Spectral display

A real-time spectral Doppler display is shown in Figure 9.1 displaying both arterial and venous flow. This displays time along the horizontal axis and the Doppler frequency shift or calculated velocity along the vertical axis. The brightness (or colour) of the display relates to the amplitude of each of the Doppler frequency components present, i.e. the relative proportion of the blood travelling with a particular velocity. The baseline indicated in the centre of the spectral display in Figure 9.1 corresponds to zero Doppler shift or zero velocity. The spectrum contains information about the speed and direction of the blood flow as well as the degree of pulsatility of the flow. Conventionally, it is arranged so that positive Doppler frequency shifts (blood flowing towards the transducer) are plotted above the baseline and negative Doppler shifts (blood flowing away from the probe) are plotted below the baseline, but the operator can invert this display as required. Both arterial and venous flow may be present within the path of the beam and when, as is usually the case, the flow in these vessels is in opposite directions, the Doppler waveforms from the arterial and venous flow appear on opposite sides of the baseline as seen in Figure 9.1. The Doppler ultrasound signal from a blood vessel will contain a range of Doppler frequency shifts.