Simple analytical model for laser-induced tissue ablation

摘要

Simple laws of physics have been applied to understand behavior of common parameters of laser induced tissue ablation. Threshold fluence for ablation is related to some of the optical and thermal properties of the tissue. For this a necessary condition of ablation is imposed that the rate of deposition of laser energy should be greater than the rate of expending energy for vaporization and at threshold they should be equal. A simple expression for resulting depth of the ablation crater is derived assuming total conversion of the optical energy above threshold into kinetic energy of the ablation products. Evaporation that should follow the ablation in the superheated exponential tail in the tissue is also considered for contributing to the crater depth. The assumption of total conversion of optical energy above threshold into kinetic energy of the ablation products is also used to find the average velocity of the ablation products ejected from the tissue. The laser induced ablation is shown to produce a pressure wave which has two components. The first component originates because of the static pressure arising due to the conversion of condensed state of the tissue into a super heated fluid which remained confined to its original volume because of its kinetics and inertia. This component has been worked out using the ideal gas laws. The second component is the recoil pressure of the ejected ablation products which at any instant of time is governed by the instantaneous intensity of the laser beam and it is calculated using Rocket effect. The values of the aforesaid parameters calculated from this model are compared with our and others experimental results and are found to be in agreement with reasonable accuracy.