Doppler sonography (synonyms: Doppler effect sonography, Doppler echography) is a medical imaging technique that can dynamically visualize fluid flows (especially blood flow). It is used to assess blood flow velocity and, in cardiology, to diagnose cardiac and valvular defects. Particularly in the case of pathological vascular phenomena, Doppler sonographic examination represents the basis of the diagnostic procedure, since both the velocity distribution in the respective vessel section is assessed and an exact representation of the direction of flow can be made. Furthermore, Doppler sonography makes it possible to reproduce the temporal change in the velocity of the blood flow. The factors obtained in this way can then be used to calculate the volume flow rate and the pathophysiologically important flow resistances. In addition to the diagnostic importance of the procedure in angiology, Doppler sonographic examination also plays a crucial role in obstetrics and gynecology. The development of Doppler sonography is largely based on the research of Austrian physicist Christian Johann Doppler, who in 1842 formulated a mathematical relationship to the phenomenon of the astronomical double star effect, which he believed was also applicable to sound waves.
The process
Doppler sonography is based on the principle that ultrasound waves are emitted into tissue at a defined frequency, where they scatter on circulating erythrocytes. Due to this scattering, a portion of the ultrasound waves returns to the transducer, which thus serves on the one hand as a transmitter and on the other hand also as a receiver of the sound waves. The erythrocytes (red blood cells) thus act as a boundary surface at which the sound waves are reflected, so that a frequency increase occurs when the distance between the transducer and the boundary surface decreases, and the frequency decreases when the distance increases. However, the so-called Doppler effects occur not only in flowing blood, but also in other moving organic structures, such as vessel walls. Doppler sonography is divided into several techniques:
- Single-channel Doppler techniques: In this method, a single beam of sound is emitted by the Doppler system, so that the resulting data arise solely from the section of vascular structure through which the beam passes.
- Continuous-wave (CW) Doppler sonography: a subset of single-channel Doppler techniques, this system represents the simplest method of collecting continuous blood flow data over the entire depth of ultrasound penetration. Each transducer has separate acoustic elements for sound transmission and reception. Continuous information acquisition is made possible by the fact that the transmitter and receiver in the transducer operate in parallel and continuously side by side. However, spatial assignment is not possible with this method. However, the advantage of this method is that the determination of high flow velocities is possible.
- Pulsed-wave (PW) Doppler sonography: as a further subgroup of the single-channel Doppler methods, a spatially selective velocity measurement is possible with this system in contrast to CW Doppler sonography. In pulsed Doppler mode, an electronic measurement window is generated to measure the flow velocity of erythrocytes flowing through the measurement window at a defined depth in the tissue. Unlike the CW Doppler method, the information is transmitted via pulses and not continuously.
- Multichannel Doppler techniques (synonyms: Color Doppler sonography, color-coded Doppler sonography, color-coded duplex sonography; combination of B-scan with PW Doppler/Pulse Wave Doppler): In this technique, as in CW Doppler sonography, the sound transmitter and the sound receiver are located as separate structures in the transducer. However, the difference is that a large number of transmitters and receivers are located in each transducer. The transmission and reception of ultrasound waves do not occur simultaneously, allowing the many sound beams to gather information from a three-dimensional cross-sectional image. All multichannel systems operate in pulsed Doppler mode. The collection of information is restricted by the limited number of evaluation channels in the Doppler sonograph.The large number of sound waves ensures accurate localization of the information sources. Due to the functional properties of the method, it is used to estimate possible flow turbulence with the help of color coding, where different flow velocities can be displayed in shades of red and blue. The turbulence itself is represented in green.
- Tissue Doppler sonography (synonym: tissue Doppler sonography): a special type of multichannel Doppler procedure in which the velocity of movement of a tissue is measured. Most commonly, an examination of the myocardium is performed to detect pathologic processes there.
To amplify the ultrasound waves in Doppler sonography can serve ultrasound contrast agents based on the technique of so-called microbubbles. Microbubbles are micrometer-sized gas bubbles that amplify the ultrasound signal because they are capable of complete reflection of sound waves. In contrast to native Doppler sonography, computed tomography (CT) and magnetic resonance imaging (MRI) allow visualization of the capillary flow area. With the use of microbubbles, it is also possible in Doppler sonographic examinations to determine the flow velocity of blood in the capillary bed by measuring and evaluating the bursting of gas bubbles caused by the occurrence of sound waves. In order to obtain the best possible information from the Doppler sonographic examination, it is necessary for the examiner to have sufficient experience as well as the ability to choose the right Doppler probe. Depending on the depth of the examination, the choice falls on a special transducer or on a special Doppler probe.
