Computed Tomography Angiography (CTA)
What is CT Angiography?
CT (computed tomography) angiography (CTA) is an examination that uses x-rays
to visualize blood flow in arterial vessels throughout the body, from arteries
serving the brain to those bringing blood to the lungs, kidneys, and arms
and legs. CT combines the use of x-rays with computerized analysis of the
images. Beams of x-rays are passed from a rotating device through the area
of interest in the patient's body from several different angles so as to
create cross-sectional images, which then are assembled by computer into
a three-dimensional picture of the area being studied. Compared to catheter
angiography, which involves placing a catheter and injecting contrast material
into an artery, CTA is a much less invasive and more patient-friendly procedure—contrast
material is injected into a peripheral vein rather than an artery. This exam
has been used to screen large numbers of individuals for arterial disease.
Most patients have CT angiography without being admitted to a hospital.
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What are some common uses of the procedure?
CTA is commonly used to:
- Examine the pulmonary arteries in the lungs to rule out pulmonary embolism,
a serious but treatable condition.
- Visualize blood flow in the renal arteries (those supplying the
kidneys) in patients with high blood pressure and those suspected
of having kidney disorders. Narrowing (stenosis) of a renal artery is
a cause of high blood pressure (hypertension) in some patients,
and can be corrected. A special computerized method of viewing the images
makes CT renal angiography a very accurate examination. Also
done in prospective kidney donors.
- Identify aneurysms in the aorta or in other major blood vessels.
Aneurysms are diseased areas of a weakened blood vessel wall
that bulges out—like
a bulge in a tire. Aneurysms are life-threatening because they can rupture.
- Identify dissection in the aorta or its major branches. Dissection
means that the layers of the artery wall peel away from each other—like
the layers of an onion. Dissection can cause pain and can be life-threatening.
- Identify a small aneurysm or arterio-venous malformation inside
the brain that can be life-threatening.
- Detect atherosclerotic disease that has narrowed the arteries
to the legs.
CTA also is used to detect narrowing or obstruction of arteries in the pelvis
and in the carotid arteries bringing blood from the heart to the brain. When
a stent has been placed to restore blood flow in a diseased artery, CT angiography
will show whether it is serving its purpose. Examining arteries in the brain
may help reach a correct diagnosis in patients who complain of headaches,
dizziness, ringing in the ears, or fainting. Injured patients may benefit
from CTA if there is a possibility that one or more arteries have been damaged.
In patients with a tumor it may be helpful for the surgeon to know the details
of arteries feeding the growth.
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How should I prepare for the procedure?
Depending on the part of the body to be examined, you may be asked to take
only clear liquids by mouth before CTA. You will be asked whether you have
asthma or any allergies to foods or drugs, and what medications you are currently
taking. If you are pregnant, you should inform the technologist before the
procedure. You probably will not have to undress if you are having an exam
of the head, neck, arms or legs but you will have to remove any jewelry,
hair clips, dentures and the like that could show up on the x-rays and make
them hard to interpret.
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What does the equipment look like?
A CT scanner is a specialized x-ray machine that looks like a large square
doughnut. It has an opening measuring about two feet in diameter that surrounds
a narrow table. Inside the frame of the scanner is a rotating device with
an x-ray tube mounted on one side and a banana-shaped detector opposite it.
Nearly all CTA studies use an advanced type of unit called a spiral CT machine
that looks like any other type of CT unit, but is able to record a large
number of pictures in a short time. This means that patients do not have
to hold their breath for a prolonged period.
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How does the procedure work?
Before the actual exam begins, you will have
a dose of contrast material injected into a vein to make the blood
vessels stand out. An automatic injector machine is used that controls the
timing and rate of injection, which may continue during part of the time
images are recorded. During the examination, the rotating device spins around
the patient, creating a fan-shaped beam of x-rays, and the detector takes
snapshots of the beam after it passes through the patient. As many as one
thousand of these pictures may be recorded in one turn of the detector. The
real work of CTA comes after the images are acquired, when powerful computer
programs process the images and make it possible to display them in different
ways, for instance, in cross-sectional slices or as three-dimensional "casts" of
the blood vessels.
