Invented 16 years ago, virtual colonoscopy has become an increasingly popular alternative to standard, or optical, colonoscopy, which is typically performed by a gastroenterologist. Initially regarded as a high-tech novelty, the new procedure has in recent months received key endorsements as a first-line screening test from influential medical groups. But in February, officials at the Centers for Medicare & Medicaid Services announced a preliminary decision not to cover the procedure as a mass screening test for Medicare recipients.