Department of Radiology

University of Cincinnati College of Medicine

Cincinnati, Ohio 45267-0761

Although no adverse effects on patients exposed to diagnostic ultrasound have been established to date, several animal models have exhibited a threshold for petechial hemorrhage in both lung and intestine. The exact mechanical mechanism that induces this damage in biological tissues has not yet been identified. However, either macroscopic or microscopic gas bodies are required in the tissue to elicit the effect (Hartman et al. 1990). Inertial cavitation has been discussed as a possible damage mechanism, though other acoustomechanical effects which act directly on the lung surface could be important. In this review, the onset of cavitation will be discussed with reference to theoretical and experimental data. The Mechanical Index, MI, which resulted from theoretical considerations of the short-pulse acoustic threshold for inertial cavitation in water populated with microbubbles of all sizes will also be reviewed. The question arises: Can the utility of the MI be extended to situations in which the threshold MI is exceeded, thereby allowing for some estimate of the quantification of a potential bioeffect due to microcavitation? Also, can MI be extended to situations in which pulses are, unlike the original formulation, not short? In vitro experiments over a broad frequency range in which the nuclei content is controlled have not been reported. However, lung damage from 1 to 6-MHz pulsed diagnostic ultrasound has been assessed in vivo in the mouse, neonatal mouse, rat, rabbit, pig, neonatal pig and monkey. The damage observed may be mediated by inertial cavitation, yet no such causal relationship has been established to date. The possible consequences of gas body activation associated with aerated lung tissue and intestinal gas pockets represent specific instances of cavitation considerations relevant to clinical practice.