Adjusting the spectral scale. (a) Color duplex US image demonstrates that when the spectral scale (or the sampling rate) is set too high (in this example, PRF = 14,286 Hz), flow is more difficult to appreciate and characterize on the scale. (b) On a color duplex US image obtained after the scale was reduced (PRF = 3,731 Hz), the range of depicted velocities is reduced and the appearance of the spectral waveform is improved, providing more visible quantitative and qualitative data. Note that the color Doppler flow US image, color bar, and color scale all remain unchanged because the spectral component is active.

http://radiographics.rsna.org/content/24/3/657.figures-only