CT colonography (virtual colonoscopy): what to expect
CT colonography is a CT scan of the colon and rectum. It needs the same bowel prep as a colonoscopy but no sedation and no scope, and the scan takes about 15 minutes. The USPSTF lists it every 5 years as one way to screen adults 45 to 75. If it shows polyps or other suspicious areas, you need a regular colonoscopy. (ACS, 2026) (MedlinePlus, 2025) (USPSTF, 2021)
What CT colonography is
A virtual colonoscopy is an imaging test that looks for cancer, polyps or other disease in the large intestine (colon). The medical name is CT colonography. A computer combines the x-ray images into 3-D pictures of the inside of the colon, which a radiologist reads. (MedlinePlus, 2025) Like a colonoscopy, it can generally see the whole colon; a flexible sigmoidoscopy sees only the lower part. (USPSTF, 2021) How it compares with stool tests, colonoscopy and the other options is on colorectal cancer screening tests compared.
Where the guidelines place it
| Group | What it says about CT colonography |
|---|---|
| USPSTF (2021) | One of the recommended screening strategies, every 5 years, for adults 45 to 75 (grade A 50 to 75, grade B 45 to 49); abnormal findings need a follow-up colonoscopy (USPSTF, 2021) |
| American Cancer Society | A preferred visual exam, every 5 years, for people at average risk starting at 45 (ACS guideline, 2026) |
| US Multi-Society Task Force (2017) | A second-tier test, every 5 years: an appropriate screening test, but with disadvantages compared with colonoscopy every 10 years and a yearly FIT, the first-tier tests (USMSTF, 2017) |
MedlinePlus says virtual colonoscopy is used for people at average risk, and that colonoscopy is the preferred test for people who have had colon polyps or colon cancer, or who are at higher risk. (MedlinePlus, 2025) RadiologyInfo says CT colonography is not recommended for people with active Crohn's disease, ulcerative colitis, inflammatory bowel disease or diverticulitis, because of a higher risk of perforating the colon, nor for people with a history of bowel perforation or with severe pain or cramps on the day of the exam. (RadiologyInfo, 2026)
Before the test: the bowel prep
The colon must be empty, or a problem that needs treatment may be missed. CT colonography needs the same type of bowel prep as a colonoscopy. (MedlinePlus, 2025) (ACS, 2026) Your provider gives you the steps. MedlinePlus says they may include no solid food for 1 to 3 days before the test, typically clear liquids for 1 day before, and enemas. Follow your provider's instructions about your usual medicines. Do not wear jewelry on the day of the exam; you change into a hospital gown. (MedlinePlus, 2025) What a full prep is like is on screening colonoscopy: what to expect.
During the scan
- The test is done in the radiology department of a hospital or medical center. No sedatives are needed and no colonoscope is used. (MedlinePlus, 2025)
- You lie on your left side and a small, flexible tube is placed in the rectum. Air or carbon dioxide is pumped in to open up the colon so it is easier to see. (MedlinePlus, 2025)
- The table slides into the CT scanner. Pictures are taken while you lie on your back and again on your stomach. You stay very still and may be asked to hold your breath briefly. (MedlinePlus, 2025)
- The scan takes about 15 minutes. The x-rays are painless; the air may cause cramping or gas pains. (ACS, 2026) (MedlinePlus, 2025)
- Afterward you may feel bloated, have mild cramps and pass a lot of gas, and you should be able to go back to your regular activities. (MedlinePlus, 2025)
Your results and C-RADS
A radiologist reads the images and sends an official report to the doctor who ordered the exam. In some cases you learn right away whether polyps were seen. (RadiologyInfo, 2026) Reports may use C-RADS (CT Colonography Reporting and Data System), now under the American College of Radiology. It has colon (C) categories and categories for findings outside the colon (E). The table restates the 2023 version as a 2026 review reproduces it. (C-RADS review, 2026)
| Category | What it means | Suggested next step |
|---|---|---|
| C0 | Inadequate study (poor prep or the colon not opened enough), or waiting for older scans to compare | Repeat CT colonography or consider another screening test (C-RADS review, 2026) |
| C1 | Normal colon or benign finding; no polyp 6 mm or larger | Continue routine screening, every 5 to 10 years (C-RADS review, 2026) |
| C2a | Intermediate polyp: fewer than three polyps of 6 to 9 mm, or an indeterminate finding | Repeat CT colonography in 3 years or a colonoscopy referral, depending on the person (C-RADS review, 2026) |
| C2b | Likely benign mass-like area linked to diverticular disease | Repeat CT colonography in 5 years if likely benign, in 3 years or less if uncertain (C-RADS review, 2026) |
| C3 | Polyp, possibly advanced adenoma: a polyp 10 mm or larger, three or more polyps of 6 to 9 mm, or a C2a polyp that grew | Colonoscopy referral (C-RADS review, 2026) |
| C4 | Likely malignant colon mass | Colonoscopy, and surgical or cancer specialist consultation (C-RADS review, 2026) |
A regular colonoscopy may also follow, on a different day, if no cause was found for bleeding or other symptoms, because a virtual colonoscopy can miss some smaller problems. (MedlinePlus, 2025) What a colonoscopy after polyps finds and when the next one is due is on colonoscopy after polyps.
