Modelling of laser irradiation of human tissue

Dye lasers have been used for the treatment of vascular lesions for some years. In fact the use of the technology has become the treatment of choice in the majority of cases. Indeed the technology often offers the only option available to the patient. The technology has been a great success story with many thousands of patients helped in ways previously impossible. The basic principle involved is that of `Selective Photothermolysis', where a carefully selected wavelength is absorbed in a target chromophore beneath the surface of tissue in order to apply thermal energy in a predefined therapeutically beneficial manner. In the case of vascular lesions the chromophore is the oxyhaemoglobin in the blood. The therapeutic benefit is the permanent removal of the target blood vessel. At the outset of this program the aim was to overcome this techniques' shortcomings, i.e. to reduce the number of treatments needed to remove a particular lesion, and to facilitate the treatment of pale stains which previously had become stubbornly resistant to the procedure. To a large extent these aims have been achieved. The program included theoretical and computer modeling analysis leading to predictive design. The design was then implemented leading to completely new laser parametric output. There followed laboratory trials with histological studies leading to full clinical trials. The model is based on a two-step process. In the first stage, Monte-Carlo simulation is used to determine how the incident laser radiation applied to a port-wine stain is distributed throughout the various skin layers. The data obtained is subsequently used in a thermal solver to determine the temperature distribution