The magnitude of this shift is also dependent on the angle at which the transducer sees the shift. This varies according to the cosign of the angle shown in the illustration. In the case of Doppler ultrasound, the transducer sends out pulses of sound that are reflected off red blood cells in the flowing blood. These act as the source of the sound and the Doppler shift from the red blood cells tells us the velocity of those cells. Therefore, Doppler ultrasound is able to tell us red cell velocity.
V is the velocity of the blood cells.
F sub r is the frequency of the reflected sound. F sub o is the frequency of the sound output of the transducer. Note that these frequencies are effected as the cosign of the Doppler angle theta. Note also that as the transducer is held at a right angle to the flow of blood, the cosign of theta becomes 90 degrees. That cosign equals 0.