The use of ultrasound in diagnostic medicine has grown at a phenomenal rate in the last few years, and ultrasonic scanners are now readily available to physicians and health care professionals. Although there have been no clinically established health risks associated with diagnostic ultrasound, there has been some scientific evidence that even microsecond length pulses of high intensity ultrasound can produce cavitation in liquids and in biological systems (e.g. Drosophila larvae that have air-filled tracheae). Since microsecond-length pulses can only produce bubbles on the scale of single biological cells, it is necessary in assessing the likelihood of cavitation from diagnostic ultrasound to develop methods for monitoring this localized cavitation. Two complementary methods are proposed. In one, the aqueous medium used in these in vitro tests is seeded with superheated drops which will vaporize if cavitation occurs in or near them. These drop vaporizations [which are also employed in neutron detection] serve to amplify the localized cavitation events so that they can be measured with common acoustic techniques. The second technique not only measures the presence of cavitation but also the strength of a cavitation event by monitoring photons produced from either sonoluminescence or chemiluminescence (if luminol is disolved in the sample). By employing these techniques we will be able to delimit the conditions of pressure amplitude, frequency, pulse duration, and pulse repetition frequency in which cavitation can occur if gas nuclei of appropriate size and concentration are present. If some of these conditions overlap with the operating conditions of commercial diagnostic equipment, then we will determine if cavitation does indeed occur in vitro using this equipment as an acoustic source. Finally, if cavitation is still observed, we will make suggestions for some extensions of the present work to in vivo situations.
