Rayleigh scattering is the backscattering of ultrasound from blood. The echoes detected from blood are created through interference between scattered wavelets from numerous point scatterers. Rayleigh Scatterers are objects whose dimensions are much less than the ultrasound wavelength. Rayleigh scattering increases with frequency raised to the 4th power and provides much of the diagnostic information from ultrasound. Doubling the ultrasonic frequency makes the echoes from blood 16 times as strong. The intensity of the backscattered echoes is proportional to the total number of scatterers, which means that the echo amplitude is proportional to the square root of the total number of scatterers.
At normal blood flow, the number of point scatterers in blood is proportional to the number of red blood cells. When blood flow is turbulent, or accelerating fast (e.g. in a stenosis), the number of inhomogeneities in the red blood cell concentration will increase.

See also Scattered Echo.

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.

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.