Pain is one of the most important physiological mechanism to ensure survival from external stimuli. In pain research, the use of lasers has been a popular tool in causing pure heat stimulation and selectively activation of A-δ and C-fibres. Different lasers have different levels of penetration throughout the skin. The higher the level of penetration the harder it is to understand how the laser-light is distributed and the heat is propagating in the skin. Through mathematical modelling it is possible to predict and describe the phenomenon. Such a model may simplify the use of lasers in research related or clinical problems where laser stimulation is being used or may be used.
The thesis is based on a previous semester project, where the goal was to build a mathematical model describing laser light absorption and heat transfer in the skin. It was constructed as 2D-axial (r,z) model and implemented via a Monte Carlo method for describing light absorption for λ=250-10600 [nm] and a Finite Element method for describing the heat transfer. The model was verified but not validated. Without a validation, it is uncertain whether the model could produce meaningful results to real problems. The motivation of this thesis is therefore to conduct an experimental validation of the model. The validation was conducted for both Carbon Dioxide (CO2) and Neodymium: Yttrium-Aluminum-Perovskite (Nd: YAP) lasers on 5 healthy subjects. Experimental data was based on thermographic recordings of the stimulation site and on subject feedback. The laser stimulations were optimized in relation nociceptive activation of A-δ pain fibers. To increase the probability of a successful validation, the model was optimized in relation to the specific stimulation site on the volar forearm and in relation to critical observations derived from the previous semester project. After the optimizations, the experimental data was compared to model data to get a basis for model adjustment as part of the validation. The modelled data and experimental data was compared through temporal heat spot profiles at (0,0) (5,0) and (10,0) [mm] over a period of 7.5 [s] from stimulation onset.
The model was successfully validated for the CO2 laser. Validation of the Nd: YAP laser was not successful, and needs therefore more adjustments to be able to be validated. Additionally, the stimulation intensities used in the experiment for activations of A-δ pain threshold, must be compared to relevant literature to ensure their reliability.