Light scattering and absorption in tissue - models and measurements
Abstract: In this work a number of theoretical models, describing light propagation in matter, have been applied to and developed for the examination of tissue. The aim was to model the light scattering and absorption in tissue in order to improve the understanding of underlying mechanisms of laser-based diagnostics and treatment modalities utilised for cancer and cardiovascular diseases. The models studied ranged from the simplest Beer-Lambert law, assuming a plain exponential behaviour of the light transport, to solutions of Maxwell's fundamental equations for the scattering of electromagnetic waves. The latter approach was specifically used for computations of light scattering by red blood cell volume-equivalent spheroids by applying the numerical T-matrix formalism. Furthermore, the adding-doubling method, based on the radiative transport equation for multiple scattering, as well as the stochastic Monte Carlo approach, describing the light transport as a random walk process of photons, were employed to model light propagation in dense tissue. The advantages and disadvantages of each model are discussed by exemplifying which applications they are useful for and valid in. Light scattering and absorption in tissue were also studied in practice. The absorption and scattering characteristics governing the macroscopic light propagation were determined in vitro in terms of the absorption and scattering coefficients as well as the g-factor, employing different integrating-sphere techniques. Changes in these fundamental optical properties were monitored for tissue being exposed to laser-based treatment modalities, such as photodynamic therapy (PDT) and continuous or pulsed thermotherapy. The observed influence on the scattering properties and the manifest increase in the absorption coefficient could be related to mainly morphological and biochemical changes in the blood and/or microvascular damage in the tissue. The latter was further confirmed by an imaging technique showing changes in laser-Doppler signals from moving red blood cells in conjunction with PDT, indicating a local increase in tissue perfusion. Differences in tissue characteristics were monitored in vivo, in order to distinguish diseased from healthy tissue alone, or in combination with photosensitive agents, employing laser-induced fluorescence (LIF) or near infrared (NIR) spectroscopy. Malignancies in the skin and in the oesophagus could be identified utilising the tumour selective agents ALA and Photofrin respectively, in conjunction with LIF. It was also shown to be possible to spectroscopically distinguish fibrous and fatty heart tissue from healthy myocardium in vitro using either LIF or NIR spectroscopy, if combined with a powerful analysis method such as Principal Component Analysis (PCA).
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