Bi-directional flow is measured in positive and negative directions.

See also Bi-directional Illumination, and Color Doppler Imaging.

For the flowprobe a vessel is positioned between transducers which generate wide beams of ultrasound to fully illuminate the vessel. The ultrasound beams alternately intersect the flowing blood in upstream and downstream directions. The flowmeter derives an accurate measure of the changes in transit time influenced by the motion of the blood.

See also Bi-directional Flow.

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.

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.

(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.