Relative Tissue Reflectivity (Brightness)

The cellular structure of soft tissue scatters sound energy in all directions.

This is similar to the scattering of light seen when a cars 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.