Computed tomographic colonography (Virtual colonoscopy): a new method for detecting colorectal neoplasms.

Computed tomographic (CT) colonography is an exciting new technique that uses volumetric CT data combined with advanced imaging software to create two-dimensional and three-dimensional images of the colon. The technique uses both three-dimensional images that simulate the endoluminal perspective of the colonoscope, as well as axial and reformatted two-dimensional images. The two-dimensional and three-dimensional images are complementary, and in combination offer the most robust performance for the detection of colorectal polyps. Currently, CT colonographic examinations are performed in the fully cleansed and air-inflated colon using a slice thickness of 5 mm, a reconstruction interval of 3 mm, a pitch of 1.3, and 70 mA. In a blinded, prospective study of 70 patients (half with a known lesion, and half from a surveillance population with a low disease prevalence) the sensitivity for the detection of polyps of 1 cm or more is 75%, and the specificity is 90%. The most commonly encountered problems include retained colonic fluid and stool, suboptimally distended colonic segments, and long interpretation times. Many of these problems can be solved using both supine and prone imaging. It is expected that the performance of this examination will improve, and that a new era of colorectal screening will begin.