Blood volume per time measured in: cm³/s.
The sonographic detection of blood flow in vascular ultrasound is limited by factors such as tissue motion (clutter), attenuation properties of the intervening tissue, and slow or low-volume flow.

Different flow types in human body:

See also Antegrade, Bi-directional Flow, Velocity, Poiseulles Law, and Venous Ultrasound.

Doppler Shift is the change in the perceived frequency relative to the transmitted frequency. 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.
Doppler equation:
Doppler shift frequency: fD = fr - f0 = 2f0v/c
Where fD is the Doppler shift frequency = the difference between transmitted and received frequencies.
Ultrasound system use the following equation:
Doppler shift frequency with incident angle: fD = 2f0v/c cosØ
Where f is the transmitted frequency, v is the blood velocity, c is the speed of sound in tissue, cosØ is the Cosine of the blood flow to beam angle.
The Doppler angle (theta) is the angle of incidence of the beam upon the object. If the beam is parallel to the flowing blood, the angle theta is zero, and the determination of flow is most accurate. If the angle of incidence is greater, the results are less reliable. Doppler shift results with an angle greater than 20° should not be used for the calculation.

See also Doppler Interrogation Frequency, Zero Crossing Detector, Doppler Effect, Doppler Ultrasound and Motion Discrimination Detector.

Doppler techniques are dependent on the transducers used. The transducer operating in continuous wave mode utilizes one half of the elements and is continuously sending sound energy while the other half is continuously receiving the reflected signals. If the transducer is being used in a pulsed wave mode, the whole transducer is used to send and then receive the returning signals.
Pulsed wave techniques allow the accurate measurement of blood flow at a specific area in the heart and the detection of both velocity and direction. Measurement is performed by timing the reception of the returning signals giving a view of flows at specific depths. The region where flow velocities are measured is called the sample volume. Errors in the accuracy of the information arise if the velocities exceed a certain speed. The highest velocity accurately measured is called the Nyquist limit.

See also Doppler Velocity Signal and Doppler Effect.

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:

Sound waves must have a medium to pass through. The velocity or propagation speed is the speed at which sound waves travel through a particular medium measured in meters per second (m/s) or millimeters per microsecond (mm/μs). Because the velocity of ultrasound waves is constant, the time taken for the wave to return to the probe can be used to determine the depth of the object causing the reflection.
The velocity is equal to the frequency x wavelength.
V = f x l
The velocity of ultrasound will differ with different media. In general, the propagation speed of sound through gases is low, liquids higher and solids highest. The speed of sound depends strongly on temperature as well as the medium through which sound waves are propagating. At 0 °C (32 °F) the speed of sound in air is about 331 m/s (1,086 ft/s; 1,192 km/h; 740 mph; 643 kn), at 20 °C (68 °F) about 343 metres per second (1,125 ft/s; 1,235 km/h; 767 mph; 667 kn)

Velocity (m/s)

Doppler ultrasound visualizes blood flow-velocity information. The peak systolic velocity and the end diastolic velocity are major Doppler parameters, which are determined from the spectrum obtained at the point of maximal vessel narrowing. Peak systolic velocity ratios are calculated by dividing the peak-systolic velocity measured at the site of flow disturbance by that measured proximal of the narrowing (stenosis, graft, etc.).

See Acceleration Index, Acceleration Time, Modal Velocity, Run-time Artifact and Maximum Velocity.