An improved wall filter for flow imaging of low velocity flow

In medical ultrasound, flow velocity estimation for blood flow imaging (color flow) relies on first filtering out signals due to stationary scatterers so that a simple autocorrelation frequency estimator can be used to determine flow. Usually, this “wall filter” is implemented as either a conventional FIR or IIR filter. Unfortunately, in the case of very low velocity flow, these wall filters may remove signals from the moving scatterers of interest. In this work we present a novel filtering scheme that involves first shifting (via a complex multiplication) the undesired signals to zero frequency and then removing these signals by simply subtracting the average of these signals from each of them. The result is a high-pass filter which is very narrow, yet does not reduce the number of samples available to the autocorrelation velocity estimator