ZEPHYR / SCIENCE PHOTO LIBRARY ZEPHYR / SCIENCE PHOTO LIBRARY
Healthy kidneys. Coloured ultrasound scans of healthy kidneys (between white crosses) in a 34-year-old person. The kidneys filter waste products from the blood and excrete them as urine. Here, coloured doppler imaging (lower images) has been used to highlight the blood supply (boxed regions) to the kidneys. Doppler ultrasound scanning detects moving fluids 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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