Objects moving away from the transducer will generate reflections with lower frequency. Objects moving towards the transducer will compress the sound waves and reflect them at a higher frequency. Figure 3.ĭoppler effect occurs when reflectors (structures reflecting sound waves) move towards or away from the transducer. Erythrocytes flowing away from the transducer will reflect sound waves with reduced frequency (Figure 3). Erythrocytes flowing towards the transducer will reflect the sound waves with higher frequency. Reflections from an erythrocyte.įlowing erythrocytes will alter the frequency of reflected sound waves. Although only a fraction of the sound waves are reflected back to the transducer, the billions of erythrocytes in the blood will collectively generate enough reflections to be detected and analyzed by the ultrasound machine. Because erythrocytes are small, round and have an irregular surface, the reflected sound waves are scattered in all directions (Figure 2). The Doppler principle, however, remains unchanged: when the sound source and reflectors move towards each other, sound waves are compressed and vice versa.Įrythrocytes reflect ultrasound waves. The moving objects are instead the blood cells (primarily erythrocytes) and tissues (primarily myocardium). The sound source in echocardiography (i.e the transducer) is stationary. The same principles can be applied to blood flow and tissue motions. When the sound source moves away from the observer, the sound waves are stretched out, which results in increased wavelength and decreased frequency. When the sound source moves towards the observer, the sound waves are compressed, which leads to a shortening of the wavelength and thus increased frequency. The Doppler principle is primarily used to study blood flow and myocardial motion. Figure 1 presents three trumpets one placed on a table, and two are mounted on ambulances driving towards and away from the observer. It can be illustrated by studying how the frequency of reflected sound waves are modified by the direction of movement of the sound source. The Doppler effect was first described in 1843 by the Austrian astronomer Christian Doppler. The change in frequency is referred to as the Doppler effect. If the reflector is in motion, however, then the frequency of the reflected sound waves will differ from the emitted sound waves. If the reflector (i.e the object reflecting the sound waves) is stationary, then the reflected sound waves will have the same frequency as the sound waves emitted by the sound source. When sound waves hit objects some of the sound waves are reflected back to the sound source.
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