CT scanning—sometimes called CAT scanning—is a noninvasive,
painless medical test that helps physicians diagnose
and treat medical conditions.
CT imaging uses special x-ray equipment to produce multiple
images or pictures of the inside of the body and a computer
to join them together in cross-sectional views of the
area being studied. The images can then be examined on
a computer monitor or printed.
CT scans of internal organs, bone, soft tissue and blood
vessels provide greater clarity than conventional x-ray
exams.
CT colonography uses CT scanning to obtain an interior
view of the colon (the large intestine) that is ordinarily
only seen with an endoscope inserted into the rectum.
The major reason for performing CT colonography is to
screen for polyps and other lesions in the large intestine.
Polyps are benign growths that arise from the inner lining
of the intestine. Some polyps may grow and turn into
cancers.
The goal of screening with colonography is to find these
growths in their early stages, so that they can be removed
before cancer has had a chance to develop. Most physicians
agree that everyone older than 50 years should be screened
for polyps every seven to 10 years. Individuals at increased
risk should be screened every five years. Risk factors
for the disease include a history of polyps, a family
history of colon cancer, or the presence of blood in
the stool.
You should wear comfortable, loose-fitting clothing
to your exam. You may be given a gown to wear during
the procedure.
Metal objects including jewelry, eyeglasses, dentures
and hairpins may affect the CT images and should be left
at home or removed prior to your exam. You may also be
asked to remove hearing aids and removable dental work.
Women should always inform their physician or technologist
if there is any possibility that they are pregnant.
It is very important to clean out your bowel the night
before your CT colonography examination so that the radiologist
can clearly see any polyps that might be present. You
will be asked to take either a set of pills or a cathartic
liquid. Some common preparations are the Fleet Prep Kit
1 (phospho-soda and Bisacodyl) and NuLytely® or Go-Lytely®
(Polyethylene glycol electrolyte solutions).
Be sure to inform your physician if you have heart, liver
or kidney disease to be certain that the bowel prep will
be safe. On the day before your exam, you should limit
your food intake to clear liquids such as broth, tea
or juice. You will be able to resume your usual diet
immediately after the exam.
The CT scanner is typically a large machine with a hole,
or tunnel, in the center. A moveable examination table
slides into and out of this tunnel. In the center of
the machine, the x-ray tube and electronic x-ray detectors
are located opposite each other on a ring, called a gantry,
which rotates around you. The computer that processes
the imaging information and monitor are located in a
separate room.
In many ways CT scanning works very much like other
x-ray examinations. X-rays are a form of radiation—like
light or radio waves—that can be directed at the body.
Different body parts absorb the x-rays in varying degrees.
In a conventional x-ray exam, a small burst of radiation
is aimed at and passes through the body, recording an
image on photographic film or a special image recording
plate. Bones appear white on the x-ray; soft tissue shows
up in shades of gray and air appears black.
With CT scanning, numerous x-ray beams and a set of electronic
x-ray detectors rotate around you, measuring the amount
of radiation being absorbed throughout your body. At
the same time, the examination table is moving through
the scanner, so that the x-ray beam follows a spiral
path. A special computer program processes this series
of pictures, or slices of your body, to create two-dimensional
cross-sectional images, which are then displayed on a
monitor.
CT imaging is sometimes compared to looking into a loaf
of bread by cutting the loaf into thin slices. When the
image slices are reassembled by computer software, the
result is a very detailed multidimensional view of the
body's interior.
Refinements in detector technology allow new CT scanners
to obtain multiple slices in a single rotation. These
scanners, called "multislice CT" or "multidetector
CT," allow thinner slices to be obtained in a shorter
period of time, resulting in more detail and additional
view capability.
Modern CT scanners are so fast that they can scan through
large sections of the body in just a few seconds. Such
speed is beneficial for all patients but especially children,
the elderly and critically ill.
For CT colonography, the computer generates a detailed
three-dimensional model of the abdomen and pelvis, which
the radiologist uses to view the bowel in a way that
simulates traveling down the colon. This is why it is
often called a virtual colonoscopy.
The technologist begins by positioning you on the CT
examination table, usually lying flat on your back or
possibly on your side or on your stomach. Straps and
pillows may be used to help you maintain the correct
position and to hold still during the exam.
