The cellular structure of soft tissue scatters sound energy in all directions.
This is similar to the scattering of light seen when a car’s headlights are raised on a foggy evening.
Rayleigh scatter
Rayleigh scatter produces the characteristic grainy appearance of solid organs on ultrasound. The image above shows that in a normal subject, the renal parenchyma appears slightly darker than the adjacent liver (arrowed). Click the image to see a larger version.
Echo texture
The scattering of sound within a soft tissue structure determines the ‘brightness’ of the organ relative to adjacent tissues.
Scattering also produces a characteristic speckle pattern or echo texture.
The image above shows the characteristic echo texture of normal liver parenchyma (arrowed). Click the image to see a larger version.