Virtual Colonoscopy (CT Colonography): A Non-Invasive Alternative

For patients who are reluctant to undergo a traditional colonoscopy, CT colonography, commonly known as virtual colonoscopy, offers an alternative approach to colorectal cancer screening. The procedure uses computed tomography (CT) scanning to produce detailed cross-sectional images of the colon without inserting a scope. While it does not replace conventional colonoscopy in all situations, it has earned a place in the screening toolkit as a recognized option endorsed by several major medical organizations.

How CT Colonography Works

During a virtual colonoscopy, you lie on a CT scanner table while a small, flexible tube is inserted a few inches into the rectum. Carbon dioxide or room air is gently pumped through the tube to inflate the colon, which helps separate the colon walls and create a clear image. You will feel pressure and mild bloating, but the process is not typically painful.

The CT scanner then takes rapid images as you lie on your back and then on your stomach. The entire scan usually takes about 10 to 15 minutes. There is no sedation, so you can drive yourself home afterward and return to normal activities immediately.

Specialized software processes the CT images to create both two-dimensional cross-sections and a three-dimensional "fly-through" view of the colon interior, which the radiologist can navigate much like a virtual video tour. This is where the name "virtual colonoscopy" comes from.

Accuracy

The clinical value of CT colonography depends on the size of the polyps it needs to detect. For polyps 10 millimeters or larger, the most clinically significant size, CT colonography has a sensitivity of approximately 90%, which is close to conventional colonoscopy. For polyps between 6 and 9 millimeters, sensitivity drops to roughly 70 to 80%. For polyps smaller than 6 millimeters, detection rates are considerably lower, though very small polyps rarely pose a near-term cancer risk.

The ACRIN 6664 trial, a large multicenter study published in the New England Journal of Medicine, found that CT colonography detected 90% of adenomas and cancers 10mm or larger. This performance was sufficient for the American Cancer Society, the American College of Radiology, and the U.S. Multi-Society Task Force to endorse CT colonography as an acceptable screening option every five years.

Advantages Over Conventional Colonoscopy

• No sedation required. You remain fully awake and alert. There is no recovery period, and you do not need someone to drive you home.
• Lower procedural risk. The risk of colon perforation with CT colonography is extremely rare (approximately 1 in 10,000), compared to roughly 1 in 1,000 for conventional colonoscopy.
• Faster procedure. The scan itself takes 10 to 15 minutes, compared to 30 to 60 minutes for a conventional colonoscopy.
• Visualization of structures outside the colon. Because CT scans image the entire abdomen and pelvis, CT colonography may incidentally detect abnormalities in other organs such as the kidneys, liver, or abdominal aorta. This can occasionally lead to the early detection of other conditions, though it can also lead to additional testing for findings that turn out to be benign.

Limitations

• Bowel preparation is still required. This is often the most surprising fact for patients considering virtual colonoscopy. You still need to follow a clear liquid diet and drink a prep solution, though some protocols use a lower-volume prep combined with a contrast agent ("tagging agent") that coats residual stool to distinguish it from polyps.
• Polyps cannot be removed during the procedure. If the CT colonography detects a polyp that needs removal, you will need a conventional colonoscopy, which means a second bowel prep and a second procedure.
• Radiation exposure. CT colonography involves ionizing radiation, typically delivering a dose of about 5 to 8 millisieverts (mSv). For context, the average annual background radiation in the United States is about 3 mSv. Low-dose protocols can reduce the exposure to 1 to 3 mSv.
• Flat polyps are harder to detect. Sessile serrated polyps, which tend to be flat and subtle, are more easily missed by CT colonography than by a skilled endoscopist performing a conventional colonoscopy.
• Insurance coverage varies. While Medicare covers CT colonography for screening in some circumstances, not all private insurers do. Check with your plan before scheduling.

Who Is a Good Candidate?

CT colonography may be particularly appropriate for:

• Patients who cannot tolerate sedation or have medical conditions that make sedation risky.
• Patients on blood-thinning medications that cannot be safely stopped for polyp removal.
• Patients who have had an incomplete conventional colonoscopy (for example, due to a tortuous or narrowed colon).
• Patients who refuse conventional colonoscopy but are willing to undergo screening by another method.

It is generally not recommended for patients with active inflammatory bowel disease, recent colon surgery, or symptoms suggestive of colorectal cancer (such as rectal bleeding or unexplained weight loss), who should undergo conventional colonoscopy for both diagnostic and therapeutic reasons.