Computed Tomography (CT) - Body
What is CT Scanning of the Body?
CT (computed tomography), sometimes called CAT scan, uses special x-ray
equipment to obtain image data from different angles around the body, and
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—lung,
bone, soft tissue and blood vessels—with great clarity. Using specialized
equipment and expertise to create and interpret CT scans of the body, radiologists
can more easily diagnose problems such as cancers, cardiovascular disease,
infectious disease, trauma and musculoskeletal disorders.
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What are some common uses of the procedure?
CT of the lungs, window level set to demonstrate the vessels and air ways
- not intended to demonstrate the heart, spine muscles etc. This is used
to look for things like pneumonia or lung cancer.Because it provides detailed,
cross-sectional views of all types of tissue, CT is one of the best tools
for studying the chest and abdomen. It is often the preferred method for
diagnosing many different cancers, including lung, liver and pancreatic cancer,
since the image allows a physician to confirm the presence of a tumor and
measure its size, precise location, and the extent of the tumor's involvement
with other nearby tissue. CT examinations are often used to plan and properly
administer radiation treatments for tumors, to guide biopsies and other minimally
invasive procedures, and to plan surgery and determine surgical resectability.
CT can clearly show even very small bones, as well as surrounding tissues
such as muscle and blood vessels. This makes it invaluable in diagnosing
and treating spinal problems and injuries to the hands, feet and other skeletal
structures. CT images can also be used to measure bone mineral density for
the detection of osteoporosis. In cases of trauma, CT can quickly identify
injuries to the liver, spleen, kidneys, or other internal organs. Many dedicated
shock-trauma centers have a CT scanner in the emergency room. CT can also
play a significant role in the detection, diagnosis and treatment of vascular
diseases that can lead to stroke, kidney failure, or even death.
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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 also be asked to remove hairpins, jewelry, eyeglasses, hearing aids and
any removable dental work, depending on the part of the body that is being
scanned. You may be asked not to eat or drink anything for one or more hours
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 table 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, and different tissues absorb 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. An x-ray
beam is emitted in a fan shape as the rotating frame spins the x-ray tube
and detector around the patient. Each time the x-ray tube and detector make
a 360 degree rotation and the x-ray passes through the patient's body, the
image of a thin section is acquired. During each rotation, the detector records
about 1,000 images (profiles) of the expanded x-ray beam. Each profile is
then reconstructed by a dedicated computer into a two-dimensional image of
the section that was 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.
A relatively new technique, spiral (helical) CT has improved the accuracy
of CT for many diseases. A new vascular imaging technique—spiral CT
angiography—is noninvasive and less expensive than conventional angiography,
and allows doctors to see blood vessels without the need for more invasive
procedures.
The term "spiral CT" comes from the shape of the path taken by
the x-ray beam during scanning. The examination table advances at a constant
rate through the scanner gantry while the x-ray tube rotates continuously
around the patient, tracing a spiral path through the patient. This spiral
path gathers continuous data with no gaps between images.
With spiral 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 elderly, pediatric or critically ill patients, populations
in whom the length of scanning was often problematic. The multidetector CT
also allows applications like CT angiography to be more successful.
With conventional CT, small lesions may 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 "doughnut." 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.
CT angiogram. Frontal or coronal view of chest-3D slab image showing pulmonary
vessels.A CT examination often requires the use of different contrast materials
to enhance the visibility of certain tissues or blood vessels. The contrast
material may be injected through an IV directly into the blood stream, swallowed
or administered by enema, depending on the type of examination. 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 different parts of the body, the patient
preparation will be different. 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 experience a sense of abdominal fullness
and may feel an increasing need to expel the liquid. Be patient; the mild
discomfort will not last long.
Commonly, a contrast material is injected into a vein to better define the
blood vessels and kidneys, and to accentuate the appearance between normal
and abnormal tissue in organs like the liver and spleen. Some people report
feeling a flush of heat and sometimes a metallic taste in the back of the
mouth. These sensations usually disappear within a minute or two. Some people
experience a mild itching sensation. If it persists or is accompanied by
hives (small bumps on the skin), the itch can be treated easily with medication.
In very rare cases, a patient may become short of breath or experience swelling
in the throat or other parts of the body. These can be indications of a more
serious reaction to the contrast material that should be treated promptly,
so tell the technologist immediately if you experience these symptoms. Fortunately,
with the safety of the newest contrast materials, these adverse effects are
very rare.
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?
A radiologist, who is a physician experienced in CT and other radiology
examinations, will analyze the images and send a signed report with his or
her interpretation to the patient's personal physician. The personal 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
- 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 trauma 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-rays, 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 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 Body?
Very fine soft-tissue details in areas such as the knee or shoulder can
be more readily and clearly seen with magnetic resonance imaging (MRI). The
exam is not generally indicated for pregnant women.
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