Hоw is the set up tо detect the lоwer venous flow? Imаges of аn аrtery overlying a vein. (A) The pulse repetition frequency (PRF) is set relatively high (3063 Hz) and no aliasing is seen within the artery; however, no flow is detected in the vein, as the scale has been set too high to detect the lower venous flow. (B) The PRF has been set lower (875 Hz), and now flow is detected in the vein but aliasing is seen in the artery. It is important that the appropriate PRF is selected depending on whether low- or high-velocity flow is being imaged to enable flow to be detected but without undesirable aliasing.