Computed Tomography (CT) - Chest
What is CT Scanning of the Chest?
Computed tomography (also known as CT or CAT scan) of the chest uses special
equipment to obtain multiple cross-sectional images of the organs and tissues
of the chest. CT produces images that are far more detailed than a conventional
chest x-ray. CT is especially useful because it can simultaneously show many
different types of tissue including the lungs, heart, bones, soft tissues,
muscle and blood vessels. Modern CT scans utilize a method called spiral
(or helical) CT that captures images of the chest from many angles and, with
the aid of a computer, processes the images to create cross-sectional pictures
or "slices" of the area of interest. The images can then be printed
out or examined on a monitor.
CT scanning is considered to be a "patient-friendly" procedure
because it involves relatively low radiation exposure and is not invasive.
Contrast material is sometimes injected through a vein to make the images
clearer. During the CT examination patients usually lie flat on their back,
but occasionally may be asked to lie on their side or on their stomach. Patients
are periodically asked to hold their breath. No preparation is required prior
to the exam. Patients generally do not require a sedative to remain calm.
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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.CT of the chest is used
to take a closer look at findings detected on conventional chest x-rays or
may be used to investigate and try to explain clinical signs or symptoms
of disease of the chest. The CT examination may provide more specific information
regarding the nature and extent of the findings or, in some cases, determine
that the chest is normal.
CT may be used to detect and evaluate the extent of tumors that arise in
the lung and mediastinum, or tumors that have spread there from other parts
of the body. CT is routinely used to assess whether tumors are responding
to treatment.
You may have heard that, in recent years, some people have chosen to have
a chest CT scan to screen for lung cancer. This makes the most sense for
those who are former or current cigarette smokers, as they are at much greater
risk of cancer than are nonsmokers. The best hope of curing lung cancer is
to find it as early as possible, making it easier to treat. CT is able to
detect even very small abnormalities that could be early lung cancer, which
would not be visible on a conventional chest x-ray. A special low-dose CT
technique is used for lung cancer screening. CT of the chest is not able
to detect every cancer.
When someone has abnormal CT findings but the cause is uncertain, a percutaneous
needle biopsy may be needed to directly examine the tissue. CT can be used
to help guide the biopsy needle to the area in question.
Chest CT also can demonstrate other lung disorders such as old or new pneumonia,
tuberculosis, emphysema, bronchiectasis, and diffuse interstitial lung disease.
When the clinical findings and regular chest x-ray are inconclusive, CT may
clarify the situation. Inflammation or other diseases of the pleura, the
membrane covering the lungs, can be seen in CT images.
Accident victims and other people with chest injury often have a CT exam
to assess damage to organs, to bones (including the spinal column), and to
large blood vessels.
An aneurysm, or ballooning out of the aorta, may be found totally unexpectedly
when chest CT is done for other reasons. Or a CT scan may be used to confirm
suspicion of aneurysm on the basis of a plain chest x-ray. It is important
to be aware of this condition so that it may be watched and, if necessary,
treated before rupturing.
CT is used to detect blood clots that travel from the deep veins in the
legs and lodge in the pulmonary arteries, blocking the normal blood flow
to the lungs. This condition, called pulmonary embolism, is usually seen
in patients who are immobile for long periods of time or who have cancer
or clotting disorders. Pulmonary embolisms can be life threatening if not
detected and treated. When pulmonary embolism is suspected, a special type
of CT scan is performed called a CT Angiogram (CTA). This involves injecting
the iodine into a vein a little faster and also more numerous and thinner
slices are made through the chest in order to see the arteries in the lungs
to better advantage.
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How should I prepare for the CAT scan?
You should dress comfortably but avoid any clothing in the chest area that
has a zipper, snaps or jewelry, since metal objects may affect the CT images.
Women should always inform their physician or the 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 unit with a hole running directly through its
center, giving the appearance of a doughnut. The patient lies on a table
that can move up or down and can slide into and out of the center of the
cavity. The computer is not in the examining room, but in an adjoining room.
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How does the procedure work?
X-rays emitted from a rotating x-ray tube pass through the patient's body.
Different tissues absorb this radiation in different amounts. Radiation leaving
the body is recorded by an array of detectors that are mounted on the gantry
along with the x-ray tube. During each rotation about 1,000 images of the
x-ray beam are recorded. These images are then reconstructed by a computer
into a very detailed two-dimensional view of the interior of the body. As
the x-ray source rotates and, at the same time, the patient lying on the
examination table advances through the scanner at a constant rate, the x-ray
beam follows a spiral path—hence, the term "spiral" CT scanning.
