(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.

Christian Johann Doppler first described the effect of motion of sound sources and the frequency change of the sound to the observer.
Doppler ultrasound uses this effect to detect and measure blood flow, and the major reflector is the red blood cell. Doppler ultrasound depends on the fact that if the reflecting surface is moving in relation to the transducer (blood flowing in a vessel) the frequency of the received ultrasound wave will be different from that of the transmitted wave. If blood cells are moving towards the transducer, they increase the frequency of the returning signal. As cells move away from the transducer, the frequency of the returning signal decreases.

See also Quadrature Detection and Doppler Techniques.

Echo ranging is the ultrasound relationship between transit time and reflector depth expressed as:
t = 2d//c

The perfect image quality is dependent on some assumptions of the propagation of ultrasound waves in tissues after generating in an imaging system. These assumptions are important for the developing of optimal ultrasound imaging systems.
A lot of steps can be taken to prevent artifacts and to improve image quality, for example beamforming is used to focus the ultrasound beam, and contrast agents decrease the reflectivity of the undesired interfaces or increase the backscattered echoes from the desired regions.

See also Coded Excitation, Validation and Refraction Artifact, Q-Value, Ultrasound Phantom, Dead Zone, Narrow Bandwidth.

Linear scattering occur from specular reflectors or tissue, in which the echo is an accurate copy of the incident ultrasound pulse. If the phase or amplitude of the transmitted sound is altered, the phase or amplitude of the echo will be also altered.
Non-linear scatterers, such as microbubbles, do not follow these rules.