Computed Tomography (CT) - Abdomen
What is CT Scanning of the Abdomen?
Computed tomography (CT)—sometimes called CAT scan—uses special
x-ray equipment to obtain image data from different angles around the body,
then uses computer processing of the information to show a cross-section
of body tissues and organs.
CT imaging is particularly useful because it can show several types of tissue
with great clarity, including organs such as the liver, spleen, pancreas
and kidneys. Using specialized equipment and expertise to create and interpret
CT scans of the lower gastrointestinal (GI) tract, the colon, and the rectum,
an experienced radiologist can accurately diagnose many causes of abdominal
pain, such as an abscess in the abdomen, an inflamed colon or colon cancer,
diverticulitis and appendicitis. Often, no additional diagnostic work-up
is necessary and treatment planning can begin immediately.
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What are some common uses of the procedure?
Because it is a non-invasive procedure that provides detailed, cross-sectional
views of all types of tissue, CT is becoming the preferred method for diagnosing
many diseases of the bowel and colon, including diverticulitis and appendicitis,
and for visualizing the liver, spleen, pancreas and kidneys. In cases of
acute abdominal distress, CT can quickly identify the source of pain. Especially
when pain is caused by infection and inflammation, the speed, ease and
accuracy of a CT examination can reduce the risk of serious complications
caused by a burst appendix or ruptured diverticulum and the subsequent
spread of infection.
CT is often the preferred method for diagnosing many different cancers,
including colon cancer, since the image allows a physician to confirm the
presence of a tumor and to measure its size, precise location, and the
extent of the tumor's involvement with other nearby tissue. CT examinations
of the lower GI tract can be used to plan and properly administer radiation
treatments for tumors, and to guide biopsies and other minimally invasive
procedures. Many dedicated shock-trauma centers have a CT scanner in the
trauma department. CT can also play a significant role in the detection,
diagnosis and treatment of vascular disorders that can lead to stroke,
gangrene or kidney failure.
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How should I prepare for the CAT scan?
You should wear comfortable, loose-fitting clothing for your CT exam. Metal
objects can affect the image, so avoid clothing with zippers and snaps.
You may be asked to remove hairpins, jewelry, eyeglasses, hearing aids
and any removable dental work that could obscure the images. You also may
be asked to refrain from eating or drinking anything for an hour or longer
before the exam. Women should always inform their doctor or x-ray technologist
if there is any possibility that they are pregnant.
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What does the equipment look like?
The CT scanner is a large, square machine with a hole in the center, something
like a doughnut. The patient lies still on a table that can move up or
down, and slide into and out from the center of the hole. Within the machine,
an x-ray tube on a rotating gantry moves around the patient's body to produce
the images, making clicking and whirring noises as the arm moves. Though
the technologist will be able to see and speak to you, you will be alone
in the room during the exam.
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How does the procedure work?
In many ways, CT
scanning works very much like other x-ray examinations. Very
small, controlled amounts of x-ray radiation are passed through the body,
while different tissues absorb the radiation at different rates. With plain
radiology, when special film is exposed to the absorbed x-rays, an image
of the inside of the body is captured. With CT, the film is replaced by
an array of detectors, which measure the x-ray profile.
Inside the CT scanner is a rotating gantry that has an x-ray tube mounted
on one side and an arc-shaped detector mounted on the opposite side. During
each full rotation, as the fan-shaped x-ray beam is emitted through the
patient's body, an image of a thin section is acquired. The detector records
about 1,000 images—or profiles—of the expanded x-ray beam with
each rotation. The profiles are then reconstructed by a dedicated computer
into two-dimensional images of the sections that were scanned. Multiple
computers are typically used to control the entire CT system.
You might think of it as looking into a loaf of bread by cutting the bread
into thin slices. When the image slices are reassembled by computer, the
result is a very detailed, multidimensional view of the body's interior.
With spiral—or helical—CT, refinements in detector technology
support faster, higher-quality image acquisition with less radiation exposure.
The current spiral CT scans are called multidetector CT and are most commonly
four- or 16-slice systems. CT scanners with 32, 40 and 64 detectors are
currently under development and are becoming available. These instruments
should provide either faster scanning or higher resolution images. Using
16-slice scanner systems the radiologist can acquire 32 image slices per
second. A spiral scan can usually be obtained during a single breath hold.
