Modelling the laser treatment of vascular lesions
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis investigates four aspects of the laser treatment of vascular lesions. The first is the use of a copper vapour laser at a wavelength of 578 nm as a means for removal of these lesions. The second is an evaluation of the physiological conditions that lead to the removal of the ectatic vessels. The third is a numerical model that predicts the optimal treatment parameters. The fourth is an evaluation of the current treatment protocols. A copper vapour laser at a wavelength of 578 nm has been used to treat over 500 vascular lesions. The chosen technique has been constrained by the available technology, but where possible has been based on histology, clinical investigation, and theoretical modelling of the physics of light interaction with vascular lesions. The treatment has been successful in the vast majority of cases and is at least as effacious as other reported techniques. The results of a patient survey indicated an emphatic approval of the treatment, with most patients feeling their treatment has resulted in a good change in appearance. Histology has established that specific damage to the ectatic vessels of these vascular lesions is achieved to a greater extent than with the wavelengths of the argon laser. This also illustrated histological differences to the pulsed dye lasers which produce purpura rather than blanching. A model of the physics of the copper vapour laser treatment suggests the damage to the ectatic vessels is thermal rather than mechanical, this is supported by the histology. The optimal damage criteria has yet to be established, but is likely to be somewhere between the part thermal/part mechanical damage caused by the pulsed dye lasers and the totally thermal damage caused by the copper vapour laser. A numerical model of the physics of illuminating ectatic blood vessels has been developed. Several criteria for optimal damage have been discussed. The model is used to produce optimal treatment parameters based on the damage criteria. These parameters are a wavelength of 577/578 nm, an irradiance of 400 to 3000 W/cm² and an illumination time of 1 to 10 ms. Additionally, this model has been applied to the present treatment protocols. The results suggest that there is likely to be vaporisation of haemoglobin and a minimum of thermal damage with the short pulses from the pulsed dye lasers. The longer illumination times of the continuous wave lasers are likely to restrict the thermal damage to vessel lumen surroundings. 585 nm has been modelled as an alternative wavelength for pulsed dye laser technique. This wavelength produces greater coagulation of individual vessels and coagulation deeper within the dermis than at 577 nm. Five papers on this work have been accepted for publication and are included in the appendices. Other parts of the work are discussed within the main body of this thesis.