Topic Overview
What is a virtual colonoscopy?
A virtual
colonoscopy uses X-rays and computers (computed tomography [CT] or
magnetic resonance imaging [MRI]) to take two- or
three-dimensional images of the interior lining of your
large intestine 
(colon) and rectum. A virtual colonoscopy can be used to
screen for precancerous and cancerous growths in the colon or rectum (colorectal cancer), such as
polyps or tumors.
Virtual colonoscopy
(more accurately called computed tomographic colonography, or CTC) is still
being studied to determine whether the results are as accurate as a
regular colonoscopy. Virtual colonoscopy is not yet
widely available.
For more information on screening tests for
colon cancer, see:
Which test should I have to screen for colorectal cancer?
Why is it done?
A virtual colonoscopy may be done
to:
- Screen for colorectal cancer or
polyps.
- Monitor the growth of polyps.
- Screen for recurrence of colorectal cancer in people who have
had surgery for this disease.
- Evaluate abnormal results from other
colon tests, such as a barium enema test.
- Replace a regular
colonoscopy if the thin lighted scope cannot be inserted in the colon for some
reason, such as when a tumor blocks the passage.
Virtual colonoscopy is not commonly done for other
conditions at this time but may prove to be useful to:
- Detect other problems or diseases of the
anus, rectum, or large intestine (colon).
- Evaluate the source of
internal bleeding.
- Evaluate the cause of chronic
diarrhea.
- Monitor the treatment of inflammatory bowel
disease.
How do I prepare for it?
Virtual colonoscopy
requires the same cleansing colon prep as a regular colonoscopy. Colon prep
takes 1 to 2 days depending on which type of prep your doctor recommends. For
many people, the prep for a colonoscopy is more trying than the actual test.
Researchers are studying methods that could be used to mark (tag) stool in the
colon before virtual colonoscopy so cleansing colon preps would not be
needed.
One to two days before a colonoscopy, you will need to
stop eating solid foods and drink only clear liquids, such as water, tea,
coffee, clear juices, clear broths, Popsicles, and gelatin (such as Jell-O). Do
not drink anything red or purple, such as grape juice or fruit punch. And do
not eat red or purple foods, such as grape Popsicles or cherry Jell-O.
Some preps, such as taking a prescription laxative tablet or drinking a
laxative solution (such as Nulytely or Golytely), are done the evening before
your colonoscopy. The colon prep causes loose, frequent stools and diarrhea so
that your colon will be empty for the test. Plan to stay home during your prep
time since you will need to use the bathroom frequently. The colon prep may be
uncomfortable, and you may feel hungry on the clear liquid diet. If you need to
drink a laxative solution, be sure to have clear fruit juices or soft drinks to
drink after the prep because the solution tastes salty.
Stop
drinking clear liquids 6 to 8 hours before the colonoscopy. Your doctor may
have you use an enema 30 to 60 minutes before the test to completely cleanse
your colon. Your doctor also may give you specific instructions before your
test that take into consideration your health, age, and other medical
conditions you may have.
How is it done?
Virtual colonoscopy may be done in
a doctor's office, clinic, or a hospital. The test is most often done by a
doctor who specializes in performing and interpreting diagnostic imaging tests
(radiologist). The doctor may also have an
assistant.
You will need to take off most of your clothes. You
will be given a gown to wear during the test.
First, you will be
asked to lie on your back. Air (or in some cases, carbon dioxide) will be used
to expand your colon. This helps the doctor see all parts of your colon. The
air is added using a tube placed in your anus. It may be uncomfortable when the
air is put into your colon. You may be given medicine to help the muscles in
your colon relax.
You will be asked to hold your breath while the
pictures of your colon are being taken. If you can't hold your breath for the
whole time, the doctor may need to take pictures a couple times in order to get
the whole colon. You may be asked to roll over and lie on your stomach. The
doctor will ask you to hold your breath again and pictures will be taken with
you lying in this position.
The test usually takes about 15 to 30
minutes.
How does a virtual colonoscopy differ from a regular colonoscopy?
Virtual colonoscopy has advantages and disadvantages
compared to a regular colonoscopy.
Advantages
- Virtual colonoscopy is less invasive, safer,
and takes less time than a regular colonoscopy.
- A thin tube to
insert air into the colon is placed in the rectum rather than a long flexible
tube that is moved up your colon. (However, there is usually some discomfort
when the air is inserted.)
- No medication to relax or sedate you is
needed, so you will be able to drive yourself home and resume normal
activities.
Disadvantages
- Virtual colonoscopy may not show polyps
smaller than 10 mm (0.4 in.).
- You will need to hold your breath for short periods
of time while the computed tomography (CT) scanner takes pictures of your
colon. CT scanning will need to be done twice: once while you lie on your back
and then again while you lie on your stomach.
- Virtual colonoscopy
is not covered by all health insurance plans; check with your insurance plan
before having the test.
- Virtual colonoscopy is done in the
radiology department and involves a small amount of radiation
exposure.
- If a polyp is found, regular colonoscopy will be needed
to confirm the diagnosis and remove the polyp so it can be looked at under a
microscope. If you cannot have a regular colonoscopy the same day, you will
need to do another bowel preparation before the colonoscopy.
What do I need to consider about a virtual colonoscopy?
If you have an average risk for colorectal cancer, virtual colonoscopy
may be an appropriate screening test for you. But if you are at an increased
risk, you may require a regular colonoscopy because tissue biopsies or polyp
removal can be done at the same time.
Virtual colonoscopy requires
the same cleansing colon prep as a regular colonoscopy. If an abnormality is
found on virtual colonoscopy, you may need a regular colonoscopy to remove and
test the abnormal tissue. Since your colon is already clean, you may have the
regular colonoscopy the same day.
Virtual colonoscopy is not
available everywhere because it requires special equipment and special training
to interpret the test. The initial evaluations on virtual colonoscopy were done
by highly trained radiologists, so the results of your virtual colonoscopy may
vary widely depending on the amount of experience your doctor's office or
clinic has with the procedure.
Virtual colonoscopy is being
studied to determine whether the results are as accurate (sensitivity) as a regular colonoscopy. More evidence
is needed to show that virtual colonoscopy is a benefit in people with average
risk for colon cancer and whether this screening method can prevent cancer.
Colon preps that are more comfortable are also being studied.
