Modeling laser treatment of port wine stains with a computer-reconstructedbiopsy

Background and Objective: The efficacy of laser treatment of port wine stai ns (PWS) has been shown to be highly dependent on patient-specific vasculat ure. The effect of tissue structure on optical and thermal mechanisms was i nvestigated for different pulse durations by using a novel theoretical mode l that incorporates tissue morphology reconstructed tomographically from a PWS biopsy. Study Design/Materials and Methods: An optical-thermal numerical model capa ble of simulating arbitrarily complex, three-dimensional tissue geometries was developed. The model is comprised of (1) a voxel-based Monte Carlo opti cal model, (2) a finite difference thermal model, and (3) an Arrhenius rate process calculation to predict the distribution of thermal damage. Simulat ions based on previous computer-based reconstruction of a series of 6 mu m sections from a PWS biopsy were performed for laser pulse durations ( tau(p )) of 0.5, 5.0, and 10.0 ms at a wavelength of 585 nm. Results: Energy depo sition rate in the blood vessels was primarily a function of vessel depth i n skin, although shading effects were evident. Thermal confinement and sele ctivity of damage were seen to be inversely proportional to pulse duration. The model predicted blood-specific damage for tau(p) = 0.5 ms, vascular an d perivascular damage for tau(p) = 5 ms, and widespread damage in superfici al regions for tau(p) = 10 ms. The effect of energy deposition in the epide rmis was most pronounced for longer pulse durations, resulting in increased temperature and extent of damage. Conclusion: Pulse durations between 0.5 and 5 ms are likely optimal for the PWS analyzed. The incorporation of a tomographically reconstructed PWS bio psy into an optical-thermal model represents a significant advance in numer ical modeling of laser-tissue interaction. Lasers Surg. Med. 24:151-166, 19 99. (C) 1999 Wiley-Liss, Inc.