What is CT colonography and what are its advantages and limitations as a colorectal cancer screening test?

• A. It is identical to colonoscopy in invasiveness and polyp removal
• B. It uses MRI to visualize colon without bowel prep
• C. CT colonography uses CT to visualize the colon after bowel prep and insufflation; less invasive than colonoscopy; good for polyps but requires bowel prep and may need follow-up colonoscopy for polyp removal; involves radiation exposure
• D. It has no radiation exposure

Answer: (C)

CT colonography, also known as virtual colonoscopy, uses CT imaging to visualize the colon after bowel cleansing and insufflation with air (or CO2) to distend the bowel. This makes the test less invasive than traditional colonoscopy because no endoscope is threaded through the rectum, and many patients tolerate it without deep sedation. It is effective for detecting polyps and cancers, providing a roadmap that can guide further evaluation. However, if a polyp is found or screening is positive, a conventional colonoscopy is usually needed to remove the polyp or obtain tissue. A key limitation is radiation exposure from the CT scan, and some small or flat lesions can be missed compared to direct colonoscopy. Additionally, it requires bowel prep, and it cannot remove polyps during the same procedure.