The gas in microbubbles is highly compressible and, when subjected to the alternating compression and refraction pressures that constitute an ultrasound pulse, microbubbles oscillate at their natural frequency at which they resonate most strongly. This is determined by their size but is also influenced by the composition of the filling gas.
Air, sulfur hexafluoride, nitrogen, and perfluorochemicals are used as filling gases. Most newer ultrasound contrast agents use perfluorochemicals because of their low solubility in blood and high vapor pressure. By substituting different types of perfluorocarbon gases for air, the stability and plasma longevity of the agents have been markedly improved, usually lasting more than five minutes.

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.

(ESWL) Extracorporeal shock wave lithotripsy is a special use of kidney ultrasound, where high intensity focused ultrasound pulses are used to break up calcified stones in the kidney, bladder, or urethra. Pulses of sonic waves pulverize dense renal stones, which are then more easily passed through the ureter and out of the body in the urine. The ultrasound energy at high acoustic power levels is focused to a point exactly on the stone requiring an ultrasound scanning gel for maximum acoustic transmission.
Air bubbles in the ultrasound couplant, regardless of their size, degrade the performance of Lithotripsy and have the following effect:
Air bubbles smaller that 1/4 wavelength cause scattering of the sound waves as omni directional scatterers and less acoustic energy reaches the focal point. The result is less acoustic power at the focal point to disintegrate the kidney stone.
Air bubbles larger than 1/4 wavelength act as reflectors and deflects the acoustic energy off in a different direction. These results in less acoustic energy at the focal point.
Microbubbles dispersed throughout the ultrasound couplant layer change the average acoustic impedance of the gel layer (which reduces the total transmitted energy) and, due to refraction, change the focal point.

Retrolenticular afterglow could occur through diffraction and refraction on interfaces. A circular object may act as a lens to the ultrasound beam, showing an artifact region of increased echogenicity.

As the sound travels through a relatively homogeneous medium it propagates in essentially a straight line. When the sound reaches an interface a part of the incident beam is reflected, and a part is refracted (transmitted). Snells law governs the direction of the transmitted beam when refraction occurs:
sin qt = (c2/c1) x sin qi (qt is the transmit and qi is the incident angle)
The amount of sound that is reflected depends on the degree of difference between the two media; the greater the acoustic mismatch, the greater the amount of sound reflected. In addition, the amount of ultrasound reflected or refracted depends on the angle at which the sound beam hits the interface between the different media. As the angle of incidence approaches 90°, a higher percentage of the ultrasound is reflected.

See also Sonographic Features.