Influence of the wall motion filter on the measurement of the Doppler frequency

Wall motion filters used in multigate blood flowmeters may induce a bias in the estimation of the Doppler frequency. In the presence of noisy signals, these filters induce a correlated noise term in algorithms based on the correlation technique. Depending on the filter characteristics, noise induces a bias in the estimation, and an effect similar to the aliasing can appear. Simple FIR (finite impulse response) filters give wrong information at a signal-to-noise ratio lower than 20 dB, especially at low Doppler frequencies. It is shown that the level of the bias is much reduced by using an IIR (infinite impulse response) filter. With a second-order IIR filter the induced bias reduces almost to zero