An ultrasound imaging system transmits a train of pulses along each scan line. The pulses are reflected at various depths and interfaces (objects). In color Doppler mode, the reflected signals from the corresponding scan line and depth correlated frame by frame. A loss of correlation is interpreted as object displacement and indicated by a color overlay. Even if an object does not move but disappears (a bubble bursts) from one image frame to another, the local loss of correlation is marked by a color. The invention is to “trick” the ultrasound imaging system by generating a localized ultrasound signal that would be interpreted as displacement and marked by a color overlay.