An experimental approach to virtual colonoscopy could eliminate the unpleasant day-before bowel prep that keeps many people from having this potentially life-saving test. Virtual colonoscopy uses computed tomography (CT) scanning with X-rays, instead of a scope, to check the colon for cancers and precancerous polyps. Earlier version have required bowel cleaning, just like regular colonoscopy. A Harvard-based team led by Dr. Michael Zalis uses sophisticated computer software to make stool in the colon disappear. It’s a little like Photoshopping blemishes from still photos. “Laxative-free CT colonography has the potential to reach some of the unscreened population and save lives,” says Dr. Zalis, an associate professor of radiology at MGH and director of CT colonography at MGH Imaging.