The Doppler effect enables ultrasound to be used to detect the motion of blood and tissue. Most Doppler ultrasound systems provide both spectral Doppler displays and colour Doppler images. Many of the features of these two modalities are common, and are described in this chapter. Specific details of spectral Doppler and colour Doppler systems are given in the chapters following this.

The descriptions that follow in this chapter refer to the detection and display of blood flow, as this is the most common application of Doppler techniques. Chapter 8 describes the use of Doppler in detection of tissue motion.

Doppler ultrasound systems

The Doppler effect

The Doppler effect is observed regularly in our daily lives. For example, it can be heard as the changing pitch of an ambulance siren as it passes by. The Doppler effect is the change in the observed frequency of the sound wave (fr) compared to the emitted frequency (ft) which occurs due to the relative motion between the observer and the source, as shown in Figure 7.1. In Figure 7.1a, both the source and the observer are stationary so the observed sound has the same frequency as the emitted sound. In Figure 7.1b, the source is moving towards the observer as it transmits the sound wave.