Cavitation thresholds and why to be wary!

Many biological studies involving ultrasound exposure report acoustic cavitation “thresholds”. The purpose of these is often to convince the reader either that cavitation activity was avoided, or that it definitely occurred as the exposure was “above the threshold”. Formally, the cavitation threshold is the minimum negative pressure amplitude at which pre-existing bubbles begin to oscillate (non-inertial cavitation) or collapse (inertial cavitation). The cavitation nucleation threshold is that for which bubbles can be drawn out of solution and driven to oscillate. Since the true physical cavitation threshold of a medium can only be measured if it is possible to detect single bubble activity, quoted thresholds are more likely to represent a threshold for detection than for cavitation activity. Furthermore, they may not be relevant beyond the specific experimental conditions tested. In interpreting a negative pressure threshold for cavitation in tissue it is important to know, amongst other things: the tissue status and sample geometry (in order that the in situ pressure may be calculated); details of the ultrasound exposure; method of cavitation detection and detector geometry; data gathering, processing and interpretation methods; threshold definition; sample statistics. Without these details, results may be misleading.