Through diffraction and refraction on intersections edge acoustic shadowing can be created. The acoustic shadowing artifact is the loss of information below a dense object because the majority of the sound energy was reflected back by the object.
Shadowing artifacts occur if decreasing of the echo amplitude is not exponential with penetration depth caused by inhomogeneous tissue layers and fluid or air-filled regions. Bone, air, foreign bodies and calcification stop the transmission of sound waves producing a 'sonic shadow' which is a dark region distal to the echogenic obstructing region. This artifact occurs also in objects like e.g. prosthetic valves.

See also Boundary Layer, and Half-Value Layer.

(Z) The acoustic impedance is dependent on the density of the material in which sound is propagated through. When an ultrasonic wave crosses an interface between tissues with different acoustic impedance, the wave divides in 2 components, and the energy of the reflected components directly relates with the acoustic impedance.
The greater the impedance the more dense the material, and the greater the difference in acoustic impedance between two adjacent tissues the more reflective will be their boundary.
The acoustic impedance (the unit is 'Rayl') is the product of the sound velocity and the physical dense.
The acoustic impedance is very high between air or bone and other body tissues, therefore not enough energy crosses these interfaces, and no information can be collected from organs placed behind them.

See also Mirror Artifact, Reverberation Artifact, Cross Talk and Ultrasound Physics.

(CPD) CPD is a type of color Doppler to visualize the presence of detectable blood flow. The flow information is based on the amplitude or strength of echoes received from moving cells and not on frequency shifts. Power Doppler is very sensitive to flowing blood but does not provide velocity or directional information.
CPD is less angle dependent than traditional color Doppler, but more sensitive to motion artifacts. Color power angio (CPA) provides better sensitivity to slow flow states.
The color maps for CPD are represented by a single continuous color (colour, Brit.). Because CPD does not provide directional information, no aliasing artifact occurs.

See also Directional Color Power Doppler.

Cross talk is an ultrasound artifact in which strong sound signals in one directional channel leak into another, appearing as a mirror image of the spectral display on the opposite side of the baseline. Cross talk distinguishes the condition of undesired crossover of transmitted sound waves into the receiving transducer in a continuous wave Doppler system.
Also called Mirror Artifact.

Diffraction is the change in the directions and intensities of a group of sound waves after passing by an obstacle or through an aperture.

See also Acoustic Shadowing, Triplication Artifact and Duplication Artifact.