The wavelength is a unit of relative distance equal to the length of a wave. This could be a light wave, a radio wave, or even a sound wave. For sound waves the formula is:
l=c/f (wavelength = propagation speed/frequency)
In ultrasound imaging is the wavelength the distance between the onset of peak compression or cycle to the next. The wave propagates as bands of compression and rarefaction. One wavelength is the distance between two bands of compression, or rarefaction. Maximum compression corresponds to maximum pressure. The wavelength (see also Angstrom) is important in image resolution.

See also Spectral Reflector.

A point scatterer is a reflector with a diameter much smaller than the ultrasound wavelength. The reflection from blood is a typical example of point scattering. Red blood cells are with 7μm versus 0.44 mm wavelength at 3.5 MHz, smaller than any US wavelength. The individual cells are not only the point scatterers, ultrasound is scattered whenever there is a change in acoustic impedance, and in blood such changes are caused by variable cell concentration. These local fluctuations in cell concentration have a spatial extent that is also much smaller than the ultrasound wavelength, and they therefore act as point scatterers.
A point scatterer gives rise to spherical wavelets spreading out in all directions with the scatterer itself at the center of the sphere. The spherical wavelets from one single point scatterer are much too weak to be detected by the transducer, but constructive interference between numerous wavelets will produce backscattering of higher amplitude echoes with parallel wavefronts, also in the direction of the ultrasound transducer.

See also Rayleigh Scattering.

(F) The number of cycles of a periodic process per unit time. Frequency and wavelength are inversely related. The higher the frequency the smaller the wavelength. The frequency of ultrasound is expressed in units of hertz (Hz), where 1 Hz = 1 cycle per second.
The effect of different frequencies on tissue penetration:
The higher the frequency the less the penetration, the lower the frequency the greater the penetration. As frequency increases, resolution improves but the imaging depth or penetration decreases. The lower the axial resolution, the more detail can be seen.
Usual frequencies for pediatric ultrasound: 5.0mHz to 7.5mHz and 10mHz.
Usual frequencies for adult ultrasound: 2.0mHz to 3.0mHz.

See also Doppler Interrogation Frequency, Multi-frequency Probe, and Huygens Principle.

(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.

Non-linear scattering describes in physics the conversion of a photon from one wavelength to one or more other wavelengths. In ultrasound, non-linear scattering is the generation of an echo containing higher harmonics. In the case of a microbubble, the oscillation is asymmetric with time, producing echoes with even harmonics.

See also Cross-section Scattering.