Non-thermal cellular effects of ultrasound

The cellular effects of ultrasound can be grouped into those which are predominantly thermal in origin and those which are, at least in part, non-thermal. Cellular effects of ultrasound generally acknowledged to be essentially non-thermal include changes associated with standing waves, acoustic streaming, microstreaming and cavitation; they have been demonstrated in vitro and may, in many instances, also occur in vivo. While some stimulate cell activity and can be exploited in ultrasonic therapy, others are potentially hazardous and must be avoided. Examples of experimentally-demonstrated cellular effects in which non-thermal, ultrasonically-induced events are involved are described, together with the mechanisms considered to be involved in producing them. The relevance of such effects to the safe and beneficial use of ultrasound in diagnosis and therapy is considered.