A colonoscopy is a procedure that allows doctors to minutely scan the large intestines for the presence of any mass, ulcers or irregular growths like polyps. It is a minimally invasive procedure that uses a fiber optic camera to enable doctors to examine the large intestines and the bowels.

According to the National Institutes of Health a colonoscopy "is most often used to look for early signs of cancer in the colon and rectum. It is also used to look for causes of unexplained changes in bowel habits and to evaluate symptoms like abdominal pain, rectal bleeding, and weight loss."

Colonoscopy can remove polyps smaller than one millimeter in diameter. These polyps are then examined microscopically to check for signs of cancer if any.

Prior to beginning colonoscopy, it is vital that the doctor is aware of any medical condition or medications being taken by the patient. According to the NIH, these include:
    * Aspirin
    * Arthritis medications
    * Blood thinners
    * Diabetes medication
    * Vitamins that contain iron

The colonoscopy procedure is a relatively simple one. Before beginning the examination, the doctor will administer a painkiller and a sedative to keep the patient relaxed and calm at all times. Thereafter a long, flexible, lighted tube called the colonoscope is inserted into the rectum and guided into the colon.. The scope bends so the doctor can move it around the curves of the colon.

Suspicious growth are then cauterized and taken to the lab to check for presence of cancer. The procedure is not without risks though. Because it is performed under anesthesia, cardiopulmonary complications such as temporary drop in blood pressure and oxygen saturation can occur. These will be taken care of by the medical team if they do occur.

The most serious complication is a condition called gastrointestinal perforation in which the instrument perforates the intestine. This is relatively rare.

Routine colonoscopies are recommended annually for people aged over 50 years as the risk of cancer is quite evident at this age.

In President Bush's case there appear to be no signs of cancer and it is just a precautionary measure. Doctors from National Naval Medical Center in Bethesda, Maryland will perform today's procedure.

Mr Bush has had polyps in his colon before he assumed presidency, but none were evident in 2002. However if polyps are detected this time around they will be removed and examined for presence of cancer.