Improvement of port wine stain laser therapy by skin preheating prior to cryogen spray cooling: A numerical simulation

Abstract

Background and Objectives: Although cryogen spray cooling (CSC) in conjunction with laser therapy has become the clinical standard for treatment of port wine stain (PWS) birthmarks, the current approach does not produce complete lesion blanching in the vast majority of patients. The objectives of this study are to: (1) experimentally determine the dynamic CSC heat flux when a skin phantom is preheated, and (2) numerically study the feasibility of using skin preheating prior to CSC to improve PWS laser therapeutic outcome. Study Design/Materials and Methods: A fast-response thin-foil thermocouple was used to measure the surface temperature and thus heat flux of an epoxy skin phantom during CSC. Using the heat flux as a boundary condition, PWS laser therapy was simulated with finite element heat diffusion and Monte Carlo light distribution models. Epidermal and PWS blood vessel thermal damage were calculated with an Arrhenius-type kinetic model. Results: Experimental results show that the skin phantom surface can be cooled to a similar minimum temperature regardless of the initial temperature. Numerical simulation indicates that upon laser irradiation, the epidermal temperature increase is virtually unaffected by preheating, while higher PWS blood vessel temperatures can be achieved. Based on the damage criterion we assumed, the depth and maximum diameter of PWS vessels that can be destroyed irreversibly with skin preheating are greater than those without. Conclusions: Skin preheating prior to CSC can maintain epidermal cooling while increasing PWS blood vessel temperature before laser irradiation. Numerical models have been developed to show that patients may benefit from the skin preheating approach, depending on PWS vessel diameter and depth.

A two-temperature model for selective photothermolysis laser treatment of port wine stains

“…Of the five assumptions mentioned earlier, 1–4 are common practice and have been widely employed in the literature [7, 14]. The local thermodynamic non-equilibrium condition in the PWS layer is introduced for the first time in laser PWS modeling.…”

“…The macroscopic energy equations derived based on the volume averaging technique are employed for the two mixtures, but one is under local thermodynamic equilibrium condition and the other is under local non-equilibrium. In the case of local equilibrium, one-energy equation is used with the properties of the mixture as the weighted averaging of the two components [14]. In the case of local non-equilibrium, however, two energy equations, one for each phase, were used [15, 16].…”

Thermal Modelling for Laser Treatment of Port Wine Stains

“…Various models have been developed to better understand the laser-tissue interaction, in particular the thermal response of biological tissues and physiological mechanisms of thermal damage of PWS (see e.g., Pfefer et al, 2000;Pickering et al, 1989;Shafirstein et al, 2004;Tunnell et al, 2003). The models have also been used for optimization of the surgery parameters and for development of new treatment protocols (Jia et al, 2006(Jia et al, , 2007. To model laser PWS, various simple skin models have been proposed to simplify the complex anatomic structure of the skin with PWS .…”

“…For laser PWS, cryogen spray cooling is widely employed by spraying volatile refrigerant such as R134a on skin surface before laser irradiation. Therefore, the cooling of skins before laser irradiation should be part of the model of laser PWS (Jia et al, 2006;Li et al, 2007a,b) In this chapter, we will first present a brief review of the development of various skin models proposed in the literature for thermal modeling of laser PWS. Next, we explain the basic principle of the multi-layer Monte-Carlo method which is widely used for simulation of light propagation in the skin with PWS.…”

“…Recently, models with randomly distributed blood vessels with varying sizes have also been utilized. Occasionally, the model with a single blood vessel was also used to test the clinic outcome of new treating protocols of laser PWS (Jia et al, 2006(Jia et al, , 2007. Concerns have been raised for possible lack of the scattering and shielding effects of numerous blood vessels in real PWS for those discrete blood vessel models (Tan et al, 1990;Verkruysse et al, 1993 The skin model with discrete blood vessels can provide direct visualization of heating of the blood vessels in laser PWS (Baumler et al, 2005;Shafirstein et al, 2004Shafirstein et al, , 2007Tunnell et al, 2003).…”