Virtual Colonoscopy

Computed tomography, more commonly known as a CT or CAT scan, is a diagnostic medical test that, like traditional x-rays, produces multiple images or pictures of the inside of the body.

The cross-sectional images generated during a CT scan can be reformatted in multiple planes, and can even generate three-dimensional images. These images can be viewed on a computer monitor, printed on film or transferred to a CD or DVD.

CT images of internal organs, bones, soft tissue and blood vessels typically provide greater detail than traditional x-rays, particularly of soft tissues and blood vessels.

CT colonography, also known as virtual colonoscopy, uses low dose radiation CT scanning to obtain an interior view of the colon (the large intestine) that is otherwise only seen with a more invasive procedure where an endoscope is inserted into the rectum and passed through the entire colon.

You should wear comfortable, loose-fitting clothing to your exam. You will be given a gown to wear during the procedure.

Women should always inform their physician and the CT technologist if there is any possibility that they may be pregnant.

The bowel-cleansing regimen for CT colonography is similar to that for a colonoscopy. Your diet will be restricted to clear liquids the day before the examination. It is very important to clean out your colon the night before your CT colonography examination so that the radiologist can clearly see any polyps that might be present. Please refer to the preparation sheet.

Be sure to inform your physician if you have heart, liver or kidney disease to be certain that the bowel prep will be safe. Your physician can advise you on dietary restrictions prior to the exam. You will be able to resume your usual diet immediately after the exam.

The CT scanner is typically a large, box-like machine with a hole, or short tunnel, in the center. You will lie on a narrow examination table that slides into and out of this tunnel. Rotating around you, the x-ray tube and electronic x-ray detectors are located opposite each other in a ring, called a gantry. The computer workstation that processes the imaging information is located in a separate control room, where the technologist operates the scanner and monitors your examination in direct visual contact and usually with the ability to hear and talk to you with the use of a speaker and microphone.

During CT colonography, you will be asked to lie on your back and then on your stomach.

In many ways CT scanning works very much like other x-ray examinations. X-rays are a form of radiation—like light or radio waves—that can be directed at the body. Different body parts absorb the x-rays in varying degrees.

In a conventional x-ray exam, a small burst of radiation is aimed at and passes through the body, recording an image on photographic film or a special image recording plate. Bones appear white on the x-ray; soft tissue shows up in shades of gray and air appears black.

With CT scanning, numerous x-ray beams and a set of electronic x-ray detectors rotate around you, measuring the amount of radiation being absorbed throughout your body. Sometimes, the examination table will move during the scan, so that the x-ray beam follows a spiral path. A special computer program processes this large volume of data to create two-dimensional cross-sectional images of your body, which are then displayed on a monitor.

CT imaging is sometimes compared to looking into a loaf of bread by cutting the loaf into thin slices. When the image slices are reassembled by computer software, the result is a very detailed For CT colonography, the computer generates a detailed 3-D model of the abdomen and pelvis, which the radiologist uses to view the bowel in a way that simulates traveling through the colon. This is why the procedure is often called a virtual colonoscopy. Two dimensional (2-D) images of the inside of the colon as well as the rest of the abdomen and pelvis are obtained and reviewed at the same time.

The technologist begins by positioning you on the CT examination table, usually lying flat on your back. We will inject into your vein an anti-spasmodic agent (Buscopan or Glucagon) to relax your bowel and avoid cramps. Gas is then insufflated (To treat medically by blowing gas, or vapor into a bodily cavity). Once that is done, the technologist will take the images while you are lying on your back then again on your stomach. The entire procedure takes about 20 minutes.

Depending on the part of the body being scanned, you may be asked to raise your arms over your head.

A very small, flexible tube will be passed two inches into your rectum to allow the carbon dioxide to be gently pumped into the colon. A small balloon is inflated on the rectal tube to help keep the tube positioned correctly. The purpose of the gas is to distend the colon as much as possible to eliminate any folds or wrinkles that might obscure polyps from the physician's view.

Next, the table will move through the scanner. Patients are asked to hold their breath for about 15 seconds or less before turning over and lying on their back or side for a second pass that is made through the scanner. In some centers the sequence of positions may be the opposite: facing upward first and then facing down. Once the scan is done, the tube is removed.

The vast majority of patients who have CT colonography report a feeling of fullness when the colon is inflated during the exam, as if they need to pass gas. The scanning procedure itself causes no pain or other symptoms.

After a CT exam, you can return to your normal activities.

Benefits:

• Minimally invasive - can depict many polyps and other lesions as clearly as when they are directly seen by conventional colonoscopy.
• Markedly lower risk of perforating the colon than conventional colonoscopy.
• Virtual colonoscopy is an excellent alternative for patients who have clinical factors that increase the risk of complications from colonoscopy, such as treatment with anticoagulants (blood thinners) or a severe breathing problem.
• Elderly patients, especially those who are frail or ill, may tolerate Virtual colonoscopy better than conventional colonoscopy.
• Virtual colonoscopy can be helpful when colonoscopy cannot be completed because the bowel is narrowed or obstructed for any reason, such as by a large tumor.
• If conventional colonoscopy cannot reach the full length of the colon—which occurs up to 10 percent of the time. Virtual colonoscopy can view the entire colon.
• No sedation or pain-relievers are required; therefore there is no recovery period.
• Able to go to work right after the procedure.

Risks:

• There is a very small risk that inflating the colon with air could injure or perforate the bowel. This has been estimated to happen in fewer than one in 10,000 patients.
• There is always a slight chance of cancer from excessive exposure to radiation. However, the benefit of an accurate diagnosis far outweighs the risk.
• CT scanning is, in general, not recommended for pregnant women unless medically necessary because of potential risk to the baby.

Disadvantages:

• No biopsies are taken with this method; if something is seen and requires biopsy a conventional colonoscopy will be required.
• Does not visualize Irritable Bowel Syndrome.