Bioheat transfer analysis of cryogen spray cooling during laser treatment of port wine stains

Background and Objective: The thermal response of port wine stain (PWS) ski n to a combined treatment of pulsed laser irradiation and cryogen spray coo ling (CSC) was analyzed through a series of simulations performed with a no vel optical-thermal model that incorporates realistic tissue morphology. Study Design/Materials and Methods: The model consisted of (1) a three-dime nsional reconstruction of a PWS biopsy, (2) a Monte Carlo optical model, (3 ) a finite difference heat transfer model, and (4) an Arrhenius thermal dam age calculation. Simulations were performed for laser pulses of 0.5, 2, and 10 ms and a wavelength of 585 nm. Simulated cryogen precooling spurts had durations of 0, 20, or 60 ms and terminated at laser onset. Continuous spra y cooling, which commenced 60 ms before laser onset and continued through t he heating and relaxation phases, was also investigated. Results: The predicted response to CSC included maximal preirradiation temp erature reductions of 27 degrees C at the superficial surface and 12 degree s C at the dermoepidermal junction. For shorter laser pulses (0.5, 2 ms), p recooling significantly reduced temperatures in superficial regions, yet di d not effect superficial vessel coagulation. Continuous cooling was require d to reduce significantly thermal effects for the 10-ms laser pulse. Conclusions: For the PWS morphology and treatment parameters studied, optim al damage distributions were obtained for a 2-ms laser pulse with a 60-ms p recooling spurt. Epidermal and vascular morphology as well as laser pulse d uration should be taken into account when planning CSC/laser treatment of P WS. Our novel, realistic-morphology modeling technique has significant pote ntial as a tool for optimizing PWS treatment parameters. (C) 2000 Wiley-Lis s, Inc.