The large bowel is anatomically divided in the following segments: rectum, sigmoid colon, descending colon, transverse colon, ascending colon and cecum. Since the colonoscopy visualizes the entire colon and rectum, it requires a very thorough and complete cleansing of the bowel.  The procedure is usually performed in the outpatient hospital or in the  endoscopy center under conscious sedation.

The colonoscope is introduced through the anus and advanced under direct vision until the tip of the instrument reaches the cecum; this is the most proximal part of the colon. The ileocecal valve is the landmark indicating that the deepest point of the colon has been reached. The procedure usually takes 15-20 minutes, however may take longer if polyps are detected and removed.

Most of the polyps of the colon and rectum, which have not turned into cancers yet, can be removed during the colonoscopy through a variety of techniques. 

Snare excision, which consists in cutting the stalk of the polyp with a snare that carries electrical current is applied for medium size polyps. 

Fulguration usually takes care of smaller polyps and biopsies could be done of polyps that are too big to be removed through the colonoscope. 

Once a patient has been diagnosed with polyps of the colon and rectum and they are successfully removed, the patient should undergo a very careful follow-up with colonoscopy every one to three years, since polyps may recur either in the same location or in another location. 

If a subsequent colonoscopy has failed to show any recurrences, it may then be repeated at longer intervals. 

Complications of the colonoscopy consist of bleeding and perforation, particularly after removal of polyps; these complications however do not occur very frequently (from 1 every 500 to 1 every 2000 procedures, according to the series).

A barium enema is a radiological technique that used to be very popular for screening of colon cancer. Because it is less sensitive, more uncomfortable than a colonoscopy, today it is used less frequently. 

A Virtual Colonoscopy is a radiological exam based on the use of Computerized Axial Tomography  technique (also known as CAT Scan). It is a strictly diagnostic tool (meaning that it is able to detect colorectal cancers and polyps), however has no therapeutic capabilities. Polyps detected by Virtual Colonoscopy still need to be removed through the use of a conventional colonoscopy.
Computed Tomographic Virtual Colonoscopy to Screen for Colorectal Neoplasia in Asymptomatic Adults.
Perry J. Pickhardt, M.D., J. Richard Choi, Sc.D., M.D., Inku Hwang, M.D., James A. Butler, M.D., Michael L. Puckett, M.D., Hans A. Hildebrandt, M.D., Roy K. Wong, M.D., Pamela A. Nugent, M.D., Pauline A. Mysliwiec, M.D., M.P.H., and William R. Schindler, D.O
New England Journal of Medicine Volume 349:2191-2200-December 4, 2003-Number 23

Methods: A total of 1233 asymptomatic adults (mean age, 57.8 years) underwent same-day virtual and optical colonoscopy. The colonoscopists were unaware of the findings on virtual colonoscopy. The sensitivity and specificity of virtual colonoscopy and the sensitivity of optical colonoscopy were calculated  and compared.
Results: The sensitivity of virtual colonoscopy for adenomatous polyps was 93.8% for polyps at least 10 mm in diameter, 93.9% for polyps at least 8 mm in diameter, and 88.7% for polyps at least 6 mm in diameter.
The sensitivity of optical colonoscopy for adenomatous polyps was 92.3%, 91.5% and 87.5%  for the three sizes of polyps, respectively
 for the three sizes of polyps, respectively.
The specificity of virtual colonoscopy for adenomatous polyps was 96.0%, 92.2%, and 79.6%.
Two malignant polyps  were detected on virtual colonoscopy, and one of them was missed on optical colonoscopy before the results on virtual colonoscopy were revealed.
Conclusions: CT virtual colonoscopy is an accurate screening method for the detection of colorectal polyps  in asymptomatic average-risk adults and compares favorably with optical colonoscopy.

Colon Diagram	Sigmoid Colon	Cecum with Appendiceal Orifice
Virtual Colonoscopy tridimensional view	Virtual Colonoscopy with a polyp