Doppler effect

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 with the medium's speed of sound. 

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 sets big requirements for dynamics when designing instruments. The Doppler frequency is normally chosen at the lower range 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 an image of the function of speed and time is displayed on the screen of the ultrasound machine, see an example in figure 7. 

Figure 7: The Doppler spectrum recorded from the aortic valve through the heart apex. Positive speed is normal, negative is a leakage.