BSIP, MENDIL / SCIENCE PHOTO LIBRARY BSIP, MENDIL / SCIENCE PHOTO LIBRARY
Doppler ultrasound scan. Doctor using Doppler ultrasound scanning to examine the carotid artery in a patient's neck. The transducer, which is held against the patient's neck, emits high-frequency sound waves (ultrasound) that echo off internal structures. The echoes are used to construct an image (seen on the screen at upper right). If the waves hit a moving object, such as flowing blood, the frequency of the echoes will be altered. The alteration of frequency due to movement is called the Doppler effect. Doppler ultrasound is used to measure the speed of blood flow and to detect defects in the blood system.
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