| Computed Tomography Angiography (CTA)
What is CT Angiography? Back to Top
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.
What are some common uses of the procedure? Back to Top
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.
How should I prepare for the procedure? Back to Top
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 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.
What does the equipment look like? Back to Top
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.
How does the procedure work? Back to Top
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.
How is the procedure performed? Back to Top
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.
What will I experience during the procedure? Back to Top
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.
Who interprets the results and how do I get them? Back to Top
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.
What are the benefits vs. risks? Back to Top
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.
Pregnant women, especially those in the first three months, should not
undergo CTA or any exam that exposes them to x-rays.
- Women should always inform their doctor or x-ray technologist if there is any possibility that they are pregnant.
What are the limitations of CT Angiography? Back to Top
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. Back to Top |
