What Is a Virtual Colonoscopy?

What to expect when undergoing this test

A virtual colonoscopy looks at the colon via CT images
What should you know about a virtual colonoscopy?. Image © Sebastian Kaulitzki/Science Photo Library/Getty Images

A virtual colonoscopy, also called CT colonography, is a special type of computed tomography scan that is offered as an alternative to a conventional colonoscopy. As a colon cancer screening test for people at average risk, a virtual colonoscopy is comparable to a conventional colonoscopy in its ability to detect colon cancer and polyps (at least those larger than 6 mm in diameter). It differs in a few ways, however, including how it is performed: A virtual colonoscopy uses a series of X-rays to create a 3-dimensional image of the inside of the colon, rather than requiring the insertion of a scope through the rectum and colon (large intestine).

Purpose of the Test

A virtual colonoscopy may be done as a screening test to look for colon cancer or precancerous polyps. A colonoscopy is somewhat unique among cancer screening tests. While many tests, such as mammography, are designed only to find cancer in the earliest of stages, a colonoscopy may also be effective in preventing cancer if a precancerous polyp is found and removed before it can progress to colon cancer.

When done according to the recommended schedule, screening colonoscopies have been found to significantly reduce the risk of a person dying from colon cancer. A virtual colonoscopy is recommended at age 50, for people of average risk, and every five years thereafter. For those who have risk factors for colon cancer, screening is often recommended earlier than age 50.

A virtual colonoscopy allows a doctor to indirectly inspect the entire inside of the colon. Findings may include masses, polyps, ulcers (areas of a breakdown of tissue), strictures (areas of narrowing), or fistulas (abnormal passageways between the colon and another region, such as the skin around the rectum). Specific conditions that may be suspected, though not formally diagnosed, based on findings include:

Conventional Colonoscopy vs. Virtual Colonoscopy

Conventional and virtual colonoscopy are considered comparable tests in detecting colon cancer in most individuals, but there are some differences.

A 2014 review of studies related found that virtual colonoscopy is not as sensitive or specific as the conventional type, but still provides similar results in average-risk individuals. A 2018 review also concluded that virtual colonoscopy is a reasonable option for screening asymptomatic people, but found that the virtual procedure was inferior in detecting advanced colorectal neoplasia (precancerous tissue).

Because of this, conventional colonoscopy is recommended for those who have risk factors for colon cancer, conditions such as inflammatory bowel disease, or symptoms of colon cancer such as:

That said, in some cases, a virtual colonoscopy can actually detect lesions that aren't found with a conventional colonoscopy, such as cancers and polyps that are found behind folds in the colon. While a conventional colonoscopy is incomplete (can't reach all the way to the beginning of the large intestine) roughly 10 percent of the time, this is not a problem with the virtual procedure.

Additionally, since a virtual colonoscopy involves a CT of the abdomen, it may detect other problems in the abdomen such as problems with organs like the liver, pancreas, or gallbladder, or conditions such as an abdominal aortic aneurysm. This can be both positive (if it identifies a real problem) and negative (if it prompts further, ultimately unnecessary testing).

A virtual colonoscopy is much faster, less invasive, and less uncomfortable than a conventional colonoscopy. Both, however, require you to restrict yourself to a clear liquid diet the day prior to surgery and undergo a colon preparation with laxatives and enemas to clean the colon. 

Virtual colonoscopy does not require sedation or anesthesia, the risks of which can be significant in people with heart disease or other medical conditions. The risk of perforation is also much lower with a virtual colonoscopy.

This method may be chosen by those who are hesitant to have a traditional colonoscopy for some reason, have certain medical conditions, have a possible bowel obstruction, or for those who are on medications, such as blood thinners, that cannot be stopped for the procedure. The thought in some of these cases is that the risks associated with conventional colonoscopy may outweigh the benefits.

Limitations

As stated, virtual colonoscopy is not as sensitive as a conventional one. Small polyps (less than 6 millimeters) and flat (sessile) lesions can be missed.

Moreover, if a polyp is found, a biopsy cannot be done during the procedure. If required, a conventional colonoscopy must be performed as a follow-up, requiring a repeat of the colon preparation process unless the procedure can be done on the same day. Since many people find that to be the most uncomfortable part of the procedure, the chance that this may need to be repeated should be considered.

