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.

Spectral analysis is the quantitative analysis method to display the distribution of frequencies. A difficult Doppler signal is separated into the frequency components so that the range of frequencies in a Doppler shifted signal can be analyzed. This allows measurement of blood flow velocity by positioning of a probing cursor in the artery (on the screen), and the signal representing blood flow velocity is generated. The peaks and ebbs create the spectrum, corresponding to systolic and diastolic blood flow. The signal is both visual and auditory.

(CDI) Color Doppler imaging depicts the mean frequency shifts of the Doppler signal. Color [colour, Brit.] Doppler imaging is a method for visualizing direction and velocity of movement, such as of blood flow within the cardiac chambers or blood vessels. The flow direction and velocity information gathered by Doppler ultrasonography is color coded onto a gray scale cross-sectional image. The sensitivity of Doppler ultrasound is increased in conjunction with the use of vascular contrast agents.
Direction and blood flow velocity are coded as colors and shades:
Red - flow coming nearer to the probe.
Blue - flow coming away of the probe.

See also Bi-directional Illumination, Color Map.

A flat velocity profile in large vessels is termed plug flow, indicating that all or most of the blood is flowing at a single velocity per time unit.

See also Blood Flow Velocity.

Vascular ultrasound obtains images and measures blood flow velocity in the carotids, abdominal aorta, and vessels of kidneys, arms, or legs. Blockages in arteries, blood clots in veins, or abdominal aortic aneurysm can be detected.
These abnormalities in blood flow are usually examined with different Doppler techniques. In addition, the speed and direction of blood flow can be color coded in a color map. Duplex techniques show both, the vessels and the surrounding tissue. The use of ultrasound contrast agents improves the left ventricular opacification in cardiac ultrasound examination. Usually, for a vascular ultrasound no special preparation is needed.

See also Echocardiography, Venous Ultrasound, Adventitia, Intima, Temporal Mean Velocity, and Intravascular Ultrasound.