What is an ultrasound test?
Ultrasound is a diagnostic test that uses reflected sound waves like those used to see the fetus during pregnancy—to create detailed images of the inside of the body, including the arteries and veins. The test does not require entering the body (it is non-invasive).
A basic ultrasound test creates pictures of the structure of the blood vessels, including the walls of the arteries or veins. Another type of ultrasound test, called Duplex Ultrasound, combines standard ultrasound images with Doppler ultrasound, which uses sound waves to track the blood as it moves through the vessels. This allows doctors to trace the speed and direction of blood flow, and to detect any changes caused by PAD or vein disease.
Duplex ultrasound can be used to diagnose and investigate:
- PAD in the legs
- Carotid (neck) artery disease
- Kidney artery disease
- Deep vein thrombosis (DVT)
- Chronic vein disease and varicose veins
To diagnose and monitor aortic disease, doctors may use duplex ultrasound or the basic ultrasound test.
Ultrasound for Carotid Artery Disease for more on how the test is used to diagnose carotid (neck) artery disease that can put you at risk for a stroke.
How does ultrasound work?
An ultrasound test evaluates the flow of blood through the vessels and can detect any narrowing or blockages caused by blood clots or atherosclerosis, the gradual buildup of fatty plaque on the artery walls. A small hand-held device (called a probe) is placed lightly against your skin. The device gives off high-frequency sound waves that pass into the body and bounce off muscle, bone, blood vessels, and organs. Each type of tissue reflects and absorbs sound differently. The probe picks up the sounds that bounce back, like echoes, and sends them to a computer that converts them into an image of the blood vessels and other tissues.
Duplex ultrasound combines a basic ultrasound test with Doppler ultrasound, which uses sound waves to track the red blood cells as they move through the vessels. This data is interpreted by a computer and displayed as a color-coded image of the speed and direction of blood flow and a tracing of the blood’s movement as it pulses through the vessels with each heartbeat.
Who might have an ultrasound test?
Because it is safe, fast, and relatively inexpensive, duplex ultrasound is often the first imaging test used to diagnose and evaluate PAD and vein disease. Duplex ultrasound can tell if PAD or blood clots are causing changes in blood flow, and help determine the location and extent of blockages. The test can also tell whether symptoms of PAD or symptoms of vein disease are caused by blood vessel problems, or if they may have another cause.
A duplex ultrasound may also be used in women who are considering a procedure to treat PAD or vein disease. In these cases, the information provided by an ultrasound test can help doctors decide which procedure may be best for you, and help develop a treatment plan. More detailed tests (such as MRA, CT angiography, contrast angiography, and venography) can provide a closer look at the structure and blood flow through the vessels. These may be necessary if the ultrasound test does not provide enough information.
Duplex ultrasound may also be used to check for complications after procedures to restore blood flow, and to monitor how well treatment is working.
Because it provides images of the structure of the artery wall, an ultrasound test (with or without Doppler blood flow measurement) is the most commonly used test to diagnose and monitor aortic aneurysms, and to check for aortic aneurysms in women who are at high risk because they have a family history of the disease. Should I be screened for an aortic aneurysm? to learn more.
How do I prepare for the test?
You should wear comfortable, loose-fitting clothing, and may need to change into a hospital gown before the test. If you are having an ultrasound that includes arteries in your abdomen, you may be asked to not eat or drink anything (except small amounts of water to take medication) overnight before the test—liquid and gas in the intestinal tract can interfere with the examination.
What happens during the test?
The ultrasound test is performed in an ultrasound lab, which may be at your doctor’s office, an outpatient testing center, or in a hospital. The exam is performed by a specially trained vascular ultrasound technologist.
You will lie down on a table with your head slightly elevated. The technologist will spread a gel over the area being tested, which helps the sound waves travel better. A hand-held device called a probe (or transducer) will be placed lightly against your skin and moved back and forth. The probe sends out sound waves and picks them up as they echo off your body tissues. This information is sent to a computer to create images of your blood vessels and the blood flow through them.
You may hear a whooshing sound during the test, which is the amplified sound of blood moving through your arteries. You may be asked to move to different positions to get a view of your vessels from different angles. If you are having an ultrasound of the renal arteries, you will lie on your side. You should stay still during the test, and may be asked to take a deep breath and hold it.
If you are having an ultrasound test to diagnose DVT, the technologist may place pressure on the veins to close them off and see how the blood flow responds. This procedure is called compression ultrasound.
A complete lower body ultrasound (including aorta, kidney arteries, and abdominal and leg arteries) can take up to two hours. If you are only having an ultrasound of your leg veins, the test will take less than 30 minutes.
What happens after the test?
When the test is complete, the gel will be wiped off your body and you can change back into your clothes. You may go back to your normal activities after the test. The technician will record the test so that it can be reviewed by a physician, who will contact your doctor with the results.
What are the risks of the test?
