How does it work?
Consider the sound of an approaching or receding train. As a moving source that produces sound with a constant frequency approaches, it becomes more high (increased frequency, the number of waves of the sound coming to the ear increased), and the farther away it is, the lower (decreased frequency, the number of waves of the sound coming to the ear decreases).
This is called Doppler shift. Shaped elements such as the red cells of the blood in the blood flowing through the veins are mobile structures. Sound is reflected from these structures. By detecting the frequency change (Doppler shift) between the sound we send and the sound reflected, we can determine the direction and speed of the current.
One of the most important purposes of Doppler examinations is to determine vascular stenosis by means of flow velocity. Another important area of use is to investigate the vascular structures and blood supply of tumors, to try to understand whether they are benign or cancerous. In addition, it helps to diagnose diseases of some organs by examining the vessels and blood flows that feed the organ.
Today, only gray scale ultrasonography devices are not used much and almost all devices produced have color Doppler feature. Both ultrasonographic examinations and color Doppler examinations are performed with the same device and are used to support each other during almost all types of examinations. The vessel, mass or organs to be examined are first analyzed anatomically with gray scale US; Irregularity, cholesterol plaques and calcification on the inner surface of the vessel are investigated. The quality and quantity of blood flow is then examined. The current is color coded, the direction and shape of the current are determined.
In this examination, which is called Color Doppler, a rough idea about the velocity of the flow can be obtained by looking at the toning of the colors. Color Doppler basically uses red and blue colors. Whichever color is chosen for the current examined, the other color indicates the flow coming from the opposite direction. The fast current is indicated by lighter shades of color. Areas where the current swirls and reverses, as observed after strictures, are coded with the opposite colour. A mosaic of colors emerges in obvious narrownesses where the speed is too high.
For a more sensitive detection of atherosclerosis, measurement of the flow velocity is required. In order for the amount of blood flowing not to change, for example, if the diameter of the vessel is halved, the flow rate must be doubled. This relationship is valid until the critical point where the internal diameter of the vessel narrows by more than 95% and the flow velocity suddenly drops. For this, a small area is selected from the middle of the examined vessel and printed in the form of current, frequency or velocity/time graph. This method, which shows the speed and quality of the current in the form of a graph, is called Spectral Doppler.
Spectral Doppler is more sensitive in determining the quantity and quality of the current. By making measurements on the graph, both the degree of stenosis can be determined and the quality of the flow can be shown numerically with some measurements (many Doppler indices).
Although high-frequency sound is used in Doppler US, the frequency change in the rotating sound is within the limits of what the ear can hear.
The ear is the most sensitive sound separator. An experienced physician can have sufficient information about the quality and quantity of the current by listening to this sound. For this reason, speakers continue to exist even in today’s most advanced color Doppler ultrasonography devices.
In recent years, with technological developments, many new features have been added to color Doppler ultrasonography devices that increase image quality and vascular flow sensitivity. In addition, a new imaging technique that uses similar working principles and computer technologies, which we will not go into detail here, is elastsonography. In this method, the softness/hardness degrees of the examined organs, tissues or masses are evaluated, and important information that helps the correct diagnosis is obtained.
Finally, another important area of application benefiting from color Doppler ultrasonography is its role in ultrasonography guidance, which is the most widely used during Interventional Radiology applications. Knowing the vascular structure of the formations such as tissue, canal, mass on the passageway or the targeted tissue, channel, mass and its surroundings while the needle or catheter is placed during the interventions is a very important factor affecting the success and risks of the applied procedure. This information is obtained with the support and help of color Doppler ultrasonography.