Doppler sonography is a special variant of ultrasound examination and allows visual and acoustic imaging of blood flow in arteries and veins. The procedure is used primarily to diagnose heart defects or vascular stenosis.
What is Doppler ultrasonography?
Doppler ultrasonography is a special variant of ultrasonography and allows visual and acoustic imaging of blood flow in arteries and veins. Doppler sonography is a special type of sonography, or ultrasound examination. The Doppler effect makes the blood flow in the blood vessels audible. Together with the so-called B-scan sonography and a special color coding, the blood flow can also be visualized. In Doppler sonography, a distinction can be made between continuous-wave Doppler (CW Doppler), pulsed-wave Doppler (PW Doppler) and color-coded Doppler sonography. Color-coded Doppler sonography is most commonly used in practice. It is also referred to as angiodynography.
Function, effect, and goals
In Doppler sonography, an ultrasound probe emits ultrasound waves at a defined frequency. When this beam of ultrasound waves hits a medium, such as a blood cell in the blood vessels, the ultrasound waves are reflected by these structures. The frequency changes depending on how fast or slow the medium is moving. The reflected waves are measured in sound form by a microphone in the ultrasound head. The results are then evaluated in the computer of the sonography device. The flow velocities measured by the ultrasound probe are color-coded. Blood flowing toward the transducer is usually displayed in red. If the blood flows away from the transducer, it appears blue. The flow velocities can now be read from the different color levels. Light colors represent higher flow velocities, while dark colors represent lower flow velocities. This means that a light blue indicates a higher velocity than a dark blue. The resulting color Doppler image is superimposed on the so-called B-mode image. The B-mode image is a variant of the normal black and gray ultrasound image. This makes it possible to localize exactly which flow velocities prevail in which vessels. The image is displayed in real time, so that changes can be displayed at any time. Doppler sonography can be used to detect various pathologies. In echocardiography, Doppler sonography is used to examine the heart. Here, the flow conditions in the area of the heart valves are examined in particular. In this way, valve defects, so-called insufficiencies or stenoses can be diagnosed. In the case of a valve insufficiency, the heart valve no longer closes properly; in the case of a stenosis, it no longer opens properly. Insufficiencies can be recognized in Doppler sonography by the incorrect direction of flow. When a valve no longer closes properly, blood flows back due to pressure conditions. In the case of a stenosis, sonography shows a higher flow velocity in the stenosis. This can be seen by a lighter color. If the valve does not open properly, the blood has to be forced through the narrowed valve. This increases the flow velocity. Carotid ultrasonography involves an examination of the carotid arteries. Here, Doppler ultrasonography is especially helpful in detecting stenosis. The most common cause of carotid stenosis is arteriosclerosis. If the stenosis is very pronounced, it can lead to circulatory disorders of the brain or eyes. If thrombi detach from the vascular deposits, a stroke can be triggered. The Doppler procedure can be used to diagnose the extent of carotid stenosis. Carotid Doppler ultrasonography is therefore also suitable for assessing the risk of stroke and also myocardial infarction. Doppler sonography is also used in the diagnosis of thrombosis or for the assessment of vessel walls. The procedure is also used in nephrology. Here, color Doppler helps to find vessels in the kidneys. In obstetrics, important vessels such as the ductus venosus or the arteria cerebri media can also be found more easily with the Doppler device.
Risks, side effects, and hazards
Actually, Doppler ultrasonography is a safe procedure.Possible sources of damage are heat generation and so-called cavitation. In the sound waves sent by the ultrasound device, there is a negative pressure phase. During one of these negative pressure phases, cavities or gas bubbles may form in the tissue under examination. If the pressure then rises again, these bubbles can collapse and damage the tissue. This effect is known as cavitation. Cavitations occur particularly frequently in air-filled areas, for example, during ultrasound of the lungs or intestines. The occurrence of cavitations can be favored by the use of ultrasound contrast media. Overall, however, the risk of cavitation occurring during Doppler ultrasonography is rather negligible. How much heat is generated during an ultrasound examination depends on the intensity of sound absorbed. Healthy tissue can tolerate a temperature increase of up to 1.5 °C even in the long term. If the tissue is already damaged beforehand, the temperature increase may possibly worsen the condition. Since these risks are also known in clinics and medical practices, the sound intensities are conscientiously adjusted there, so that a health hazard is rather unlikely. The advantages of Doppler sonography therefore lie in its low-risk and, above all, painless application. In contrast to X-rays, patients are not exposed to any radiation. From an economic point of view, Doppler sonography also has advantages. The acquisition costs for sonography equipment are significantly lower than for procedures such as computer tomography or magnetic resonance imaging. In addition, Doppler sonography is the only procedure that can visualize fluid flows. The disadvantage with Doppler sonography is that there is no standardized training. Therefore, the results are strongly influenced by the skill and diagnostic ability of the examiner.
