Doppler Ultrasound is really just normal Ultrasound signals that have been processed in a different way.
Instead of detecting the amplitude of a returning echo, in Doppler ultrasound, the change in frequency of returning echoes is used to determine a bodies movement.
To explain, consider an ambulance passing by. As the vehicle races towards and then away from you the siren's pitch changes. First the pitch becomes higher and then lower.
Originally discovered by the Austrian mathematician and physicist, Christian Doppler (1803-53), this change in pitch results from a shift in the frequency of the sound waves, as illustrated in the following picture.
As the ambulance approaches, the sound waves from its siren are compressed towards the observer. The intervals between waves diminish, which translates into an increase in frequency or pitch. As the ambulance recedes, the sound waves are stretched relative to the observer, causing the siren's pitch to decrease. By the change in pitch of the siren, you can determine if the ambulance is coming nearer or speeding away. If you could measure the rate of change of pitch, you could also estimate the ambulance's speed.
So, the Doppler effect is a change in the frequency of a wave, resulting from motion of the wave source or receiver, or in the case of a reflected wave, motion of the reflector.
In medicine, Doppler US is used to detect and measure blood flow, and the major reflector is the red blood cell. The Doppler shift is dependent on the insonating frequency, the velocity of moving blood, and the angle between the sound beam and direction of moving blood, as expressed in the Doppler equation.
where Df is the Doppler shift frequency (the difference between transmitted and received frequencies), f is the transmitted frequency, v is the blood velocity, c is the speed of sound, and q is the angle between the sound beam and the direction of moving blood. The equation can be rearranged to solve for blood velocity, and this is the value calculated by the Doppler US machine:
The Ultrasound scanner has no idea whether the blood is arterial or venous but displays movement away from the transducer in one colour and movement away in another.