This allows allows scanning of the chest or abdomen in 10 seconds or less.
Such speed is beneficial in all patients, but especially in populations
in which the length of scanning was often problematic, such as elderly,
pediatric or critically-ill patients. The multidetector CT also allows
applications like CT angiography to be more successful.
With conventional CT, small lesions may frequently go undetected when a
patient breathes differently on consecutive scans, as a lesion may be missed
by unequal spacing between scans. The speed of spiral scanning and a single
breath hold increase the rate of lesion detection.
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How is the CAT scan performed?
The technologist begins by positioning the patient on the CT table. The
patient's body may be supported by pillows to help hold it still and in
the proper position during the scan. As the study proceeds, the table will
move slowly into the CT scanner. Depending on the area of the body being
examined, the increments of movement may be so small that they are almost
undetectable, or large enough that the patient feels the sensation of motion.
A CT examination of the gastrointestinal tract requires the use of a contrast
material to enhance the visibility of certain tissues. The contrast material
may be swallowed or administered by enema. Before administering the contrast
material, the radiologist or technologist will ask whether the patient
has any allergies, especially to medications or iodine, and whether the
patient has a history of diabetes, asthma, a heart condition, kidney problems
or thyroid conditions. These conditions may indicate a higher risk of reaction
to the contrast material or potential problems eliminating the material
from the patient's system after the exam.
A CT examination usually takes five minutes to half an hour. When the exam
is over, the patient may be asked to wait until the images are examined
to determine if more images are needed.
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What will I experience during the procedure?
CT scanning causes no pain, and with spiral CT, the need to lie still for
any length of time is reduced. For examinations of the abdomen and lower
gastrointestinal tract, you may be asked to swallow either water or a positive
contrast material, a liquid that allows the radiologist to better see the
stomach, small bowel and colon. Some patients find the taste of the contrast
material mildly unpleasant, but most can easily tolerate it. Your exam
may require the administration of the material by enema if the colon is
the focus of the study.
You will be alone in the room during the scan; however, the technologist
can see, hear and speak with you at all times. In pediatric patients, a
parent may be allowed in the room with the patient to alleviate fear, but
will be required to wear a lead apron to prevent radiation exposure.
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Who interprets the results and how do I get them?
Radiologists viewing CAT scansA radiologist, who is a physician experienced
in CT and other radiologic examinations, will analyze the images and send
a signed report with his or her interpretation to the patient's primary
care physician. The physician's office will inform the patient on how to
obtain their results. New technology also allows for distribution of diagnostic
reports and referral images over the Internet at some facilities.
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What are the benefits vs. risks?
Benefits
- Viewing a CT scan, an experienced radiologist can diagnose many causes
of abdominal pain with very high accuracy, enabling faster
treatment and often eliminating the need for additional, more
invasive diagnostic procedures.
- Unlike other imaging methods, CT scanning offers detailed views of
many types of tissue, including the lungs, bones, soft tissues and
blood vessels.
- CT scanning is painless, noninvasive and accurate.
- CT examinations are fast and simple. For example, in emergency cases,
they can reveal internal injuries and bleeding quickly enough to
help save lives.
- Diagnosis made with the assistance of CT can eliminate the need for
invasive exploratory surgery and surgical biopsy.
- CT scanning can identify both normal and abnormal structures, making
it a useful tool to guide radiotherapy, needle biopsies and other
minimally invasive procedures.
- CT has been shown to be a cost-effective imaging tool for a wide range
of clinical problems.
Risks
- CT does involve exposure to radiation in the form of x-ray, but the
benefit of an accurate diagnosis far outweighs the risk. The effective
radiation dose from this procedure is about 10 mSv, which is about
the same as the average person receives from background radiation in
three years. See the Safety page for more information about radiation
dose.
- Special care is taken during x-ray examinations to ensure maximum
safety for the patient by shielding the abdomen and pelvis with a lead
apron, with the exception of those examinations in which the abdomen
and pelvis are being imaged. Women should always inform their doctor
or x-ray technologist if there is any possibility that they are pregnant.
- Nursing mothers should wait for 24 hours after contrast material injection
before resuming breast feeding.
- The risk of serious allergic reaction to iodine-containing contrast
material is rare, and radiology departments are well-equipped to deal
with them.
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What are the limitations of CT Scanning of the Abdomen?
The exam is not generally indicated for pregnant women.
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