The most recent spiral CT units produce higher-quality images in a shorter
time. This is an especially important feature for children and for patients
who are elderly or critically ill, as well as those who cannot hold their
breath for a long time. Modern scanners are able to image large regions of
the body, such as the lungs, during a single breath hold of about 20 seconds.
If a suspicious nodule is found, additional scans may be done to obtain greater
detail. It now is possible to combine multiple CT images to produce a three-dimensional
display that provides additional information.
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How is the CAT scan performed?
CT angiogram. Frontal or coronal view of chest-3D slab image showing pulmonary
vessels.The first step is for the technologist to make certain that you are
correctly positioned on the CT table. Pillows may be used to help maintain
the correct position during the examination. For the initial scans, the table
will move rapidly through the scanner to determine the correct starting position.
The rest of the scans are made as the table moves more slowly through the
cavity in the scanner. The best chest CT scans are obtained when you are
able to hold your breath. If this is not possible, you will be asked to breathe
quietly and regularly.
Under certain circumstances, as when evaluating blood vessels, you will
have contrast material injected into a vein shortly before scanning begins.
If this is necessary, the radiologist, a physician specially trained to obtain
and interpret medical images, or a staff member will ask whether you have
any allergies to medications or iodine (which is a part of many contrast
materials) and whether you have a history of asthma, diabetes, a heart disorder,
multiple myeloma, or kidney disease. These conditions may indicate a greater
risk of an adverse reaction to contrast material. Kidney disease makes it
harder to eliminate contrast material from the bloodstream.
After the CT examination is completed you may have to wait a short time
while the radiologist checks the scans to make sure the quality is good enough
to be correctly interpreted. If necessary, a few additional scans will be
obtained.
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What will I experience during the procedure?
CT scanning is a pain-free procedure. If contrast material is injected you
may feel a flush of heat or a metallic taste in your mouth, usually lasting
no more than a minute or two. You also may notice mild itching. If this persists
or hives develop, effective medication is available. Very rarely a patient
becomes short of breath or has swelling in the throat or another part of
the body, indicating a more serious reaction to contrast material that must
be promptly treated. If you experience any of these symptoms, inform the
technologist immediately.
You will be alone during the scan, but the technologist can see and hear
you and can speak to you at any time from the adjacent control room. The
examination usually takes 15 to 30 minutes, including preparation time. The
actual scan time is less than 30 seconds.
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Who interprets the results and how do I get them?
A radiologist experienced in CT scanning will examine and interpret the
CT findings. After reaching a conclusion, the radiologist will send a detailed
report to your primary care or referring physician, who will give you the
results.
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What are the benefits vs. risks?
Benefits
- Unlike conventional x-rays, CT scanning provides very detailed images
of a wide range of body organs and tissues.
- CT is fast. This is especially important for patients with chest injury,
because internal damage or bleeding can be diagnosed in time to give
life-saving treatment.
- There is no pain from CT. Apart from an intravenous injection if needed,
the procedure is non-invasive.
- CT can identify lung tumors when they are smaller in size than will
be visible in conventional x-rays and are easier to treat.
- Because it identifies both normal and abnormal tissues, CT scanning
is a convenient way of guiding needle biopsies.
- CT can help in planning radiotherapy.
- A CT diagnosis may eliminate the need for exploratory surgery.
Risks
- Chest CT requires exposure to radiation, but the risk is considered
so low that it is far outweighed by the benefit of making an accurate diagnosis.
Radiologists use the smallest radiation dose that will provide
accurate results. The effective radiation dose from this procedure is about
8 mSv, which is about the same as an average person receives from background
radiation in 3 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.
- If you are a nursing mother and received contrast material for your
CT exam, you should wait 24 hours before resuming breast-feeding to allow
your body time to eliminate the material.
- There is a very small but real risk of having a serious allergic reaction
to contrast material that contains iodine, but CT staff will be able
to deal with it.
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What are the limitations of CT Scanning of the Chest?
* Magnetic resonance imaging (MRI) may be better than CT for showing very
fine soft tissue detail.
* Motion can lessen the quality of a CT study.
* CT scanning is generally not recommended for pregnant women.
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