Lack of an acoustic shadow on scans of gallstones: a possible artifact

The high attenuation of gallstones results in the formation of an acoustic shadow on the ultrasound scan. Such shadows are best seen when the stone lies within the focal zone of the transducer and is large in comparison to the beam width or wavelength employed. Inadequecies in the dynamic range of available TV display units necessitate the use of compression-amplification signal processing which may preclude perception of such a shadow and seriously interfere with diagnostic accuracy.