Changing the color Doppler wall filter. (a) Color Doppler flow US image obtained with the highest possible wall filter setting shows how color signal arising from low-velocity flow may be filtered out. (b) Color Doppler flow US image obtained with a low filter setting demonstrates filling in of flow in the hepatic veins (blue), which indicates minimal filtering of color signal. The change in the filter setting appears as a change in the width of the horizontal black line in the center of the color bar.