Disadvantages of Virtual Colonoscopy

The doctor cannot take tissue samples or remove polyps during virtual colonoscopy, so a conventional colonoscopy must be performed if abnormalities are found. Also, virtual colonoscopy does not show as much detail as a conventional colonoscopy, so polyps smaller than 10 millimeters in diameter may not show up on the images.

source: http://digestive.niddk.nih.gov/ddiseases/pubs/virtualcolonoscopy/index.htm