The optimum laser treatment of port-wine stains
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A port-wine stain is a visually obtrusive cosmetic defect of the skin. There is a desire for a removal process that is effective with a minimal risk for the patient. The laser treatment of port-wine stains has been the most effective method so far. Further improvements in this technique may be attained by adjustment of the treatment parameters. In this thesis, we numerically model the treatment process. The passage of the laser light through skin is simulated with the Monte Carlo method. Our simulation is superior to those of other workers as we use a more realistic model of skin with the minimum of assumptions. These results are used to predict the temperature change in skin. The effect of conduction is included, so we can calculate the ideal illumination time. The illumination time should be in the range of 1-10 ms. For vessels with a diameter of 50 μm, the ideal illumination time is close to 4 ms. The calculations show that the fluence should be around 3 J /cm². When the vessel diameter is larger, a longer illumination time and a higher fluence will be necessary. To provide these illumination times, a computer controlled scanning device was designed and built. The light from a copper vapour laser is scanned over the portwine stain to form a rasterscan like pattern on the lesion. The user determines what part of the lesion is treated, so the light never scans over normal skin. Following treatment with illumination times of 4 ms and a fluence of 10 J/cm², the skin blanches, and then turns red as a result of erythema. The blanching response is not related to non-selective damage within the skin but is attributed to constriction of the blood filled vessels. We describe the probable mechanism that causes the constriction. Our calculations predict that a fluence that is approximately one fifth of the fluence that is used in the clinic. We discuss the validity of the optical parameters that were used in our calculations. The effect of changing these parameters by a factor of five is shown. One possible explanation is that the model of skin used does not include the effect of the superficial layers of the epidermis. We suggest that these layers prevent some of the incident beam from reaching the vessels below the laser beam. The time before the blanching response commences is calculated from measurements of the quantity of light backscattered from the skin. The blanching occurs between 8 and 13 ms after the spot is incident on a region of skin. This is 4 ms, or more, after sufficient energy has been deposited in the skin to cause the desired response.