Colonoscopy vs CT Colonography

What is CT colonography?

CT colonography (CTC) — also called virtual colonoscopy or CT pneumocolon — is an X-ray-based scan that creates detailed three-dimensional images of the inside of your colon without putting a camera inside you. A radiologist then reviews these images to look for polyps, growths, or other abnormalities. It's a real and legitimate investigation, not a lesser substitute.

You do still need to do a bowel preparation beforehand — the colon has to be empty and clean for the images to be useful, just as with a regular colonoscopy. A small, flexible tube is placed briefly into the rectum to gently fill the bowel with carbon dioxide gas so it opens up for the scan. The scan itself takes only a few minutes, and no sedation is needed at all.

CT colonography is well established in Australia and attracts a Medicare rebate in certain specific situations — for example, after an incomplete conventional colonoscopy, or if a regular colonoscopy is considered medically unsafe for you.

How the two tests compare

Accuracy: how do they compare?

For larger polyps — 10 mm or more — CT colonography is genuinely comparable to a regular colonoscopy. It picks up roughly 90–95% of large polyps, which is why it's considered a reasonable alternative when a regular colonoscopy isn't possible for you.

For smaller polyps (6–9 mm), the accuracy drops to around 70–80%. For polyps under 6 mm, it drops further. There are also certain flat polyps called sessile serrated lesions — which are increasingly recognised as important because they can progress to cancer through a different pathway — that CT colonography is more likely to miss.

A regular colonoscopy is the gold standard because it can both find and remove a polyp in the same session. With CT colonography, if something is found that needs to be removed, you'll still need to go through a colonoscopy — which means two rounds of preparation and two procedures instead of one.

When is CT colonography the preferred choice?

For most people, a regular colonoscopy is the better test. But CT colonography is genuinely the right choice for a specific group of patients:

• Incomplete colonoscopy: If your colonoscopy couldn't be completed — because your bowel is particularly long or bendy, or because there's a narrowing or a large growth blocking the way — CT colonography can image the parts of the colon that couldn't be reached, using the same preparation you've already done.
• Too unwell for sedation: If you have significant heart or lung disease, or other medical conditions that make receiving sedation risky for you, CT colonography is a much safer option because no sedation is needed.
• Blood thinners you can't safely stop: If you're on anticoagulant medication (blood thinners) that can't be paused without putting you at real risk — for example, after a recent blood clot or with a mechanical heart valve — CT colonography avoids the need to stop those medications.
• Strong preference against colonoscopy: If you genuinely cannot face a colonoscopy and are at risk of simply not having any investigation at all, CT colonography is a reasonable path that can still find important problems.
• Suspected blockage: If a cancer may be blocking part of the bowel before surgery, CT colonography can map the colon above the blockage to guide the surgical plan.
• Looking at other organs at the same time: As a bonus, CT colonography images your whole abdomen and pelvis — including your liver, kidneys, and other structures — which can occasionally pick up something important outside the bowel.

Limitations of CT colonography

It's important to go in with a clear understanding of what CT colonography can't do. Here's the honest picture:

• It can't remove polyps or take biopsies. This is the biggest limitation. CT colonography is purely a looking test — if something is found that needs to come out or be tested, you'll still need a conventional colonoscopy.
• You still have to do the bowel preparation. A lot of people hope that CT colonography means skipping the prep. Unfortunately, it doesn't. You need a clean bowel for the images to be accurate, and the preparation is essentially the same. Some centres use a technique called "faecal tagging" that reduces the prep slightly, but results are less reliable.
• There is a small radiation dose. CT scans use X-rays. The dose for a CT colonography is roughly 2–8 millisieverts — about the same as one to four years of the natural background radiation we all receive from the environment. For most adults having an occasional examination, this is considered low and acceptable. It's a slightly more relevant consideration for younger patients.
• It can find things outside the bowel — not always helpfully. Because CT colonography images your whole abdomen, it often picks up incidental findings in other organs — things like small kidney cysts or adrenal nodules. These usually turn out to be nothing significant, but they can trigger further tests, further anxiety, and further cost. This is worth being aware of.
• Small and flat polyps are easier to miss. Polyps under 6 mm and flat-type polyps are more likely to be overlooked on CT colonography compared with a regular colonoscopy.
• It's usually not Medicare rebated for routine screening. Medicare only covers CT colonography in specific situations. If you're an average-risk person wanting it as your first-choice screening test, it will likely cost you out of pocket. Ask your GP or the imaging centre about costs before you book.

The bowel preparation for CT colonography

You'll need to do essentially the same bowel preparation for CT colonography as for a regular colonoscopy — a low-residue diet in the days before, followed by a laxative solution to clear everything out. Some CT centres use a technique called "faecal tagging," where you take a small amount of oral contrast with your meals, so any residual material shows up differently on the scan and can be subtracted electronically. This can sometimes reduce how much preparation is needed, but it's not available everywhere and the results aren't quite as reliable as a fully clean colon.

During the scan, a small tube is placed briefly in your rectum to gently fill the bowel with carbon dioxide gas — this opens it up so the camera can see the walls properly. Most people feel bloated and a bit uncomfortable during this part, and you may feel the urge to pass wind. It's temporary, and the gas escapes quickly once the scan is done. No sedation is needed, and the actual scanning part only takes a few minutes.

Cost and Medicare in Australia

A colonoscopy performed by a specialist for a clinically appropriate reason attracts a Medicare rebate in Australia. Your out-of-pocket cost will vary depending on the facility and whether an anaesthetist is involved — your doctor's rooms can give you an estimate.

CT colonography attracts a Medicare rebate in specific circumstances, including:

• Investigating a suspected abnormality in the colon
• Following up on a previous incomplete colonoscopy
• When a conventional colonoscopy is medically contraindicated (not safe for you)

If you simply prefer CT colonography as your first-choice bowel cancer screening test and don't have one of those specific indications, you'll generally pay out of pocket for it. Make sure you ask about the cost before you book — your GP or the imaging centre can help you understand what to expect.

Which test should you choose?

For most people who have been referred for a colonoscopy — whether for bowel cancer screening, investigating symptoms, or checking for new polyps — a conventional colonoscopy remains the recommended test. It finds small polyps more reliably, and crucially, it can remove them on the spot, giving you a definitive answer and a treatment in a single procedure.

CT colonography is genuinely useful and evidence-based for the specific situations described above. The right choice for you depends on your overall health, your medical history, your personal circumstances, and what you feel comfortable with. This is worth discussing openly with your specialist.

If you're not sure which path is right for you, Mr Ba Nguyen is very happy to talk through the options with you and help you feel clear about the decision.

Frequently asked questions