The Doppler ultrasonography - A type of ultrasound scanning in which shifts in the frequency of ultrasonic waves are used to measure the velocity of moving structures. An emitter sends out pulses of ultrasound (inaudible, high-frequency sound) of a specific frequency.
When the pulses bounce off a moving object (for example, blood flowing through a blood vessel), the frequency of the echoes is changed from that of the emitted sound. A sensor is able to detect the frequency changes and converts the data into useful information (about how fast the blood flows, for example).
Doppler ultrasonography is widely used to detect narrowing of arteries in the neck due to atherosclerosis (accumulation of fatty deposits on the artery walls) and to detect blood clots in veins (as occurs in deep vein thrombosis).
In addition, Doppler ultrasound techniques are used to monitor the fetal heartbeat, to detect air bubbles during dialysis and in heart–lung machines, and to measure blood pressure. (See also Doppler echocardiography.)
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