Pictorial review of colonic polyp and mass distortion and recognition with the CT virtual dissection technique

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.