(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.

Ultrasound waves are reflected when there is a change in acoustic impedance. The larger the change, the more ultrasound is reflected. Microbubbles have an enormous difference in acoustic impedance as compared to surrounding fluid due to the large differences in density, elasticity and compressibility.
At low acoustic power (mechanical index less than 0.1), the mechanism of ultrasound reflection is that of Rayleigh scattering and the microbubbles may be regarded as point scatterers. The scattering strength of a point scatterer is proportional to the sixth power of the particle radius and to the fourth power of the ultrasound frequency;; the echogenicity of such contrast agent is therefore highly dependent upon particle size and transmit frequency. The backscattered intensity of a group of point scatterers is furthermore directly proportional to the total number of scatterers in the insonified volume. The concentration of the contrast medium is of importance.

See also Backscatter Energy, Cross-section Scattering.

Reflection of the sound beam occurs when it hits a boundary between materials having different acoustic impedance. The reflection (echo) is the portion of a sound that is returned from the boundary. The reflection time (the time taken for the wave to return to the probe) can be used to determine the depth of the object.
The reflection within the body produces the ultrasound image, but should be minimized at an ultrasound couplant to skin boundary where the couplant acts as an acoustic window through which the image is seen. The amount of sound waves, which are reflected back at the interface between two tissues is depend on the angle of incidence and the difference between the acoustic impedance values of the two tissues.
If the difference is great, a large part of the sound waves will be reflected back. If too much sound is reflected back and not enough waves are remaining to be able to penetrate the tissue, the imaging will be poor.
If the difference is small, a small amount will be reflected back. Enough sound signal remains to continue with ultrasound imaging.
If the ultrasound beam meets a rough surface or small object, the beam is scattered in all directions and only a small amount will be received by the probe.

See also False Distance Artifact, Target Strength, and Snells Law.

The acoustic lens is placed at the time the transducer is manufactured and cannot be changed. The acoustic lens is generally focused in the mid field rather than the near or far fields. The exact focal length varies with transducer frequency, but is generally in the range of 4-6 cm for a 5 MHz curved linear probe and 7-9 cm for a 3.5 MHz curved transducer.
Placing the elevation plane (z-plane) focal zone of the acoustic lens in the very near or far field would improve the beam width at precisely those depths. However, this would degrade the beam width to a much greater and unacceptable degree at all other depths.
There are some chemicals in ultrasound couplants that can degrade the acoustic lens, destroy bonding, or change the acoustic properties of the lens. Problematic chemicals include mineral oil, silicone oil, alcohol, surfactants, and fragrances. Fragrance can affect the transducer's acoustic lens or face material by absorption over time into elastomer and plastic materials, thus changing the material's weight, size, density, and acoustic impedance. Surfactants can degrade the bond between the lens and the piezoelectric elements and contribute to the accelerated degeneration of the lens.

See also Retrolenticular Afterglow.

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.