 Also , unlike the arterial ultrasound examination , blood velocity in veins has no diagnostic meaning . Veins are a draining system similar to a low pressure hydraulic system , with a laminar flow and a low velocity . This low velocity is responsible for the fact that we can only detect it spontaneously with the Doppler effect on the proximal and larger femoral and iliac veins . Here the flow is either modulated by the respiratory rhythm or is continuous in cases where the flow is high . The thinner veins do not have a spontaneous flow . However , in some circumstances the blood flow is so slow that we can see some <unk> material moving within the vein , in " spontaneous contrast " . This material can easily be mistaken for a thrombus , but can also easily be discounted by testing the vein 's compressibility . 
