Specifics of Doppler Ultrasound
Doppler ultrasound is a form of ultrasound that can detect and measure blood flow. Doppler ultrasound depends on the Doppler Effect, a change in the frequency of a wave resulting here from the motion of a reflector, the red blood cell.
Doppler ultrasounds have two primary applications:
• Detection of fetal heart rate. The fetal heart rate can be detected with a hand held Doppler between eight and 12 weeks of pregnancy. Depending on the model of Doppler used, the physician may manually count the heartbeats or view them on a digital display.
• Evaluation of blood flow in adults, primarily in the legs (although it can be used in the neck, heart and abdomen). It is particularly effective in checking for peripheral arterial disease.
Although some Dopplers can only be used either for fetal heart rate detection or vascular examinations, some models offer interchangeable probes, and can be used for both applications.
Detection of Arterial Disease:
Dopplers are often used to determine whether blood flow in veins and arteries is normal or obstructed. In normal arteries, for example, the Doppler sound has multiple beats, due to several physiological factors, such as the closing of the aortic valve, resistance and elasticity of the arterial wall. A Doppler sound that lacks the multiple beats may indicate a problem in the artery’s blood flow. (Veins, on the other hand, always exhibit a mono-phasic sound.)
One of the most common tests performed with the Doppler is the ankle brachial index, or ABI. In this test, the physician uses the Doppler to monitor the blood flow in both arms and both legs. If the systolic pressures are the same, no blockage exists. However, if the ratio of the flow in the ankles to that of the flow in the arms is less than one, then peripheral arterial disease may be present.
The Society of Interventional Radiology recommends that all patients being evaluated for peripheral vascular disease have their ABI measured. In addition, the American Heart Association and American Diabetes Association recommend that physicians caring for diabetic patients be able to measure the ABI with a Doppler ultrasound instrument.
Dopplers may also be used to diagnose chronic venous insufficiency, a condition in which a valve fails to close properly, causing blood to pool in the vein. The result can be varicose veins or blood clots on the wall of the vessel, a condition called Deep Vein Thrombosis.
There are several kinds of Doppler ultrasound: Color Doppler -- This technique estimates the average velocity of flow within a vessel by color coding the information. The direction of blood flow is assigned the color red or blue, indicating flow toward or away from the ultrasound tranducer. Pulsed Doppler -- This method allows a sampling volume or "gate" to be positioned in a vessel visualized on the gray-scale image, and displays a graph of the full range of blood velocities within the gate versus time. The amplitude of the signal is approximately proportional to the number of red blood cells and is indicated, not in color, but simply as a shade of gray.
Power Doppler -- This device depicts the amplitude, or power, of Doppler signals rather than the frequency shift. This allows detection of a larger range of Doppler shifts and thus better visualization of small vessels, but at the expense of directional and velocity information.
Buying and Selling:
Like more traditional ultrasound, Dopplers must be used in conjunction with a gel between the probe and the patient’s skin. This is because high frequency sound waves do not travel readily through air.
Dopplers use a variety of probes, ranging from 2 MHz to 10 MHz. The lower the number, the more sensitive the probe. So a 2 MHz probe should be used to detect the fetal heartbeat. However, to detect blood flow in a vein in the ankle, 8 MHz probes are often used. As noted earlier, many Dopplers may be purchased with interchangeable probes, making them appropriate for multiple applications.
Customers should be aware that in order to receive reimbursement for a Doppler exam , the instrument must be bi-directional (meaning that it measures blood flow in both directions), and the practice must produce a printout of the waveform. Printers specifically geared to Dopplers are available from Doppler manufacturers.