(Å* or A*) A unit used to measure the wavelength of X-rays.
Definition: 1A* = 0.1 nanometer or 10^-10 meter.

Cross-section scattering is a measure of the scattering strength of a point scatterer. The scattering strength is dependent on the size of the scatterer, the density and compressibility of the scatterer and the surrounding medium, and the ultrasound wavelength.
If a transducer emits ultrasound with a total acoustic power of P, and the power is assumed to be uniform distributed over the US beam cross-sectional area, then the ultrasound intensity at a certain range, is defined by:
I = P/A
where I is the intensity, and A is the cross-sectional beam area at that range.
A point scatterer located in the ultrasound beam at this range, will scatter the ultrasound with a total acoustic power of Ps, defined by:
Ps = I s
where s is the scattering cross-section of the point scatterer.

The divergence is an ultrasound beam characteristic of the far field. The beam divergence angle q, depends on the transducer frequency and diameter according to the following approximation:
sin q 1.22 ld
where l is the wavelength of the ultrasound in the medium of propagation and d is the diameter of the transducer element.

Harmonic B-mode imaging takes advantage of the non-linear oscillation of microbubbles. During harmonic imaging, the sound signal is transmitted at a frequency of around 1.5 to 2.0 MHz and received at twice this frequency. The microbubbles also reflect waves with wavelengths different from the transmitted one, the detectors can be set to receive only the latter ones and create only images of the contrast agent.
Using bandpass filters the transmitted frequency is separated from the received signal to get improved visualization of vessels containing ultrasound contrast agents (USCAs). The signal to noise ratio during the presence of microbubbles in tissue is four- to fivefold higher at the harmonic compared with the basic frequency.
Using harmonic B-mode imaging, harmonic frequencies produced by the ultrasound propagation through tissue have to be taken into account. The tissue reflection produces only a small amount harmonic energy compared to USCAs, but has to be removed by background subtraction for quantitative evaluation of myocardial perfusion.

See also Non-linear Propagation.

Huygens principle states that an expanding sphere of waves behaves as if each point on the wave front were a new source of radiation of the same frequency and phase. The principle explains how a flat ultrasound transducer can transmit a narrow ultrasound beam, which in the near field is confined to the dimensions of the transducer surface.
Spherical wavelets are emitted from numerous point sources on the transducer surface. They interfere to form a narrow, slightly converging beam of ultrasound in the near field. The wavefronts in the beam are nearly parallel. A precondition for this interference is that the transducer surface is much larger than the ultrasound wavelength.

See also Interference Artifact.