4.1 The Doppler Effect

Doppler ultrasound is based on application of the Doppler effect, also called the Doppler shift. This physical phenomenon is related to change in wave frequency that is observed when the wave is moving or when it is reflected off a moving object. The name of the effect comes from the Austrian mathematician and physicist Christian Andreas Doppler.

The most common example of Doppler shift in change in perceived sound of an ambulance siren which is different when the vehicle is approaching and going away (Fig. 4.1). In comparison with the original emitted frequency, the received one is higher when the object is moving towards the source of waves, equal at the moment of passage, while lower when the object is moving away. This is due to change in emission of subsequent waves crests which are first closer to each other, then more and more distant from each other. Consequently, the time required for the wave crest to reach the observer on approach is shorter which results in an increase of frequency. On the contrary, during recession the time is longer thus the frequency is decreased.

The Doppler shift is observed when the source of waves is moving and observer motionless, when it is the observer who is moving and the source of waves constant, finally the medium serving for propagation of sound waves can be mobile. However, light waves and gravitational waves are exceptions as they do not require any medium for propagation.

In medicine, the ultrasound wave is reflected off the moving objects which are erythrocytes in blood vessels. Instead of change of frequency of wave coming and going, it is the difference in time of returning echoes of subsequent waves (decreased or increased) which is registered as the frequency shift. The bigger the frequency of the transmitted wave, the bigger the frequency shift is. Angulation of ultrasound beam is essential for Doppler measurements. If the angle of incidence equaled 0 and ultrasound wave was parallel to the examined blood vessel, the frequency shift would be the greatest. However, in clinical practice it is not possible to obtain such an angle of incidence therefore angulations between 0 and 60° are applied, and those closer to 60° are preferred as with angulation of 0° no Doppler signal is picked up [1].

4.2 Applications of the Doppler Shift in Diagnostic Imaging

The following types of application of the Doppler shift in diagnostic imaging are distinguished:

• Spectral Doppler, including Continuous wave Doppler and Pulsed wave Doppler (Fig. 4.2).

• Color Doppler, also called Color flow Doppler (Fig. 4.3).

• Power Doppler (Fig. 4.4).