Doppler ultrasound is a medical imaging technique for calculating the relative velocity between two points by measuring the frequency shift of a sound wave transmitted from one point to the other, based on the Doppler effect. Continuous or pulsed Doppler is frequently used to examine cardiovascular blood flow. The combination of routine 2D-mode and Doppler ultrasound allows a complete evaluation of the heart's anatomy and function (including the fetal heart). See also Doppler Fluximetry in Pregnancy.
Doppler ultrasound depends on the fact that if a moving object reflects the ultrasound waves, the echo frequencies are changed. A higher frequency is created if the object is moving toward the probe//transducer and a lower frequency if it is moving away from it. How much the frequency is changed depends upon how fast the object is moving. Doppler ultrasound shows the different rates of blood flow in different colors on a monitor in real time.
The major Doppler parameters are the peak systolic velocity and the end-diastolic velocity. The peak systolic velocity ratio compensates the variability between different patients and instrumentations.

Different Doppler and duplex techniques:

Pressure is the force per unit area applied on a surface in a direction perpendicular to that surface.
Pressure can also be described as a form of potential energy in a fluid. The maximum pressure of the fluid medium obtained during propagation of an ultrasonic pulse.
The negative peak pressure is the peak rarefaction pressure attained during the negative portion of a propagating ultrasound pulse in a medium such as tissue.
Sound pressure can be measured using a microphone in air and a hydrophone in water. The SI unit for sound pressure is the Pascal.
Blood pressure is the pressure exerted by the blood on the walls of the blood vessels.

See also Rarefactional Pressure, Low Intensity Pulsed Ultrasound, and Projector.

(RI) The resistive index is a measure of resistance to arterial flow and is calculated from the power Doppler arterial waveform.
The equation of this index is:
RI = (peak systolic velocity - end diastolic velocity) : peak systolic velocity

(AT) The acceleration time is the duration of upstroke from end-diastole to peak systole.

See Acceleration Index.

The velocity of flowing blood is usually measured in cm/s. It is always zero at the vessel wall and the velocity profile across a vessel can have various shapes depending upon the type of flow being observed (see also pulsatile flow). Laminar flow giving rise to a laminar velocity profile, plug flow giving rise to a flat velocity profile and disturbed flow can be distinguished.
Normal peak systolic velocities in the femoral and popliteal arteries vary from 90 to 110 cm/s in the femoral artery and from 40 to 70 cm/s in the popliteal artery.

See also Bi-directional Flow, Pulsatility Index, and Acceleration Index.