CT (computed tomography) angiography (CTA) is an examination
that uses x-rays to visualize blood flow in arterial and
venous 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 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 sizable catheter
and injecting contrast material into a large artery or
vein, CTA is a much less invasive and more patient-friendly
procedure—contrast material is injected into a small peripheral
vein by using a small needle or catheter. This type of
exam has been used to screen large numbers of individuals
for arterial disease. Most patients undergo CT angiography
without being admitted to a hospital.
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
renal CT 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 arteriovenous malformation
inside the brain that can be life-threatening.
- Detect atherosclerotic disease that has narrowed the
arteries to the legs.
- Detect thrombosis (clots) in veins, for example large
veins in the pelvis and legs. Such clots can travel to
the lungs and result in pulmonary embolism.
CTA is also used to detect narrowing or obstruction of
arteries in the pelvis and in the carotid arteries,which
bring blood from the heart to the brain. When a stent has
been placed to restore blood flow in a diseased artery,
CTA 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.
Depending on the part of the body to be examined, you
may be asked to take only clear liquids by mouth before
CTA. You may 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 undergoing 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.
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.
An example of the CT equipment that may be used is shown
above.
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.
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
you 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 may be 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. Afterward, the images
will be reviewed and, if necessary, some will be repeated.
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, 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 an area of your body 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.
A radiologist, who is a physician experienced in CTA and
other radiology examinations, will analyze the images and
send a report to your personal physician, who in turn will
discuss the findings with you. Typically the results of
CTA are available within 24 hours, although in complicated
cases special computer analysis may take somewhat longer.
Benefits
- CTA can be used to examine blood vessels in many key
areas of the body, including the brain, kidneys, pelvis,
and the lungs. The procedure is able to detect narrowing
of blood vessels in time for corrective therapy 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 undergo
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 is also 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 material
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 material leaks
out under the skin where the IV is placed, skin damage
can result. If you feel any pain in this area during
contrast material injection, you should immediately inform
the technologist.
- If you are breastfeeding 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.
- Women should always inform their doctor or x-ray technologist
if there is any possibility that they are pregnant.
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 may
also make the images hard to interpret. CTA is not yet
able to reliably image small, twisted arteries or vessels
in organs that move rapidly.
Reference:
http://www.radiologyinfo.org/
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Diagnostics 