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How is the procedure performed?
Most of the time for a CTA examination is spent setting everything up. Actually
recording the images takes only seconds. After changing into a hospital gown
and having an IV set up, you will answer questions about things that might
complicate the exam (such as allergies) and then will lie down on a narrow
table. The part of your body to be examined will be placed inside the opening
of the CT unit with the aid of criss-crossed positioning lights. A test image
is taken to determine the best position, and a small dose of contrast material
is given to see how long it takes to reach the area under study. Then the
IV is hooked up to an automatic injector, contrast material is injected,
and the scan begins. Afterwards, the images will be reviewed and, if necessary,
some will be repeated. No special measures are needed after the procedure.
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What will I experience during the procedure?
CTA takes about 10 to 25 minutes from the time the actual examination begins.
Overall, you can expect to be in or near the examining room for 20 to 60
minutes. You may feel warm all over when contrast material is injected before
the scan, but you should not feel pain at any time. Any CT study requires
that you remain still during the exam. Pillows and foam pads may help make
it more comfortable. At the same time the nurse or technologist may use pads
or Velcro straps to keep the area from moving. The examination table will
move into and out of the scanner opening, but it is not enclosed and only
a small part of your body will be inside at any one time. You may be asked
to hold your breath for 10 to 25 seconds to be sure that the images will
not be blurred. During the time that no actual imaging is taking place you
are free to ask questions or talk to the technologist, but friends or relatives
will not be allowed in the examining room. Once the needed images have been
recorded, you will be free to leave. You can eat immediately and it is a
good idea to drink plenty of fluids in the hours after the exam to help flush
contrast material out of the system.
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Who interprets the results and how do I get them?
Typically the results of CTA are available within 24 hours, although in
complicated cases special computer analysis may take somewhat longer. The
radiologist will report the findings to your physician, who in turn will
discuss them with you.
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What are the benefits vs. risks?
Benefits
- CTA can be used to examine blood vessels in many key areas of the body,
including the brain, kidneys, pelvis, and the arteries serving
the lungs. The procedure is able to detect narrowing of arteries in time
for corrective surgery to be done.
- This method displays the anatomical detail of blood vessels more precisely
than magnetic resonance imaging (MRI) or ultrasound. Today many patients
can have CTA in place of a conventional catheter angiogram.
- CTA is a useful way of screening for arterial disease because it is
safer and much less time-consuming than catheter angiography and is a cost-effective
procedure. There also is less discomfort because contrast material
is injected into an arm vein rather than into a large artery in the groin.
Risks
- There is a risk of an allergic reaction—which may be serious—whenever
contrast material containing iodine is injected. If you have
a history of allergy to x-ray dye, your radiologist may advise that
you take special medication for 24 hours before CTA to lessen
the risk of allergic reaction. Another option is to undergo a different
exam that does not call for contrast injection.
- CTA should be avoided in patients with kidney disease or severe diabetes
because x-ray contrast material can further harm kidney function.
- If a large amount of x-ray contrast leaks out under the skin where the
IV is placed, skin damage can result. If you feel any pain in this
area during contrast injection, you should immediately inform the technologist.
- Women should always inform their doctor or x-ray technologist if there
is any possibility that they are pregnant.
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What are the limitations of CT Angiography?
CTA images of blood vessels anywhere in the body may be fuzzy if the patient
moves during the exam or if the heart is not functioning normally. Blocked
blood vessels also may make the images hard to interpret. CTA is not yet
able to reliably image small, twisted arteries or vessels in organs that
move rapidly. If you are breast feeding at the time of the exam you should
ask your radiologist how to proceed. It may help to pump breast milk ahead
of time and keep it on hand for use after CTA contrast material has cleared
from your body. Pregnant women, especially those in the first three months,
should not have CTA or any exam exposing them to x-rays.
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