What is CT angiography?
Computerized tomographic angiography, also called CT angiography or CTA, is a test that combines the technology of a conventional CT scan with that of traditional angiography to create detailed images of the blood vessels in the body.
In a CT scan, x rays and computers create images that show cross-sections, or slices, of your body. Angiography involves the injection of contrast dye into a large blood vessel, usually in your leg, to help visualize the blood vessels and the blood flow within them. When the contrast dye is used to visualize your veins, the study is called a venogram, and when it is used to visualize your arteries, it is known as an arteriogram. CT angiography is similar to a CT scan, but the contrast dye is injected into one of your veins shortly before the x ray image is performed. Because the dye is injected into a vein rather than into an artery, as in traditional angiography, CT angiography could be considered less invasive.
Your physician may order CT angiography to help diagnose a narrowing or obstruction of the arteries, an aneurysm, deep vein thrombosis, pulmonary embolism, or another vascular condition.
During the study, you will lie down on a table, which passes through a donut-shaped device. Inside the device, a machine takes x rays in arcs around the area of your body being examined. Tissues of varying densities absorb these x rays in varying amounts. The computer assigns these densities different numerical values and then plots an image based on these values, in shades of gray. During the CT angiogram, a dose of contrast dye will be injected into one of your veins. As the dye flows through your circulatory system, it will highlight your blood vessels on the scan. A computer will produce 3-dimensional (3D) images of your blood vessels from the x ray images.
How do I prepare?
Your physician will ask you to avoid foods and liquids for about 4 hours before the test. You will probably be allowed to continue taking any prescribed medications, but ask your physician before the exam to be sure.
Before the test is performed, you will be asked to change into a hospital gown and remove all jewelry and other metal objects, which can interfere with x rays.
Am I at risk for complications?
Because CT angiography is an x-ray test, it sends radiation through your body. Although the amount of radiation for each scan is small, it can accumulate and damage body cells over time with repeated exposures.
Contrast material carries a slight risk of causing an allergic reaction. If you know that you are allergic to contrast material or dye, let your physician know before you receive the contrast material. In addition, allergies to iodine or shellfish may place you at increased risk for having an allergy to the contrast dye. If you are allergic to iodine or shellfish, alert your physician prior to the test. Medications can sometimes be given before the contrast material is administered to lessen the risk of allergic reactions in susceptible patients.
The contrast material is also eliminated by the kidneys and can damage kidney function, especially if you have kidney problems already. Let your doctor know if you have damaged kidney function. In this situation, medication and fluids, called hydration, can sometimes be given before the contrast material is administered to decrease the effects of the contrast dye on the kidneys. Blood tests may be done to evaluate your kidney function before the CT scan.
You may be unsuited for CT angiography if you:
- Have an allergy to contrast dye
- Have kidney problems
- Have severe diabetes
- Are pregnant, because radiation may harm the fetus
- Have unstable vital signs
- Weigh more than 300 pounds, because some x ray tables cannot support the weight
What happens during a CT angiography?
Before the x rays are taken, the contrast material will be injected into a vein in your arm or hand using an automatic injector machine, which controls the timing and rate of injection. The machine may continue to inject contrast dye into your vein throughout the test. The contrast material may make you feel flushed and warm, and it sometimes can produce a mildly sick-to-your-stomach sensation.
You will need to lie very still on the scan table that slides into the gantry, which is the donut-shaped device that houses the scanning equipment, in order to get the best quality images. The machine is quiet and relatively open; only the part of the body being examined lies inside the gantry. As a medical technician operates the scanning machine in another room, he or she watches and speaks to you through speakers in the CT scan room.
An x ray tube slides around the gantry, passing narrow beams of low-dosage x-rays in an arc over the body. These beams reflect onto a detector positioned opposite the x-ray source. After the x ray source completes an arc, the scanning table moves forward a small distance and the x ray source transmits another arc of x rays. Most CT angiographies use a type of machine called a spiral CT machine, which is able to record a large number of pictures–as many as 1,000 pictures in each arc — in a short time. The detector transmits the x ray energy to a computer, which transforms the information about the reflected energy into a 3D image. To create images from different angles, the technician may adjust the position of the scanning table.
You must remain very still as the CT scanning machine operates. The technician may ask you to hold your breath for 10 to 25 seconds at a time, because even the motion of breathing can blur the images.
The entire procedure usually takes 20 minutes to 1 hour to complete.
What can I expect after a CT angiography?
There are no restrictions after a CT angiography and you can resume normal activities immediately.
You should drink plenty of fluids following the test to speed excretion of the contrast agent and to guard against dehydration.
Are there any complications?
The most serious early complication of CT angiography is an allergic reaction to the contrast dye. Reactions usually occur immediately and include flushing, itching, or, rarely, difficulty breathing or swallowing. Notify your physician if you experience any of these symptoms. Sometimes contrast dye leaks under your skin at the injection site. This can cause redness, swelling, or pain. The contrast dye can also damage kidney function depending upon the amount of the dye used and whether you have any kidney problems that already exist.