ATHENAIS, ISM / SCIENCE PHOTO LIBRARY ATHENAIS, ISM / SCIENCE PHOTO LIBRARY
Venous blood flow. Colour Doppler ultrasound scan of blood flow (horizontal colour) through a normal popliteal (knee) vein. The venous system drains blood from the body back towards the heart. Doppler ultrasound scanning detects moving fluids, such as blood, by the frequency change they cause when reflecting high-frequency sound waves. Colour Doppler scans colour-code the moving fluid to show the direction and speed of the flow.
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