When the object being imaged is moving, the received frequency will be different from the transmitted. This is the Doppler effect, the same effect one can hear when a train whistles. The tone is high (high frequency) when the train gets closer and low when the train moves away. The relationship between frequency and speed are shown in a fact box. The Doppler shift increases with transmitted frequency and the object's speed, and is inversely proportional to the medium's speed of sound. 
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The red blood cells are the most important spreaders of the Doppler shift. They give a quite weak signal compared to the echo of surrounding stationary tissue. This results in big requirements for dynamics in the design of the instruments. The Doppler frequency is normally chosen at the lower end of a probe's frequency bandwidth, for example, a 7.5 MHz probe uses 5-6 MHz as the Doppler frequency. With normal speed of blood flow, the difference between transmitted and received frequency appear in the audible range. It is therefore normal to  play this on speakers while the velocity spectrum as a function of time is displayed on the screen of the ultrasound machine, see an example in figure 7. 
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<img src="/elaring/fag/ultralyd/teknologi/figur7.jpg" height="250"> 
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Figure 7: The Doppler spectrum recorded from the aortic valve through the heart apex. Positive speed is normal, negative represents a leakage.