Causes of shadowing in breast sonography.

As ultrasonic beams propagate through tissues, there is a loss of energy by absorption, reflection and scattering. This loss is displayed in the image as shadowing and is an important sonographic sign for the detection and diagnosis of breast disease. Shadowing may result because of reflection of most of the energy by a large impedance discontinuity. The degree of shadowing by interfaces is dependent on their size relative to the ultrasonic beam. Interfaces smaller than the beam may allow sufficient energy to bypass the reflector to permit good visualization of posterior detail. Shadowing may also result from beam spreading due to refraction. Attenuation is the acoustic parameter which specifies total loss of energy and is due predominantly to absorption. Echogenicity is determined by the reflecting and scattering characteristics and therefore attenuation and echogenicity should be regarded as independent entities in the interpretation of breast images.