A contrast medium (or contrast agent) is a chemical substance applied to the anatomical or functional region being imaged, to increase the differences between different tissues or between normal and abnormal tissue.
The chemical composition of the contrast media determines the specific usage. The intention in the development of ultrasound contrast media is a high contrast between blood and tissues in different organs or even tumors.

See also ultrasound contrast agents, the info sheet gives an overview and more in-depth information about different types of ultrasound contrast medium.

(CAP) Contrast agent particles are filled with gas and coated by a shell. The reflectivity of a particle increases with the diameter. The used particles are too large to cross the endothelium, so that there is no interstitial phase of enhancement. They are essentially markers for the blood pool and their distribution is similar to those of tagged red cells. In addition, any body cavity that can be accessed can be injected with vascular contrast.

(CCI) A major limitation of the use of ultrasound contrast agents is the problem that signals from the microbubbles are mixed with those from tissue, so that the distribution of the microbubbles is not optimally displayed either in Doppler or gray scale.
Coherent contrast imaging is a high frame rate implementation of inverting the phase of alternate sound pulses and summing the resulting echoes. The symmetrical signals from linear reflectors are cancelled leaving those from non-linear scatterers, with the advantage that the cancellation is performed without the need to transmit two pulses per image line so that bubble destruction is minimized. Coherent contrast imaging yields best results in the vascular phase of phospholipid microbubbles (such as Definity and SonoVue).

See also Coherence.

(CHI) Contrast harmonic imaging is an ultrasound technique to improve the measurement of blood perfusion or capillary blood flow. Based on the nonlinear properties of contrast agents, CHI transmits at the fundamental frequency but receives at the second harmonic. Contrast enhanced echo signals contain significant energy components at higher harmonics (bubbles acts as harmonic oscillators), while tissue echoes do not. Caused by that contrast signal can be separated from tissue echoes by the characteristic signal.
In combination with the pulse inversion technique, CHI promises very high contrast agent sensitivity with high spatial resolution.

See also Ultrasound Contrast Agent Safety and Hemoglobin.

(CPS) Contrast pulse sequencing is a technique to exploit contrast agent properties with series of three pulses that differ in phase and amplitude. CPS allows bubble specific imaging with non-linear fundamental and higher order harmonics, low MI, and extremely high microbubble-to-tissue background ratio.

See also Ultrasound Contrast Agent Safety.