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When high amplitude ultrasound is used, it is possible to detect weak echoes from much smaller structures, including those from red blood cells.
When the blood is moving relative to the direction of the ultrasound beam, the frequency of the returning waves will be changed according to the Doppler principle (this states that the shift in frequency of the perceived waveform relative to the frequency of the transmitted waveform is proportional to the velocity of the source (i.e. the reflecting surfaces of moving blood cells) relative to the observer).
In doppler studies the ultrasound should be aligned as accurately a possible in parallel with the direction of flow (rather than at right angles to it).