When a sound wave interacts with a medium, several possibilities exist for what can happen to the wave. The wave may be reflected in a variety of ways (described below), or it can continue through the medium and become attenuated.
The two most common types of reflection are specular reflection and diffuse reflection (backscatter). With diffuse reflection, the ultrasound wave is reflected in multiple directions upon striking a particle in the medium. Some of this reflected ultrasound returns to the probe and is detected. Diffuse reflection (backscatter) is responsible for generating the bulk of the ultrasound image and in particular the fine detail of the image.
When the ultrasound beam strikes a smooth edge or surface, specular reflection can occur. Specular reflection is resonsible for forming the edges of the ultrasound image and occurs best when the beam is perpendicular to the edge in question and worst when the beam is parallel to the edge in question. Perhaps the best example is the laterall wall of the left ventricle in the apical 4 chamber view, when there is frequent dropout of the edge of the lateral wall as the edge is formed by specular reflection and the utrasoud beam is often quite parallel to the lateral wall itself.
The final form of reflection which is important for medical ultrasound is Rayleigh scattering. This occurs when the reflector is very small relative to the wavelength of the sound beam. The classic example of Rayleigh scattering is scattering generated by red blood cells.
If sound is not reflected in the medium, it may continue on through the medium and become attenuated. Attenuation refersto a depth dependent decrease in the amplitude and therefore the intensity of the sound. Units of attenuation are given in Decibels. The degree of attenuation is determined both by the medium and by the ultrasound itself. Transducers with a higher frequency have a greater degree of attenuation and therefore cannot image at deeper depths whereas those with lower frequencies attenuate less in any given medium and can be used to image at deeper depths. An attenuation coefficit describes the decrease in intensity per cm of depath in the medium and is roughly the frequency (MHz)/2 in dB/cm.
Attenuation varies depending on the medium as well. For example, attenuation in air is much higher than that in blood or water. This is in large part the reason why we are unable to image through lung (for example in patients with COPD) but we could image through a pleural effusion if present.