Doppler studies are used to identify occlusion of the veins. Venous
patency is demonstrated through Doppler ultrasound by detecting
moving red blood cells (RBCs) within the vein. A conductive gel is
applied to the skin on the areas being investigated, typically the
ankle, calf, thigh, and groin. The Doppler transducer is placed on
the conductive gel and directs an ultrasound beam at the vein. Moving
RBCs reflect the beam back to the transducer, which then transforms
the flow velocity into a "swishing" noise augmented by an audio
speaker. If the vein is occluded, no swishing sounds are detected.
