polyp is identified by means of its stalk, whereas a sessile polyp appears as a polypoid soft-tissue mass projecting into the air-filled lumen of the colon. However, flat adenomas (lesions raised less than 2 mm from the surface of the colon) are difficult to detect with CT colography. In cases of colorectal cancer, both intraluminal and extraluminal disease can be evaluated with CT colography. Although CT colography does not allow differentiation between hyperplastic and adenomatous polyps, lipomas can be confidently diagnosed because of their fatty attenuation on reformatted 2D images. Pseudolesions that can produce false-positive findings at CT colography include the ileocecal valve, retained stool, retained barium, respiratory artifacts, and a stool-filled diverticulum.