Arterial Doppler Ultrasound
Arterial Doppler Ultrasound
Doppler ultrasound, also called Doppler ultrasonography, is a noninvasive, painless procedure used to evaluate blood flow in major arteries and veins in the arms and legs. It uses high frequency sound waves that are above the level of human hearing (ultrasound). In Doppler ultrasound, sound waves are transmitted through the body and are echoed back to produce images of blood flow in body tissues and organs.
Doppler ultrasonography is used to assess the direction, velocity, and turbulence of blood flow. A Doppler study of blood flow can be used to diagnose many conditions, such as blood clots, incompetent valves in leg veins, which cause fluid to accumulate (venous insufficiency), deep vein thrombosis, a blocked artery (arterial occlusion), and narrowing (stenosis) of an artery. The test can also be used to evaluate injuries to arteries such as damage due to myocardial infarction (heart attack) or to monitor arterial reconstruction and bypass grafts. The test is done as an alternative to arteriography and venography, which are both invasive procedures with greater risks to the patient than Doppler ultrasound. Other uses of the procedure include evaluation and diagnosis of tumors with vascular involvement, abdominal aortic aneurysms, heart valve defects and congenital heart disease.
Doppler ultrasonography is harmless, as it uses no ionizing (x ray) radiation. There is also no pain associated with Doppler ultrasound testing.
Cigarette smoking may alter the results of the test, as nicotine can cause arteries in the extremities to constrict.
Doppler ultrasound testing is performed using an ultrasound scanner with Doppler imaging capabilities and a transducer. A transducer is a hand-held device that converts electrical signals into ultrasound waves, directs the high-frequency sound waves to the artery or vein being tested, and then converts the returning ultrasound waves back into electrical signals. Ultrasound waves used for medical imaging are usually in the range of 2 to 10 megahertz (MHz).
Images are produced by a computer using the frequency shift caused by the Doppler effect. The Doppler effect is a physics principle that states that relative to an observer, the frequency of any sound or light wave will vary as the source of the wave approaches or moves away. For example, the pitch of a siren changes as an police car moves toward, past, and then away from the listener. When used to evaluate blood flow, sound waves increase in frequency when they echo from red blood cells moving toward the transducer and decrease in frequency when they move away from the transducer. The change in frequency is related to the velocity of the moving red blood cells and can be measured to determine the velocity of the blood flow. Blood flow also produces a "swishing" sound that can be heard with Doppler ultrasound.
Doppler ultrasonography can be operated in either continuousor pulsed-wave modes. Continuous-wave Doppler is the simplest mode, whereby flow information is received from all of the moving reflectors in the path of the beam. In pulsed-wave mode, a transducer sends a series of short sound pulses into the body, with pauses between each pulse to allow for the detection of the returning sounds that are echoing back from the red blood cells. Pulsed-wave Doppler allows the operator to select a specific area of interest for flow analysis.
Most ultrasound scanners also have color Doppler imaging capability, where color is superimposed over the gray-scale images. Blood flow changes the pitch of the sound beam, so that the Doppler effect can also be detected on the image as color and displayed graphically. For example, red and blue in a blood vessel image indicate that the flow is away from the transducer, while blue and green indicate that the blood is flowing towards the transducer. Color Doppler imaging can be used to identify areas of arterial narrowing.
More advanced ultrasound scanners utilize power Doppler, which is more sensitive than color Doppler. Power Doppler can produce images of abnormalities that are not normally produced with ultrasound, such as signs of inflammation.