Why a positive result needs a colonoscopy
CT colonography cannot remove polyps or take tissue samples. If polyps or other suspicious areas are seen, a colonoscopy is needed to remove them or look further, and the USPSTF says abnormal findings need a follow-up colonoscopy for screening to bring its benefits. (MedlinePlus, 2025) (ACS, 2026) (USPSTF, 2021) Some imaging centers can do the colonoscopy, and remove polyps if needed, on the same day as the CT colonography. (RadiologyInfo, 2026) More on what follows any abnormal screening result: abnormal result? What usually happens next.
Findings outside the colon
Virtual colonoscopy often finds problems with organs other than the colon, including the kidneys, chest, liver, ovaries, spleen and pancreas. Some of these findings lead to more tests that may not improve health. (NCI, 2025) In the studies the USPSTF reviewed (27 studies, 48,235 people), 1.3% to 11.4% of exams found something outside the colon that needed more workup, and 3% or less of people with such findings needed medical or surgical treatment for it. These findings may be more common with age, and it is not yet known whether they help or harm on balance. (USPSTF, 2021) RadiologyInfo notes they can include early cancers in other organs and an abdominal aortic aneurysm. (RadiologyInfo, 2026)
C-RADS sorts these findings into E categories: E1/E2 means nothing clinically important or a known stable finding, with no more workup; E3 a likely unimportant finding where more workup may be warranted; E4 a likely important finding, such as an abdominal aortic aneurysm larger than 3 cm, that needs more workup. (C-RADS review, 2026) Why extra findings can be a mixed blessing is on false positives and overdiagnosis.
Risks and limits
- Radiation. Reported doses range from 0.8 to 5.3 mSv, compared with an average yearly background dose of 3.0 mSv per person in the US. (USPSTF, 2021) The NCI says the harms of this radiation are not known. (NCI, 2025) Tell your doctor and the technologist if there is any chance you are pregnant. (RadiologyInfo, 2026)
- Perforation. Inflating the colon with air carries a very small risk of injury, estimated at fewer than 1 in 10,000 patients, much lower than with colonoscopy. (RadiologyInfo, 2026) (ACS, 2026)
- Small polyps. It often cannot detect polyps smaller than 10 mm, and it may not tell stool from small polyps as well as colonoscopy. (MedlinePlus, 2025) (RadiologyInfo, 2026)
- Prep side effects. The prep can cause nausea, vomiting, bloating or rectal irritation. (MedlinePlus, 2025)
- Scanner limits. The scanner table usually has a weight limit, usually 450 pounds. (RadiologyInfo, 2026)
Coverage in general terms
Medicare Part B covers screening CT colonography from age 45 when your doctor or another clinician orders it: once every 60 months if you are not at high risk, or 48 months after a flexible sigmoidoscopy or colonoscopy, and once every 24 months at high risk. You pay nothing if your provider accepts assignment. (Medicare.gov) RadiologyInfo notes that some insurers do not cover it as a screening test, and that Medicare has covered screening CT colonography since January 2025. (RadiologyInfo, 2026) Coverage of a follow-up colonoscopy is on colorectal cancer screening tests compared.
Where they differ:
- The USPSTF lists CT colonography every 5 years among its recommended strategies without ranking them, and the American Cancer Society lists it as a preferred visual exam. (USPSTF, 2021) (ACS guideline, 2026)
- The US Multi-Society Task Force ranks it second-tier, behind colonoscopy and a yearly FIT. (USMSTF, 2017)
- MedlinePlus says colonoscopy is preferred for people with past polyps or colon cancer, or at higher risk. (MedlinePlus, 2025)
Your doctor decides what your result means for you.
Questions people ask
Is CT colonography the same as a virtual colonoscopy?
Yes. Virtual colonoscopy is the common name; the medical name is CT colonography. It is an imaging test that uses x-rays and a computer to look for cancer, polyps or other disease in the colon, without a scope. (MedlinePlus, 2025) (ACS, 2026)
Do I need a bowel prep for CT colonography?
Yes. It needs the same type of bowel prep as a colonoscopy, because the colon has to be empty for clear pictures. Your provider gives you the steps. (ACS, 2026) (MedlinePlus, 2025)
How long does a virtual colonoscopy take, and can I drive afterward?
The scan takes about 15 minutes. No sedatives are needed, so you can usually go back to your normal activities right away. You may feel bloated, have mild cramps and pass a lot of gas for a while. (ACS, 2026) (MedlinePlus, 2025)
What happens if CT colonography finds a polyp?
Polyps cannot be removed during CT colonography. If polyps or other suspicious areas are seen, a regular colonoscopy is needed to remove them or look further. Some imaging centers can do the colonoscopy on the same day. (ACS, 2026) (USPSTF, 2021) (RadiologyInfo, 2026)
How much radiation is in a CT colonography?
The USPSTF reports doses from 0.8 to 5.3 mSv, compared with an average yearly background dose of 3.0 mSv per person in the US. The NCI says the harms of this radiation are not known. (USPSTF, 2021) (NCI, 2025)
Does Medicare cover a virtual colonoscopy?
Part B covers screening CT colonography from age 45 when your clinician orders it: once every 60 months, or 48 months after a flexible sigmoidoscopy or colonoscopy, and once every 24 months at high risk. You pay nothing if your provider accepts assignment. (Medicare.gov)