Duplex ultrasound involves using high frequency sound waves to look at the speed of blood flow, and structure of the leg veins. The term "duplex" refers to the fact that two modes of ultrasound are used, Doppler and B-mode. The B-mode transducer (like a microphone) obtains an image of the vessel being studied. The Doppler probe within the transducer evaluates the velocity and direction of blood flow in the vessel.
For example, a carotid duplex scan may be performed to assess occlusion (blockage) or stenosis (narrowing) of the carotid arteries of the neck and/or the branches of the carotid artery. This type of Doppler examination provides a 2-dimensional (2-D) image of the arteries so that the structure of the arteries and location of an occlusion can be determined, as well as the degree of blood flow.