Other Colon Cancer Tests

There are a number of different tests that have been used to find colon cancer in the earlier stages of the disease. Tests such as a guaiac smear test (done as part of a physician's rectal exam during a physical) and a fecal occult blood test (done on stool specimens at home) can sometimes find blood, but miss far too many cancers to be considered effective screening tools when compared with colonoscopy.

A barium enema, an X-ray study in which the colon is observed after inserting barium into the colon, can also detect some cancers, but misses significantly more cancers than either type of colonoscopy. Either a rigid or flexible sigmoidoscopy has also been used as a screening test for colon cancer, but only it looks at the sigmoid colon (the rectum and lower large intestine). Since roughly 40 percent of colon cancers are right-sided colon cancers, which would be missed on a sigmoidoscopy, the use of this procedure has declined. In addition, it appears that right-sided colon cancers are increasing.

Risks and Contraindications

As with many medical procedures, there are potential risks associated with a virtual colonoscopy, as well as times when the test should not be ordered.

Risks

The colon preparation process, though mostly a nuisance for those who are healthy, has the potential to cause problems, such as fluid overload, in those who have congestive heart failure or kidney disease.

There is a very small risk of perforation with a virtual colonoscopy (less than one in 10,000) and, if this occurs, surgery will be needed to repair the perforation. Perforation may occur due to the air or carbon dioxide that is used to inflate the colon.

There is also the risk of drug allergy to the contrast dye used in the procedure.

In recent years, the risk of leukemia related to CT scans has been called into question. Even though virtual colonoscopy exposes people to radiation, the amount of radiation is less than that involved in an abdominal CT scan.

To reduce your risk of complications, be sure that a thorough colon preparation is done according to your doctor's specifications and answer any questions she may have about your health fully.

Contraindications

Because of the risk of perforation, virtual colonoscopy is not recommended for people with active inflammatory bowel disease (ulcerative colitis or Crohn's disease) or diverticulitis. It should also be avoided by those who are experiencing abdominal pain or severe cramps.

Again, a conventional colonoscopy is preferred for people who have significant risk factors for colon cancer, such as a strong family history of the disease.

Due to the use of X-rays, virtual colonoscopy is not recommended for women who are pregnant or may be pregnant.

Before the Test

Before you have your virtual colonoscopy, you will have an appointment with your physician during which she will ask you about any symptoms and medical conditions you have. She will also explain the risks and benefits of the virtual procedure compared with a conventional colonoscopy, what you can expect during the procedure, and the preparation that is necessary beforehand.

If you have had any previous abdominal scans, she will ask you to gather them to bring to your appointment. Most often, radiologists require the actual scans or a CD of previous scans, rather than a typed report from another radiologist.

Timing

A virtual colonoscopy itself takes only 10 to 15 minutes, but it's important to leave extra time when you plan your test day. You may be asked to fill out forms at the hospital or surgicenter, unless you are given these to complete ahead of time. Prior to the procedure, it will take some time for you to change into a hospital gown, as well as to get positioned on the table. After the exam, you may be asked to wait for the radiologist to look at your images, or instead, you may be released to go right home.

Location

A virtual colonoscopy may be done in a hospital radiology department or at an outpatient facility.

Colon Preparation

Preparing for a colonoscopy involves cleaning out your bowels so they are empty of stool, and you will need to be near a bathroom throughout the preparation. Physicians differ in the precise preparation, but most of the time you will need to use a combination of laxatives and an enema the day prior to the procedure.

Laxatives may be taken in pill form, such as bisacodyl tablets, or as a liquid, in preparations such as Go-Lytely or NuLytely (polyethylene glycol solutions). Some of these laxatives require you to drink upwards of a gallon of fluid, and you should talk to your physician ahead of time if this volume of fluid could be a problem, such as if you have congestive heart failure or kidney disease.

Food and Drink

Starting around three days before your procedure, you be asked to avoid certain foods. This includes raw fruits and vegetables (canned and cooked are OK), popcorn, grains and seeds, nuts, and potato skins.

Most of the time, you will be instructed to follow a clear-liquid diet for one to two days before the procedure, then to refrain from eating or drinking anything after midnight the night before your test.

