Arterial blood flow. Coloured Doppler ultrasound scan of healthy blood flow through the junction of the common carotid artery. The carotid arteries carry blood from the heart to the brain and head. Ultrasound scanning uses high-frequency sound waves to build up a picture of body structures. Sound is emitted by a transducer, which also receives the echoes from internal structures. If the structures are moving (as is the blood here), then the frequency of the sound is changed slightly (the Doppler effect). This allows measurement of the speed of blood flow. Doppler ultrasound can be used to diagnose disorders of blood flow, such as atherosclerosis.