MODEL-BASED PLANNING AND REAL-TIME MONITORING FOR LASER THERMAL THERAPY IN OPHTHALMOLOGY

Severe vision impairment and blindness represent one of the major medical challenges of the next decades, especially in western countries due to the population ageing. Today, the World Health Organization estimates that at least one billion people worldwide suffer from moderate or severe distance vision impairment or blindness due to unaddressed refractive error (88.4 million), cataract (94 million), age-related macular degeneration (8 million), glaucoma (7.7 million) and diabetic retinopathy (3.9 million). This numbers are expected to worsen considering that the prevalence of severe impairment leading to blindness is larger in over 50-year-old people. The social impact of such diseases is very high because it limits working capacities and, in the case of elder adults, contributes to social isolation, difficulty in walking, with the consequence of higher risk of falls and fractures. At present there are no definitive therapies capable of reversing these eye diseases; therefore, the treatments are aimed at stopping or at least slowing their evolution. One of the most promising approaches is the use of lasers to provide thermal therapies, especially since the development of micro-pulse technology, which has opened a range of new treatments. However, the widespread of these techniques is still limited for several reasons, one of the most important being the need for accurate predictions and measurements of the induced temperature to optimize the treatment outcome. Therapy planning methods, which can estimate the thermal response of the target area, are critical in adjusting laser power and treatment duration. Moreover, real-time monitoring systems are key to directly control the temperature induced during the thermal processes. The thesis investigates the optimisation of thermal therapies based upon tissue absorption of laser radiation to generate a localised temperature increase. The purpose of this thesis is to study a method for estimating and controlling the temperature distribution during laser treatment processes, helping the ophthalmologist in planning, and monitoring thermal therapies. This has required the development of a suitable parametric full-eye 3D model, analyzed with COMSOL Multiphysics finite element software, to provide insights about the eye-laser interaction in view of proposing personalized treatments for each patient. The model is composed of two parts: the light-tissue interaction and the heat diffusion in tissues. In particular, the thesis work has developed a “heat pulse method” to recover the thermal parameters of biological tissue. Relying on the intrinsic correlation between biological properties and thermal outcomes, the response to a short heat pulse, not intense enough to cause cell damage, is fitted with the analytical solution of the bioheat transfer equation. The temperature is measured through all-optical fiber sensors based on fiber Bragg gratings. The method has been validated first through numerical simulations and the impact of different types of uncertainties has been analyzed. Experimental tests have been carried out on phantoms and ex-vivo tissue to demonstrate the viability of the proposed approach.