Physics of laser-induced stress wave propagation, cracking, and cavitation in biological tissue

Abstract

In the regime where the specific time for propagation of stress waves is longer than the laser pulse duration, but shorter than the heat dissipation time, stress can be one of the governing mechanisms of laser-induced ablation of biological tissue. In such inertially confined regimes, knowing the mechanical properties of biological tissue an the kinetics of cracking (in hard tissue represented by bone) and cavitation (in soft tissue represented by meniscus) are important to understand the ablation process. An experimental technique has been developed to study laser-induced stress generation and mechanical properties of tissue in such regimes. This technique is based on monitoring the tissue surface after laser irradiation, using an interferometer that can measure submicron surface displacements on a nanosecond time scale. The subablation threshold laser-induced surface displacements can be related to the stress within the tissue and mechanical properties of the tissue. The surface movement of aqueous solution and meniscus tissue irradiated by 7.5-ns pulses of 355 nm light was consistent with growth and collapse of cavitation bubble. Bone movement was qualitatively consistent with theoretical predictions obtained by solving the equation of motion both analytically and numerically. In the regime where laser beam radius and optical absorption depth are comparable, it is shown that a full 3D analysis is necessary to understand the observed results.