Acoustic cavitation and the safety of diagnostic ultrasound.

A massive body of screening studies attests to the safety of diagnostic ultrasound. It appears that further progress in analysis of the safety question, will require mechanistic guidance. In theory, transient cavitation can occur under diagnostically relevant conditions if appropriate nuclei exist in the exposed medium. Low-temporal-average-intensity, pulsed ultrasound equivalent to that used in diagnosis has been shown to cause deleterious effects in lower organisms. All available evidence supports the conclusion that these effects are mediated by acoustic cavitation. It is impossible to extrapolate from these observations to human beings. However, all of the information available to us at the present time suggests that if transient cavitation occurs under diagnostic conditions, the events are rare, highly localized and probably would be of minor importance in almost any part of the body with the possible exception of the developing embryo or fetus. Transient cavitation is characterized by sharply defined thresholds. If future studies demonstrate the occurrence of cavitation related effects, they can be eliminated simply by keeping the maximum pressure amplitudes below those thresholds.