Simulation of laser-tissue thermal interaction and plasma-mediated ablation
Creators: name: Jiao, Jian
Publisher: name: No Publisher Supplied
Year: 2011

description: In this dissertation, two types of laser-tissue interactions - ultra-short pulsed (USP) laser-tissue thermal interaction and plasma-mediated ablation - are numerically investigated. The thermal interaction is focused on laser hyperthermia skin cancer treatment. A combined ultrafast radiative transfer and bioheat transfer model is developed to predict the temperature distribution in a 2-D axisymmetric skin tissue cylinder subjected to a train of short pulsed irradiations. An appropriate mathematical model to describe the focused laser beam transport is considered. The laser propagation in the tissue is simulated via solving the transient equation of radiative transfer with the Transient Discrete-Ordinates Method. The bioheat transfer equation is calculated by the Alternative Directional Implicit Method. A numerical scheme is introduced to deal with the multi-time-scale heat transfer problems in a three-layer model skin tissue exposed to multiple short laser pulses. Parametric studies including influences of lens diameter, laser focused depth, wavelength and laser power on killing different types of skin tumors are carried out. For the study of USP laser induced plasma-mediated ablation in transparent media, an ionization rate equation is adopted, which consists of the multiphoton and avalanche ionizations, and diffusion and recombination effects. The present 2-D axisymmetric numerical model is validated via extensive comparisons with experimental measurements and other 1-D numerical models available in the literature.  The dynamic process of plasma formation for ablation in pure water can be better visualized by the current modeling.  To determine plasma-mediated ablation threshold, we propose that a certain number of free electrons are required to trigger the avalanche ionization. Based on this assumption, the ablation thresholds for pulse widths down to the femtoseconds range at wavelength 800 nm are calculated for transparent corneal epithelium. It is found that the critical seed free electron-density decreases as the pulse width increases, obeying a tp-5.65 rule. A complete numerical model with all ionization and loss mechanisms is also employed to elucidate and validate the conditions when avalanche ionization occurs and dominates in the plasma-mediated ablation process. Moreover, the crater sizes ablated in a PDMS by a 900 fs pulsed laser at wavelength 1552 nm are modeled using the present model, and the results match with the existing experimental measurements.  The study of plasma-mediated ablation is further extended to turbid media. For ablation in absorbing and highly-scattering biological tissues, combined modeling of ultrafast radiative transfer and plasma formation is needed.  We studied plasma-mediated ablation in a model human skin tissue, in which the rate equation is modified with chromophore ionization in addition to the multiphoton and avalanche ionizations. A parametric study on the influences of band gap, critical free-electron density and recombination rate on the prediction of ablation threshold in the model skin tissue is carried out. Moreover, investigation on crater formation is conducted, and the modeled crater sizes are compared with available experimental measurements on corundum and sapphire materials. For ablation in corundum and sapphire, the free electrons are generated via photoionization (multiphoton and tunneling ionizations) and avalanche ionization. The onset of mass ablation in the material is assumed when the local absorbed laser energy is sufficient to cause evaporation. Good agreements between the modeling and experiments are observed.