What is CT Colonography?

CT (computed tomography) colonography, also referred to as a virtual colonoscopy, is a minimally invasive diagnostic test or technique that utilises low-dose radiation CT scans to inspect the colon and the rectum for colorectal cancer.

Colorectal cancer is a cancer that forms in the tissues of the colon (the longest part of the large intestine) and the rectum (the last few inches of the large intestine near the anus). It is the third-leading cause of cancer-linked deaths in the United States among both men and women.

CT colonography involves taking a series of X-ray images of the colon to create cross-sectional images (slices) to provide more detailed information about the colon than plain X-rays do in order to detect cancer.

Indications for CT Colonography

Your physician typically recommends a CT colonography to screen for cancers and other conditions affecting the colon. In general, CT colonography may be used for:

  • Screening for colorectal cancer
  • Screening for growths called polyps before they turn cancerous 
  • Evaluation of colon after unsuccessful or incomplete optical (standard) colonoscopy
  • Assessment of strictures
  • Individuals with contraindications to or who do not want a standard colonoscopy
  • Better examination of the colon proximal to obstructing neoplasms identified by standard colonoscopy

Preparation for CT Colonography

In general, CT colonography preparation will involve the following:

  • You are recommended to have a clear liquid or low-fibre diet for at least 24 hours prior to the procedure.
  • Your bowel will be completely emptied using a laxative or enema the night before your exam or just before the procedure so that your radiologist can clearly see the inside of your colon and rectum for any polyps or abnormalities that might be present.
  • Your physician may also recommend a special liquid drink (contrast medium) called gastrografin over 1 or 2 days. Gastrografin is a form of dye containing iodine that assists to make scan pictures clearer. It also functions as a laxative and may cause diarrhoea.
  • You should disclose information on your current medications as your doctor may need to adjust them prior to the procedure or may ask you to stop taking certain medicines for a defined period.
  • You should also bring your medical records and imaging pertaining to your current problem.
  • You should inform your doctor if you:
    • Are allergic to any medications
    • Have diabetes, heart, or lung condition
    • Are taking any blood thinning medications (anticoagulants) such as aspirin that may affect blood clotting
    • Are pregnant
  • Signing an informed consent form after the pros and cons of the procedure has been explained

Procedure for CT Colonography

CT colonography is typically performed as an outpatient procedure in the radiology (CT scanning) department at the hospital. A specialist doctor (radiologist) or a radiographer performs this test using a CT scanner machine. The CT scanner is generally a large, doughnut-shaped machine with a short tunnel in the centre. You will be placed on a table that slides in and out of this short tunnel. Rotating around you, the X-ray tube and electronic X-ray detectors are positioned opposite to each other in a ring known as a gantry. The computer workstation that processes the imaging information is in a separate control room. This is where your technologist operates the scanner and monitors your exam in direct visual contact. Your technologist will be able to hear and talk to you using a speaker and microphone.

In general, CT colonography will involve the following steps:

  • Your radiographer will place you on the scanning table in a position that is best suited for conducting the exam.
  • A small tube is then inserted into the rectum and a carbon dioxide gas or air is used to distend (inflate) the colon and rectum to obtain clear scans of the inside of your bowel. You may feel uncomfortable and bloated with the gas.
  • Your radiographer will then move the scanning table into the CT scanner machine. Several X-ray beams and electronic X-ray detectors rotate around you. Each rotation yields several images of thin slices of your colon. You may hear buzzing and whirring noises.
  • A special computer program processes this large volume of data to generate a detailed three-dimensional (3-D) model of the colon, which your radiographer utilises to see the colon in a way that simulates travelling through the colon. This is why the procedure is often referred to as 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 taken and reviewed at the same time.
  • The entire exam typically takes around 30 minutes.
  • A radiologist, a physician specially trained to supervise and interpret radiology exams, will analyze the images and send an official report to the doctor who ordered the exam.

Post-Procedure Care

After the exam, you will be shifted to an observation room where you will stay for a short time to ensure that you feel well after the exam. You can return to your normal activities once you are comfortable. If you were administered contrast material, you may receive special instructions. You will likely be told to drink lots of fluids to help your kidneys remove the contrast material from your body after the scan.

Risks of CT Colonography

Some of the possible risks of CT colonography include:

  • Dehydration
  • Nausea
  • Stomach bloating
  • Stomach pain
  • Diarrhoea
  • Blurred vision
  • Tear in the bowel
  • Radiation exposure

Benefits of CT Colonography

Some of the benefits of CT colonography over conventional colonoscopy include:

  • Provides both 2-D and 3-D images that can represent many polyps and other lesions as clearly as when they are directly seen by conventional colonoscopy
  • Much lower risk of colon perforation compared to colonoscopy
  • Excellent alternative for individuals with a risk of complications from colonoscopy
  • A better diagnostic alternative for elderly patients who cannot tolerate standard colonoscopy
  • Provides clearer and more detailed images than a conventional colonoscopy
  • Less expensive than colonoscopy
  • Less exposure to radiation
  • Requires no sedation or pain reliever, hence well tolerated by most patients
  • No immediate side effects
  • Can detect abnormalities outside of the colon

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