(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.

Albunex and Infoson, used mainly in cardiac evaluations, are first generation one-pass-only contrast agents and have been replaced by the new-generation contrast media. Albunex and Infoson are the same sonicated human serum albumin microbubbles. Infoson is licensed and manufactured in Europe, while Albunex was produced in the USA.
Albunex, an air-filled microbubble with a denatured albumin shell (modified from air-filled albumin microspheres prepared from sonicated 5% human serum albumin), was the first FDA-approved contrast agent, but is no longer in production.
Cardiac shunts and valve regurgitations are often evaluated with Color Doppler Imaging (CDI), which also improved with injections of Albunex, but this agent is pressure-sensitive and does not recirculate. It is effectively a one-pass-only agent, limiting its clinical efficacy.

See also First generation USCA, Echocardiography and Contrast Enhanced Ultrasound.

The Doppler signal from any range gate is proportional to the number of red blood cells moving through during the periods of its read out, so the amplitude (or loudness) works as an indicator of the number of blood cell reflectors.
High Doppler amplitudes are generated by large pulsed Doppler or color Doppler imaging range gates and large vessels.

See also Amplitude Map, Color Amplitude Imaging.

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

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

The persistence of microbubbles is depended of the shell stability and the density of the gas.
This is defined by the equation:
(R x d)/(DIFS x const_sat)
where R is the bubble radius, d the gas density, DIFS the gas diffusivity and const_sat the saturation constant.
Microbubbles are stabilized with thin coatings of substances such as palmitic acid or by encapsulation in microspheres made with albumin, lipids, or polymers. Low-solubility low-diffusibility gases dramatically improve the persistence. Most recently developed ultrasound contrast agents combine these two approaches to prolong contrast enhancement.
Persistence is also a type of temporal smoothing used in both gray scale and color Doppler imaging. Successive frames are averaged as they are displayed to reduce the variations in the image between frames, hence lowering the temporal resolution of the image.