A virtual colonoscopy (VC) is also known as a CT colonography. VC is an alternate screening option for some people. A radiologist uses CT scan technology to create 2-D and 3-D images of the colon. It does not require sedation. VC has limitations, however. For example, it requires the same preparation as a conventional colonoscopy: a clear-liquid diet and medication to clear out the colon. In addition, if VC detects a polyp or other abnormality in the colon, you may need a colonoscopy to remove it. VC can sometimes miss small or flat polyps.