JOHN MCLEAN / SCIENCE PHOTO LIBRARY JOHN MCLEAN / SCIENCE PHOTO LIBRARY
Doppler ultrasound scan. Patient having a doppler ultrasound scan taken of her iliac artery during an investigation into blood flow. Ultrasound uses sound waves to build up an internal image of the body. A transducer held on the woman's abdomen emits high-frequency sound waves and picks up their reflections off body structures. This allows a 2-D or 3-D image of the structures to be created. Doppler ultrasound measures blood flow using the doppler effect. Sound waves reflected by moving objects have their frequencies shifted by an amount related to the speed of movement. The bright patches on the screen show the fastest blood flow.
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