Inertially confined ablation of biological tissue

Abstract

An experimental study to determine the role of laser-induced pressure and the mechanical properties of biological tissue should yield important information concerning the fundamental mechanisms of short pulsed (less than a microsecond) laser ablation. An understanding of these mechanisms could have important implications in clinical ablation procedures. The surface movement of tissue immediately after irradiation with a short pulse of laser light is intimately related to the distribution of absorbed energy, the instantaneous pressure generated by this energy, and the mechanical properties of the tissue. Measuring the surface movement for fluences both below and near the threshold can thus reveal information on these important aspects initiating the ablation process. We have developed a physical model to describe this movement and an experimental technique to measure it. The model is based on the equation of state and the coupled differential equations for the conservation of mass, momentum, and energy in a continuous medium. The results from this model impose stringent requirements on the temporal and spatial resolution of the experimental technique used to determine the surface displacement. An interferometric surface monitoring tool, with the ability to measure nanometer movements on a nanosecond time scale, has been developed to satisfy these requirements [Albagli et al., 1993]. Using this tool, the laser-induced surface displacement of several samples has been measured. Results are presented for bovine shank bone, acrylite, human meniscus and an aqueous dye solution for incident fluences several times below ablation threshold. >