A clear liquid diet includes things such as water, bouillon or broth (without the fat), gelatin, plain coffee or tea (no cream), sports drinks, apple juice, or white grape juice (no pulp or liquids that are colored red or purple). It's important to try to drink at least two quarts of fluid during this time, unless you are instructed otherwise.

Contrast Medium

The night before the procedure, you will also be asked to drink the contrast medium that helps the radiologist visualize the inside of your colon. It's important to let your doctor know if you have had any reaction to radiology dyes in the past. If so, a conventional colonoscopy may be a better option, or, alternatively, some physicians may prescribe a medication such as prednisone to reduce your risk of a reaction.

Medications

If you take regular medications, your doctor will let you know if it is alright to take these with a few sips of water on the day of your procedure.

Medications that may need to be stopped, sometimes a week or two before the procedure, include:

  • Anticoagulants, such as Coumadin (warfarin), Fragmin (dalteparin), Pradaxa (dabigatran), and Lovenox (enoxaparin)
  • Anti-platelet medications, such as Plavix (clopidogrel), Effient (prasugrel), and Pletal (cilostazol)
  • Aspirin and products containing aspirin
  • Nonsteroidal anti-inflammatory medications (NSAIDS), such as Advil (ibuprofen) and others
  • Some arthritis medications (in addition to NSAIDS)
  • Iron supplements or vitamins that contain iron

It's important for your treating physician to be aware of your plan to have a virtual colonoscopy and make recommendations on whether it is wise to stop these medications or not. Keep in mind that some dietary supplements have blood thinning properties as well, and should be stopped well before the procedure.

If you have diabetes and use insulin or medications to control your blood sugar, talk to your regular doctor about any changes in your regimen that will be needed both while on a clear liquid diet and the day of the procedure.

What to Wear

Prior to your procedure, you will be asked to undress and change into a gown. It's best to wear comfortable clothes to the procedure that are loose in the abdomen, as you may have some cramping and bloating after the procedure. Leave jewelry at home.

Cost and Health Insurance

Insurance coverage may differ from that for conventional colonoscopy, and it is important to check with your insurance company. Prior authorization may be required and can take some time. The average out-of-pocket cost for a virtual colonoscopy is $2,400, but can range from less than $750 to more than $5,000, depending on the facility and its geographic location. While the procedure itself costs less than a conventional colonoscopy, it may be costlier due to co-insurance requirements and copays. Costs may also differ on whether the procedure is ordered as a screening test or due to symptoms related to the colon.

Private insurance companies are now mandated to provide coverage for virtual colonoscopy according to provisions in the Affordable Care Act. Medicare does not currently cover virtual colonoscopy, though it's thought this will change soon.

For those who do not have health insurance, there are now several colonoscopy options for the uninsured provided by organizations that support colon cancer screening.

What to Bring

On the day of your appointment, you should bring any paperwork you were asked to complete, your health insurance card, and any previous scans that were requested. It's also helpful to bring reading material to pass the time if you end up waiting. Unlike a conventional colonoscopy, you can usually drive yourself home, but many people like to bring a companion. That said, if a polyp is found and a conventional colonoscopy can be done the same day to remove it, you will be required to have a driver who can take you home before your medical team will do the procedure.

During the Test

When you arrive for your test, a radiology technician will meet you and talk about what to expect. He will remain with you throughout the procedure.

Pre-Test

Before the test, your radiology technician will make sure that you completed your colon preparation and haven't had anything to eat or drink for the specified amount of time, and confirm that you do or don't have any allergies.

He will then help you lie on the CT table, and you will see the doughnut-shaped hole that you will slide into during the scan. While the technician will not be in the same room with you during the procedure (to avoid radiation exposure), there is a window through which she can see you right next to the CT room. An intercom will allow him to ask you how you are doing and convey special instructions.

Throughout the Test

When the test begins, you will lie on your side on the table, and the technician will insert a thin tube roughly 2 inches into your rectum. The tube is used to inflate your colon, either with air, through a squeeze bulb, or with carbon dioxide, via an automated pump. Inflating your colon reduces the risk that small tumors or polyps could hide behind the folds in the colon. Sometimes a small balloon is inflated to hold the tube in place once it is in your rectum.

