Doppler ultrasound is used to assess the quality of blood flow through the vessels in the patient’s leg and foot. The process uses the Doppler effect (the shifting sound you hear as an ambulance passes by) by sending sound waves through the skin which are reflected depending on the volume and velocity of blood flow. This provides a lot of vital info to the podiatrist, and Doppler ultrasound can very accurately pinpoint blood flow problems, which can then be treated. The process is completely pain-free and non-invasive, using a stick-like transducer and ultrasound gel (just like a pregnancy ultrasound) to glide over the skin.

http://www.centrus.com.br/DiplomaFMF/SeriesFMF/doppler/capitulos-html/chapter_01.htm

http://www.webmd.com/a-to-z-guides/doppler-ultrasound