Optimizing gate size and position. Color duplex US image obtained with a wide gate placed in a suboptimal location shows sampling of flow in both the portal (above the baseline) and hepatic (below the baseline) veins. Too large a gate size may result in sampling from too large an anatomic region. By reducing the gate size and improving the position for sampling, a normal spectral waveform is obtained.


http://radiographics.rsna.org/content/24/3/657.figures-only