A very small, flexible tube will be passed two inches
into your rectum to allow air to be gently pumped into
the colon using a hand-held squeeze bulb. Sometimes an
electronic pump is used to deliver carbon dioxide gas
into the colon. In either case, you will be able to control
the amount of air or gas passing into the colon. The
purpose of the gas is to distend the colon a little to
eliminate any folds or wrinkles that might obscure polyps
from the physician’s view.
Next, the table will move through the scanner. Patients
are asked to hold their breath for about 15 seconds before
turning over and lying on their back for a second pass
is made through the scanner. In some centers the sequence
of positions may be the opposite: facing upward first
and then facing down. Once the scan is done, the tube
is removed.
The entire examination is usually completed within 15
minutes.
The vast majority of patients who have CT colonography
report a feeling of fullness when the colon is inflated
during the exam, as if they need to pass gas. Significant
pain is uncommon, occurring in fewer than 5 percent of
patients. A muscle-relaxing drug may be injected intravenously
to lessen discomfort, but this is seldom necessary. After
the tube is inserted, your privacy will be respected.
The scanning procedure itself causes no pain or other
symptoms.
When you enter the CT scanner, special lights may be
used to ensure that you are properly positioned. With
modern CT scanners, you will hear only slight buzzing,
clicking and whirring sounds as the CT scanner revolves
around you during the imaging process.
You will be alone in the exam room during the CT scan,
however, the technologist will be able to see, hear and
speak with you at all times.
After a CT exam, you can return to your normal activities.
A radiologist, a physician specifically trained to supervise
and interpret radiology examinations, will analyze the
images and send a signed report to your primary care
or referring physician, who will share the results with
you.
Benefits
- This new minimally invasive test provides three-dimensional
images that can depict many polyps and other lesions
as clearly as when they are directly seen by optical
colonoscopy.
- CT colonography has a markedly lower risk of perforating
the colon than conventional colonoscopy. Most of those
examined do not have polyps, and can be spared having
to undergo a full colonoscopy.
- CT colonography is a less invasive option for patients
who do not wish to have colonoscopy, which involves
inserting a flexible tube into the colon to view the
bowel wall.
- CT colonography is an excellent alternative for patients
who have clinical factors that increase the risk of
complications from colonoscopy, such as treatment with
a blood thinner or a severe breathing problem.
- Elderly patients, especially those who are frail
or ill, will tolerate CT colonography better than conventional
colonoscopy.
- CT colonography can be helpful when colonoscopy cannot
be completed because the bowel is narrowed or obstructed
for any reason, such as by a large tumor.
- If conventional colonoscopy cannot reach the full
length of the colon—which occurs up to 10 percent of
the time—CT colonography can be performed on the same
day because the colon has already been cleansed.
- CT colonography provides clearer and more detailed
images than does a conventional barium enema x-ray
examination.
- In 5 percent of patients, CT colonography shows abnormalities
outside the colon which would otherwise be missed because
colonoscopy only looks at the interior surfaces.
- CT colonography is tolerated well. Sedation and pain-relievers
are not needed, so there is no recovery period.
- CT colonography is less costly than colonoscopy.
- No radiation remains in a patient's body after a
CT examination.
- X-rays used in CT scans usually have no side effects.
Risks
- There is a very small risk that inflating the colon
with air could injure or perforate the bowel. This
has been estimated to happen in fewer than one in 2,000
patients.
- There is always a slight chance of cancer from radiation.
However, the benefit of an accurate diagnosis far outweighs
the risk.
- The effective radiation dose from this procedure
is about 5 mSv, which is about the same as the average
person receives from background radiation in 20 months.
- Women should always inform their physician or x-ray
technologist if there is any possibility that they
are pregnant.
- CT scanning is, in general, not recommended for pregnant
women because of potential risk to the baby.
A person who is very obese may not fit into the opening
of a conventional CT unit.
CT colonography is strictly a diagnostic procedure. If
any significant polyps are found, they will have to be
removed by conventional colonoscopy.
Many insurance companies do not cover CT colonography
as a screening test for colonic polyps, but they may
cover the cost if a patient has symptoms related to the
colon.
Reference:
http://www.radiologyinfo.org/
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Diagnostics 
