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To assess the added benefits of prone positioning in addition to supine positioning and oral iodinated contrast medium for help in the detection of colonic polyps at computed tomographic (CT) colonography.CT colonography was performed in prone and supine positions in 180 patients with polyps or risk factors for colonic neoplasia. Patients were randomly assigned to receive a standard bowel preparation or a standard preparation plus oral iodinated contrast medium. One radiologist interpreted supine images alone, and another analyzed supine and prone images. All patients subsequently underwent colonoscopy.At colonoscopy, 121 large (or =1-cm-diameter) polyps and 142 smaller (0.5-0.9-cm) polyps were identified. Prone positioning resulted in increased sensitivity for identification of patients with large (or =1-cm) polyps (increase from 70% to 85%, P: =.004) and of patients with polyps 0.5 cm or larger (increase from 75% to 88%, P:.005), with no change in specificity. Use of oral contrast medium did not significantly improve polyp detection even in the subset of patients in whom colonic fluid attenuation was markedly increased.Acquisition and review of supine and prone CT colonographic images significantly improves the ability to identify patients with polyps 0.5 cm in diameter or larger. Administration of oral iodinated contrast medium does not significantly improve polyp detection.

Virtual dissection is a three-dimensional (3D) display technique for CT colonography that could potentially reduce interpretation times. With virtual dissection, the 3D model of the colon is "sliced" open along a centerline trace, rendering a 360° view of the endoluminal mucosa as a rectangular image. However, one must be familiar with several pitfalls and limitations to avoid errors in interpretation. One of the main limitations is the anatomic distortion that results. Polyp shape and colonic folds can be distorted and colonic or polyp mobility can lead to mischaracterization of polyps. Distorted folds, which frequently occur at flexures, can mimic polyps. Annular constricting masses can lead to skip areas, where the abnormality is not displayed. Various entities including diverticula and stool can mimic polyps at virtual dissection. Finally, technical errors such as an inadequate centerline trace can render a polyp occult. The purpose of this review is to demonstrate the spectrum of appearances of polyps at virtual dissection, with an emphasis on more difficult to detect polyps. In addition, 10 interactive virtual dissection quiz cases are presented along with corresponding two-dimensional and 3D endoluminal fly-through views.