Doppler sonography/physical principle

Physical principles of Doppler-Sonography

The Doppler-Sonography is a special type of Sonography that enables the physician to display the the bloodflow in blood vessels and the heart in acoustic and visual qualities using the Dopplereffect of soundwaves. In Doppler Sonography it is still necessary to use the method of usual Sonography to find the target structure. The target structures of Doppler Sonography need to be vessels or the heart since a moving object or fluid (in this case blood) is required to create the Dopplereffect. Common examined organs or tissues are for instance the common carotic artery, the heart and also the vessles of the brain. Nowdays almost every modern medical ultrasound device can use Doppler Sonography in order to evaluate the bloodflow of the visualized structures. Because Doppler-Sonography is fast, relativley cheap, and non-invasive it has become essential in the diagnostic of vessles in various areas of medicine such as internal medicine, gynaecology, angiology and cardiology.

In this part of the article we are going to discuss the physical principles of the Doppler Sonography, the Dopplereffect, first in general and then how the Dopplereffect is applied to the medical ultrasound device. To fully comprehend this article the principle of sonography must be understood.

Doppler effect in tissues

Fd = 2 (v*Fo)/c

Fd = Doppler shift (The difference in the frequency sent from the transducer (Fo) and the received from the returned echo) Fo = original frequency v  = velocity of blood c  = propagation speed

In Sonography the Dopplereffect works slightly different compared to the examples provided above as the blood does not emmit any sound but rather reflects the soundwaves produced by the probe. Through the reflection of the soundwaves by the erythroctes and their movement due to bloodflow the blood behaves like a moving source of sound. Therefore the soundwaves reflected by the erythrocytes will undergo the Dopplereffect and there will be a change of frequency in comparsim with the emmited frequency by the transducer. If we use the human - car analogy the transducer would be the person standing at the road and listening while the erythrocytes are the car emminting a sound. Therefore the transducer has to be again, like in sonography, the producer of sound and at the same time the