The intense conditions generated in the core of a collapsing bubble have been the subject of intense scrutiny from fields as diverse as marine biology and nuclear fusion. In particular, the phenomenon of sonoluminescence, whereby a collapsing bubble emits light, has received significant attention. Sonoluminescence has been associated predominantly with millimeter-sized bubbles excited at low frequencies and under conditions far removed from those associated with the use of ultrasound in medicine. In this study, however, we demonstrate that sonoluminescence is produced under medically relevant exposure conditions by microbubbles commonly used as contrast agents for ultrasound imaging. This provides a mechanistic explanation for the somewhat controversial reports of "sonodynamic" therapy, in which light-sensitive drugs have been shown to be activated by ultrasound-induced cavitation. To illustrate this, we demonstrate the activation of a photodynamic therapy agent using microbubbles and ultrasound. Since ultrasound can be accurately focused at large tissue depths, this opens up the potential for generating light at locations that cannot be reached by external sources. This could be exploited both for diagnostic and therapeutic applications, significantly increasing the range of applications that are currently restricted by the limited penetration of light in the tissue.
Keywords: microbubbles; sonodynamic therapy; sonoluminescence; ultrasound.