Dependence of power Doppler image on a high pass filter instrumented in ultrasound machine.

The aim of this study was to examine the influence of flow velocity, beam incident angle and a wall motion filter on the intensity of power Doppler image of steady flow in vitro. Power Doppler images of flow were recorded using a 7.5-MHz linear transducer with the fixed repetition frequency, gain and frame rate settings at the velocity levels of 15, 20, 35 and 65 cm/s with the beam incident angles of 30 degrees and 60 degrees. The power (P) of received Doppler signal was digitized to 25 gray values (G) for imaging using a wall motion filter with the transfer function: G = a x log P + b (a, b = constant values). Image intensity was measured at the center of the flow image off-line and compared with the flow velocity measured by conventional pulsed wave Doppler. Below the Nyquist limit, G increased as velocity increased, and G with the incident angle 30 degrees was lower than those with 60 degrees. Overall relationship between the Doppler shift frequency and the image intensity exhibited an excellent correlation when fit to the theoretical curve derived from the filter property (r = 0.97). The signal intensity of power Doppler image of flow depends upon the mean Doppler frequency shift or the flow velocity and was clearly affected by the wall motion filter.