Scattering (ultrasound)

Scattering occurs when a sound wave strikes a structure with a different acoustic impedance to the surrounding tissue and which is smaller than the wavelength of the incident sound wave. Such structures are known as “diffuse reflectors,” with examples being red blood cells and non-smooth surfaces of visceral organs.

In contrast to “specular reflectors”, tissues with smooth interfaces from which ultrasound waves are reflected in a specular fashion, “diffuse reflectors” cause ultrasound waves to scatter in all directions thus resulting in multiple echoes propagating from the numerous tiny structures. Not only does this scattering result in echoes with smaller amplitudes (compared to specular reflection) but the scattered echoes also interact with each other. This interaction causes constructive and destructive interference of the waves. The resultant image is termed “speckle” due to the various intensities of the echoes received by the transducer, and this is seen as an irregularity in the greyscale of the image.

Most echoes from ultrasound imaging arise from scattering, rather than the reflection from specular reflectors. The speckle arising from this scatter results in the grainy appearance of the parenchyma of organs and also the signal in doppler ultrasound.