When the air or carbon dioxide is injected, you will feel a sense of fullness and that you need to pass gas or defecate. It should not be painful. You will then be told to turn onto your back and the table will slide into the CT tube. (A CT tube is much larger than an MRI tube and most people do not experience claustrophobia.)

As the scanning begins, you will hear some clicking and whirring sounds from the machine, but these are not loud like with an MRI. Over a period of 10 to 15 minutes, the technician will take different images of your colon, having you roll to one side or the other and on to your stomach. He will also have you hold your breath for a short while periodically to make sure the images are as clear as possible and not blurred by movement.

In some cases, medications are given to relax the colon. As the exam continues, you may feel some cramping and bloating, but this usually resolves shortly after the procedure is completed. It's important to let your technician know if you experience any discomfort or if you have difficulty changing into any of the positions.

Post-Test

When the test is done, your technician will remove the tube from your rectum and help you get up from the table. He will go over your discharge instructions and have you change back into your clothes. Depending on the facility, and whether any polyps or other problems are seen on the images, the radiologist may talk to you about how it went.

Most often, however, you will go home and hear about your results later on. In locations where a conventional colonoscopy can be done the same day if polyps are found, a physician will talk to you about proceeding with that other study if needed.

After the Test

After your virtual colonoscopy, you can usually resume your regular diet and activity level right away, though it is helpful to have easy access to a bathroom while you still feel crampy and bloated. The radiologist will look at the images of your scan and contact your doctor.

Managing Side Effects

Other than some cramping and bloating for a few hours, you should feel normal after the procedure, and there are no special instructions. Walking can be helpful in reducing gas and may help with any cramping that remains.

You should call your doctor if you develop severe pain in your abdomen, a fever, notice blood in your stool, or feel lightheaded, dizzy, or weak.

Interpreting Results

The time it takes to get your results can vary with the facility where you had the test done, and whether the doctor performing the test tells you about your results, or instead sends a report to your primary care doctor who will notify you. In some cases, such as if a polyp is found, you may learn about your results right away.

There are no references for normal and abnormal results. Rather, your radiologist and doctor's professional opinions about what the images show are what gets reported. This is one of the many reasons why choosing a reputable facility that performs a significant volume of these studies is important.

Follow-Up

If your exam is normal, the radiologist will send a note to your doctor who will call you. In this case, a repeat exam in five years would usually be recommended. If your colon preparation wasn't sufficient or if the images were inconclusive for some reason, your doctor will talk to you about the next steps.

If there is any evidence of polyps or masses, a conventional colonoscopy is usually recommended. If this wasn't already discussed and performed the day of your test, one would be scheduled for a week or two after it. If it is likely that an abnormality is indeed colon cancer, other diagnostic tests, such as serum markers, may be done as well.

In the event there is evidence of diverticuli, your doctor will talk to you about how to manage diverticular disease. If any other abnormalities were noted, tests to evaluate those concerns will be recommended.

Since virtual colonoscopy allows radiologists to view other areas of the abdomen, further evaluations of any abnormalities found in other organs may also be required.

Other Considerations

You may have questions about the accuracy of your virtual colonoscopy and the chance that a polyp or tumor could have been missed. It's important to mention these concerns to your doctor and talk about any further follow-up that might be recommended.

If you have concerns, such as colon cancer symptoms but a normal test, you may wish to consider getting a second opinion. If you are found to have polyps, it's also helpful to ask questions; some, but not all, polyps are considered precancerous and can progress to cancer. Your doctor can talk to you about the cancer risk related to polyps and what your results mean in this regard.

A Word From Verywell

As with other cancer screening tests, scheduling your virtual colonoscopy, going through the preparation, and waiting for results can cause anxiety and stress. That stress can be magnified if any findings are abnormal and you need to wait for the results of further testing to know more.

Fortunately, screening colonoscopies often find precancerous polyps when they can be removed. Even when colon cancer is found, these tumors are often in the earlier and very treatable stages of the disease.

Unfortunately, there are many people who resist having either kind of colonoscopy done, and colon cancer continues to be the third leading cause of cancer deaths in the United States. Scheduling your test is an excellent way to take charge of your health, and the temporary discomfort of going through the preparation and procedure is very worth the effort over the long run.

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