The Difference Between Colour Doppler Velocity Imaging and Power Doppler Imaging

The Doppler effect, which occurs on the reflection of ultrasound from moving blood or tissue, is observed as a shift in frequency of the reflected ultrasound from that of the incident ultrasound. This Doppler shift has been widely used in clinical practice as a means of measuring blood velocity. Two types of Doppler Imaging are often discussed as facilities on ultrasonic scanners, namely Colour Doppler Velocity Imaging and Power Doppler Imaging. In this article the difference between the two will be described and it will be noted that, although both are used in vascular disease, Power Doppler Imaging is not extensively used at present in cardiology. A number of other names are used for these Doppler techniques, for example ’Normal Doppler’ for Colour Doppler Velocity Imaging and ’Energy Doppler’ for Power Doppler. The echocardiographer is obliged to ascertain what exactly each name stands for. Simple Doppler devices that provide velocity information when their beam is directed at a blood vessel have been available for many years. Indeed images can be produced by these devices if a narrow ultrasound beam is slowly moved across a blood vessel and the blood velocity at each position displayed on a screen. However real-time Doppler imaging only became available when a breakthrough was made in fast signal processing. By ’real-time imaging’ we mean the production of several images per second of the scanned region. The fast signal processing rapidly produces values of the mean blood velocity at neighbouring points along the ultrasound beam and allows the beam to be swept quickly to generate a cross-sectional image of regions of blood flow. Static or slow moving tissue does not produce significant Doppler shifts in the reflected ultrasound and is therefore not presented in a Doppler image. Of course a grey shade B-mode image of these tissues is usually combined with the Doppler image. With early instruments, 15 Doppler images per second were typical but now the rate can be in excess of 50 per second.