During an ultrasound study, the patient is placed on a bed or table that can be tilted such that the area to be imaged is easily accessible. Blood flow is examined by placing a water-soluble acoustic coupling gel (a special gel that enhances transmission of ultrasound waves and produces a secure contact and eliminates air pockets between the transducer and the skin) on the transducer and on the skin over the arteries or veins of the limb being tested. The transducer is then placed firmly on the skin and may be swept over the area of interest to acquire images of the blood vessels. Usually gray-scale images, which use different shades of gray to indicate differences in the strength of echoes, are acquired first. Echoes from blood are of lower strength and appear darker than surrounding tissue. Next, Doppler mode is selected to acquire the Doppler images, which are superimposed over the gray-scale ultrasound images. The sonographer then uses the computer associated with the scanner to mark areas and calculate parameters of interest, such as blood flow velocity in vessels with narrowing or blockage. The live images can be recorded, and still frames of the moving picture can be obtained by stopping the moving picture and capturing a series of images. These can be printed using a medical image printer. Most ultrasound exams take less than 30 minutes, although more complicated examinations can take longer.
Clothing and jewelry must be removed from the extremity that will be tested. The health care provider should be informed of any medications that the patient is taking, especially blood pressure and vascular medications that could interfere with interpretation of results. To reduce anxiety or resistance to the procedure that children may experience, an explanation of what the procedure will entail as well as support during the procedure may help to reduce stress.
No aftercare is required.
There are no risks associated with using ultrasound waves on the body. Ultrasound does not affect cardiac pacemakers or metal implants or fragments within the body.
A normal finding in ultrasound testing will show an absence of blood flow restriction or structural abnormalities. Deep veins will show flow that varies with breathing. The arteries will show flow that corresponds to the normal beating and resting of the heart. Disrupted, obstructed, or restricted blood flow or other abnormalities indicate abnormal results that require follow up treatment. Examples of follow up treatment may include angioplasty, grafts, anticoagulant therapy, and filters to prevent clots from traveling to the lungs.
Health care team roles
Doppler ultrasonography can be performed in a hospital ultrasound, radiology or cardiology department, a peripheral vascular laboratory or outpatient imaging center. In an emergency it can be performed at the patient's bedside or in the operating room. The examination is performed by an ultrasonographer with special training in ultrasound techniques, including cardiac and vascular imaging. The sonographer should be a registered vascular technologist or a registered cardiac sonographer. A radiologist, cardiologist, or other physician experienced in ultrasound imaging techniques will interpret the results of the imaging procedure. Results may also be provided to the patient's primary care physician.
Arteriography— Also call angiography. A procedure to x-ray arteries. A dye is injected into the artery believed to be damaged so that the artery can be viewed by x ray.
Extremities— The arms and hands (upper extremities) and the legs and feet (lower extremities).
Noninvasive— A diagnostic procedure or treatment that does not require the skin to be broken or a body cavity to be entered.
Vascular— Referring to a person's system of blood vessels: arteries, veins, and capillaries.
Venography— X rays taken after dye is injected into a small vein, to show areas of normal and abnormal blood flow.
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Kerut, Edmund Kenneth, Elizabeth F. McIlwain, and Gary D. Plotnick. Handbook of Echo-Doppler Interpretation. Oxford, UK: Blackwell Publishing Ltd., 2004.
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Myers, Kenneth and Amy Clough. Making Sense of Vascular Ultrasound: A Hands-On Guide. London, England: Hodder Headline Group, 2004.
Shung, K. Kirk. Diagnostic Ultrasound: Imaging and Blood Flow Measurements. Boca Raton, FL: CRC Press, 2005.
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American Institute of Ultrasound in Medicine. 14750 Sweitzer Lane, Suite 100, Laurel, MD 20707-5906. (301) 498-4100. http://www.aium.org.
American Registry of Diagnostic Medical Sonographers. 51 Monroe Street, Plaza One East, Rockville, MD 20850-2400. (301) 738-8401. http://www.ardms.org.
American Society of Radiologic Technologists (ASRT). 15000 Central Avenue SE, Albuquerque, NM 87123-3917. (800) 444-2778. http://www.asrt.org.
Society of Diagnostic Medical Sonography. 2745 Dallas Parkway, Suite 350, Plano, TX 75093-8730. (800) 229-9506. http://www.sdms.org.
Society of Vascular Technology. 4601 Presidents Drive, Suite 260, Lanham, MD 20706. (301) 459-7550. http://www.svunet.org.