A duplex ultrasound test poses no significant risks. Ultrasound is safe and painless because it uses harmless sound waves and does not involve radiation, injected dyes, or entering the body.
What do the results mean?
The results of a duplex ultrasound can tell doctors how blood is flowing through your arteries and determine the location and severity of any narrowing or blockages. When used to diagnose DVT, the ultrasound can detect areas where blood flow through the veins is stopped or slowed by a blood clot, and sometimes the clot itself will be visible on the ultrasound image.
The images produced by a duplex ultrasound include a color-enhanced picture of the speed of blood flow through the vessels. In addition, the readout includes a tracing of the blood as it is pushed through your body in “pulses” by each beat of the heart (called a Doppler waveform analysis). The shape of the pulses can help doctors determine how smoothly blood is flowing.
A duplex ultrasound image of blood flow through a normal artery.
The red area means blood is flowing quickly through the arteries.
A duplex ultrasound image of an artery with mild narrowing caused by PAD.
The large red areas show that most blood is flowing quickly, but the small blue sections indicate areas of slow blood flow.
A duplex ultrasound image of an artery with limited blood flow caused by moderate PAD.
The large blue area indicates that PAD is causing blood to flow slowly. The dark areas beyond the blue blockage indicate areas where blood has not reached because PAD is partially blocking the artery.
A duplex ultrasound image of an artery that has been almost completely blocked by PAD.
The blue areas indicate slow blood flow, and not much blood flow is visible beyond the blockage because PAD has nearly closed off the artery.
How accurate is duplex ultrasound?
Duplex ultrasound is 90% to 95% accurate at diagnosing severe blockages from the abdomen to the knee, and is also extremely accurate for the diagnosis of renal (kidney) artery disease. Ultrasound is less reliable (about 83% accurate) for diagnosing PAD in the lower legs because of the smaller arteries.
Duplex ultrasound may not be as accurate as other tests (such as CT angiography, MRA, and contrast angiography) for planning procedures to treat PAD. However, some studies find that is it often accurate enough to develop a reliable treatment plan. Some women may need more detailed tests to help plan PAD treatment.
The ultrasound test does have some limitations. It may be difficult to diagnose blood vessel problems in the lower abdomen because bowel gas or body fat can block some of the sound waves. The buildup of hard calcium on the walls of the arteries can also interfere with ultrasound images. In addition, ultrasound can miss artery narrowing that is downstream of a more severe narrowing or blockage because of limited blood flow past the first blockage.
Ultrasound with leg vein compression is usually the first imaging test to diagnose DVT because it is safe and simple to perform. A negative ultrasound result means you are very unlikely to have a DVT.
While it is 86% to 99% accurate at diagnosing DVT above the knee, the ultrasound test is less accurate for diagnosing DVT in the calf veins. DVTs in the abdomen can sometimes be missed on an ultrasound test because of interference by body fat or bowel gas.
How does duplex ultrasound compare to other imaging tests?
Ultrasound tests are relatively inexpensive, easy to perform, and do not involve magnetic fields, radiation, dye injections, or entering the body. For these reasons, they are the most commonly used initial imaging test to diagnose PAD and DVT.
The images produced by ultrasound are not as detailed as those from a CT angiogram, MRA, or contrast angiogram, and therefore may be slightly less accurate for diagnosing PAD. You may need additional tests to evaluate PAD and develop a treatment plan. One study of 514 patients (one third were women) found that doctors were less confident making treatment decisions based on ultrasound compared with CT angiography and MRA. A study of 352 patients with PAD (32% were women) found that patients who received MRA as the first diagnostic test were 42% less likely to have additional tests compared with those who had a duplex ultrasound first. Click here for a quick comparison of ultrasound with other imaging tests used to diagnose and evaluate PAD.
Ultrasound is less accurate than the gold-standard contrast venogram test for diagnosing DVT and vein disease. However, it is a useful first imaging test to diagnose DVT, and if a blood clot is detected you will probably not need any additional tests.
|Imaging Tests for Vein Disease|
|Test||Radiation?||Injected Dye?||Invasive?||Time||Other Considerations|
|Ultrasound||No||No||No||30 min||Less accurate in lower legs and pelvis|
|Venography||Yes||Yes||Yes||30 to 45 min||Most accurate but carries risks|
What happens next?
Women with PAD who have a duplex ultrasound that shows blockages may be considered directly for treatment. Your doctor may also request an additional test, such as a CT angiogram, MRA, or contrast angiogram to get a closer look at the arteries before deciding on a treatment plan.
Women with a suspected DVT who have a positive ultrasound test should be treated for DVT with blood-thinning drugs. See DVT Treatment Overview (coming soon) for more. If your ultrasound test was negative but you are considered at high risk for DVT, you may need additional tests to make sure the ultrasound did not miss a blood clot. Further investigation may include a D-dimer test, a second ultrasound test 3 to 7 days later, or a venogram.