Ex vivo investigation of tissue optical properties using an optical fibre sensor

Biomedical research has become a strong growing sector in recent years. Moreover the interdisciplinary background involves novel possibilities and measurement techniques. Light propagation in turbid media like human tissue is a central aspect to many medical and biomedical applications. This is a very complex process and depends on parameters, which are called optical properties. The spatial distribution of light is determined by those optical properties. A major difficulty in this field can be explained by the forward and inverse problem. There are several theories and approximations that are used to describe the propagation of light in scattering media. Those approaches are often applied to get qualitative results that can be helpful in fields like laser surgery, photodynamic therapy and diagnostic purposes. This thesis presents the development of an optical fibre based system that uses diffuse reflectance data to determine the optical properties of tissue. The optical properties obtained are intended to be used as indicators in the characterization of burn wounds. Since the absorption of light by tissue has a minimum in the therapeutic window, which is situated in the visible to near-infrared region (600-1000 nm), this wavelength region is of special interest in reflectance measurements. The development of the sensor includes designing a sensor head with an appropriate geometry, electronics providing light sources and detecting the reflectance as well as the creation of a procedure to determine the optical properties from the detected reflectances. To solve the inverse problem and extract the tissue optical properties from the recorded reflectances a neural network was trained with Monte Carlo simulation data. The neural network has been improved several times until an appropriate range of optical properties was achieved. The RMS error obtained within the range of absorption of the training data set (0.05 – 4.5 cm-1) was 1.26 and 2.2 % for the absorption and scattering coefficient, respectively. For an extended range of absorption (0.001 – 5.5 cm-1) the RMS error slightly increases to 2.4 and 4 % for the absorption and scattering coefficient, respectively. Measurement series performed with the calibrated sensor include in vivo measurements on human test subjects with different skin colour and ex vivo measurements on healthy and burned skin of pigs. The results obtained agree very well with values from the literature. This method of optical property determination is in vivo and offers instantaneous results.