The rarefactional pressure is the amplitude of a negative instantaneous sound pressure in an ultrasound beam. Rarefaction is the reduction in pressure of the medium during the acoustic cycle.

See also Mechanical Index.

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.

Side lobes are secondary and smaller acoustic beams falling outside at predictable angles located around the main lobe.

See also Grating-lobe Artifact, Amplitude Shading, and Apodization.

(SNR) Signal to Noise Ratio is the ratio of the amplitude of a signal to undesired noise. The larger the signal noise ratio, the easier is the detection and measurement of a signal. The sensitivity of any device is limited by the SNR. The SNR for sound is often expressed in decibel.

Sonazoid™ is an ultrasound contrast agent (UCA) consisting of stabilized gas microbubbles in an aqueous suspension. Sonazoid™ has overcome the stability problems of first generation USCA and can produce myocardial perfusion images. Myocardial imaging using ultrasound contrast agents provides diagnosis of chronic heart disease and assessment of the coronary arteries and of the coronary blood flow reserve.
Sonazoid™ is taken up by healthy Kupffer cells in the liver and spleen, but break down in high amplitude ultrasound imaging modes such as color Doppler imaging. The bubble rupture produces a transient pressure wave, which results in a characteristic mosaic color pattern from tissues containing the microbubbles (induced acoustic emission). Liver tumors without Kupffer cells will not display the mosaic pattern and can therefore be identified easily.