Theoretical analysis of the role played by tissue-optical parameters in the laser ablation process

The purpose of this work was to gain better overall picture to the thermal process involved in ablation of biological media performed by means of both continuous waves (C.W) and pulsed laser irradiations. The theoretical analysis is based upon a computerized laser evaporative model. This scheme has been applied in order to clarify the following situations : - Ablation of tissue assimilated as water with finite absorption coefficient. - Tissue ablation by C.W Argon laser using gel (tissue-like phantom material) as a model system for tissue. - Skin ablation performed by pulsed Er:YAG laser radiation. In each case the role played by optical absorption in the dynamics of the ablation process is depicted. Typically, it appears that the position of the ablation front (or crater depth) and the transient ablation velocity are both influenced by tissue absorption while the steady-state stays independent on this parameter and evolves linearly with power density. Additionally, the ablation temperature beyond the moving front can reach a maximum higher than ablation threshold temperature. The peak temperature and its location are mediated by tissue type. Calculations show that for small absorption coefficients higher temperatures are reached at deeper levels. In contrast, at fixed absorption coefficient, the growing of the power density increases the peak temperature but reduces the penetration depth of the heated volume. The whole of computed data confirms that thermal laser ablation of tissue can be described as an explosive event and that a decrease of water content in the target alters the penetration depth which control the ablation rate.