Displacement of the diaphragmatic echo by hepatic cysts: a new explanation with computer simulation

Previous demonstrations of distal displacement of interfaces deep to cystic structures have been based on only the slower speed of sound through the cystic structure, which has been postulated to cause an apparent discontinuity in the echo from the diaphragm deep to a cyst. The authors found cysts whose identification could not be explained by this theory because they were too small or the diaphragm was displaced in such a manner that the explanation would not hold true. They propose that, in addition to the slowing of the speed of sound through the cyst, refraction at the edge of the cyst can cause the apparent discontinuity. Since the machine does not distinguish the sound beam as having been refracted, the echo is displayed at the appropriate distance along the vector in the direction in which the transducer was pointing at the time. A computer simulation is used to confirm this theory.