Moving phase transitions in laser-irradiated biological tissue

Abstract

A theory of laser-induced phase transitions in an absorptive medium with internal scattering is developed in order to improve our understanding of the multiplicity of phenomena occurring in the application of lasers in medicine. The redistributive internal scattering phenomenon, which has successfully explained several anomalies in tissue heating without phase transitions, is studied in detail. Several interesting results including general profile independent formulas for the back- and forwardscattered power and an explanation of the popcorn effect or laser-induced decrepitation commonly observed in high-intensity irradiation are obtained, the application of the latter to the theory of laser drilling being touched on in a footnote. The behavior of moving phase transitions in one dimension (plane incoming wave) is studied. The heat equation is solved for the pure absorption model in terms of the unknown depth of the phase front; this solution is used to derive a numerical procedure to find the time dependence of the phase-front depth as well as an analytic expression of its initial time dependence.