A bolus is a rapid infusion of high dose contrast agent. Dynamic and accumulation phase imaging can be performed after bolus injection. Since the transit time of the bolus is only a short time, images with high frame rate show the wash in and wash out of the contrast material. The injection rate and the total injected volume modifies the bolus peak profile. Substantial changes in the concentrations during signal acquisition induce artifacts. Furthermore, the hemodynamic parameters (cardiac output, blood pressure) influence the bolus profile. However, the characteristics of ultrasound contrast agents are favorable with a continuous perfusion.

See also Negative Bolus.

Cavitation is any activity of highly compressible transient or stable microbubbles of gas and/or vapour, generated by ultrasonic power in the propagation medium. Cavitation can be described as inertial or non-inertial. Inertial cavitation has the most potential to damage tissue and occurs when a gas-filled cavity grows, during pressure rarefaction of the ultrasound pulse, and contracts, during the compression phase. Collapses of bubbles can generate local high temperatures and pressures. Transient cavitation can cause tissue damage.
The threshold for cavitation is high and does not occur at current levels of diagnostic ultrasound. The introduction of contrast agents leads to the formation of microbubbles that potentially provide gas nuclei for cavitation. The use of contrast agents can lower the threshold at which cavitation occurs.

Types of cavitation:

Composite arrays are combinations of piezoelectric ceramics and polymers that form a new material with different properties. Piezocomposites improve the performance of usual arrays such as the mechanically scanned annular array and the linear phased array.
Piezocomposites reduce the acoustic impedance with a better impedance match with tissue. The result is a reduction of the reverberation level in the near field. Unwanted surface waves propagating laterally over the transducer are suppressed. The composite materials allow to vary the electromechanical coupling constant, and to give better control over the trade-off between sensitivity and bandwidth.

See also Narrow Bandwidth, Dead Zone, Ultrasound Phantom.

(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.

(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.