Colonoscopy is the endoscopic examination of the large colon and the distal part of the small bowel with a CCD camera or a fiber optic camera on a flexible tube passed through the anus. It may provide a visual diagnosis (e.g. ulceration, polyps) and grants the opportunity for biopsy or removal of suspected lesions.
Virtual colonoscopy, which uses 2D and 3D imagery reconstructed from computed tomography (CT) scans or from nuclear magnetic resonance (MR) scans, is also possible, as a totally non-invasive medical test, although it is not standard and still under investigation regarding its diagnostic abilities. Furthermore, virtual colonoscopy does not allow for therapeutic maneuvers such as polyp/tumor removal or biopsy nor visualization of lesions smaller than 5 millimeters. If a growth or polyp is detected using CT colonography, a standard colonoscopy would still need to be performed.
Colonoscopy can remove polyps as small as one millimeter or less. Once polyps are removed, they can be studied with the aid of a microscope to determine if they are precancerous or not.
Colonoscopy is similar to but not the same as sigmoidoscopy, the difference being related to which parts of the colon each can examine. While colonoscopy allows an examination of the entire colon (measuring four to five feet in length), sigmoidoscopy allows doctors to view only the final two feet of the colon. A sigmoidoscopy is often used as a screening procedure for a full colonoscopy, in many instances in conjunction with a fecal occult blood test (FOBT), which can detect the formation of cancerous cells throughout the colon. Other times, a sigmoidoscopy is preferred to a full colonoscopy in patients having an active flare of ulcerative colitis or Crohn's disease to avoid perforation of the colon. Additionally, surgeons have lately been using the term pouchoscopy to refer to a colonoscopy of the ileo-anal pouch.