The effect of echo suppression on the mean velocity estimation range of the RF cross-correlation model estimator

Ultrasound systems are widely used to visualize in real-time internal structures and blood flow velocity distributions. The latter are estimated from the received ultrasound radio frequency (RF) signals using a mean frequency estimator, capable of estimating the temporal and spatial mean frequency within a given depth and time window (estimation window). Since ultrasound RF-signals are composed of three major signal components (reflections, scattering and noise) it is necessary to suppress the power of the reflections or reverberations (stationary echo removal) to estimate the temporal mean frequency of the signal component induced by scattering originating from red blood cells. However, the stationary echo removal filter in front of the mean frequency estimator will restrict the temporal mean frequency estimation range because the power of the signal component induced by the slowly moving blood is suppressed as well. This article describes a stationary echo removal filter, acting in the RF-domain and adaptive to the temporal mean frequency of the signal component induced by stationary or slowly moving reflections and reverberations. Furthermore, a comparison in performance of this adaptive stationary echo removal filter and several static stationary echo removal filters is presented. For direct comparison of these filter algorithms as far as the effect on the temporal mean frequency estimation range is concerned, the same signal conditions in combination with the same RF-domain mean frequency estimator were used in this comparison. It could be demonstrated that the adaptive echo removal filter, in combination with the RF-domain mean frequency estimator used, exhibits the best performance.