Study On Cutaneous Melanin Near-Infrared Autofluorescence And Raman Spectrum Properties

A number of diagnostic techniques, based on optical imaging and spectroscopy, have been developed over the years to provide objective evaluation of the skin both in health and in disease. In this Medical Physics PhD project, a set of fluorescence imaging appliances, spectral measurement systems and theoretical models was built to study the fluorescence properties of cutaneous melanin and human skin under Near-infrared laser excitation.Experimental studies were completed in terms of macroscopic, in vivo fluorescence imaging of pigmented skin lesion under continuous laser exposure. A NIR fluorescence imaging system has been built to observe and characterize the melanin distribution in human skin, with particular utility for achieving a better clinically feasibility and clearer image. And also, in terms of microscopic ex vivo fluorophores distribution and spectral difference, we present a simple method for building an economical and modular near infrared Raman microspectroscopy system that combines a microscope with a Raman spectrometer using an optical fibre bundle. Several interesting physical phenomena were discovered, after two new methods have been developed for deriving auto fluorescence information of pigmented skin lesion and normal skin from in vivo and ex vivo measurement. The most interesting finding was that, under near-infrared laser (785nm) continuous excitation, pigmented human skin lesions emitted higher NIR fluorescence than the surrounding normal skin due to the presentation of higher concentrations of cutaneous melanin within the lesions.A seven-layer skin optical model was developed based on the structural anatomy of skin, the published optical parameters of different skin layer, melanin and blood, and measured microscopic skin fluorophore distribution. With this skin model, the autofluorescence spectra of the human skin and pigmented skin lesion in near-infrared spectral range were reconstructed using Monte Carlo method. It provides detailed information on the light-tissue interactions of scattering, absorption and anisotropy propagation of regenerated autofluorescence photons in the skin tissue. The theoretical modeling, which was performed to explain the measured spectral differences and applied in other non-invasive optical studies of the normal skin in Near Infrared spectral range, unified the microscopic ex vivo properties and macroscopic in vivo measurement. It was shown that the fluorophore detection efficient could be used to estimate the fractional contributions of different skin layers to the observed in vivo skin autofluorescence. And also, it could be observed that the content of melanin contributes skin autofluorescence in NIR spectral range, which provides a theoretical explanation of in vivo experimental results.In this project, two technical innovations were achieved. For the first time, an optimized in vivo NIR auto-fluorescence imaging was built and used in clinical dermatology usage. With a special system setup of microscopic Raman system, we present the microscopic auto-fluorescence image of skin tissue section for the first time, too.