Virtual. You are in a cat scanner, with air and sometimes contrast pumped into you. Unpleasant and not as accurate as a regular colonoscopy. Also, if something is found, youmwill need a colonoscopy anyway.
CT. Virtual colonoscopy involves a bowel prep just like a regular colonoscopy. However, rather than a scope inserted via the anus to look at the inside with a camera, air is infused into the colon and a ct scan obtained. The radiologic images give a 3d picture of the inside of the colon to determine if any polyps or other ptoblems exist. If they do, you will still need a colonoscopy to check them out.