Laser Spectroscopy in Scattering Media for Biological and Medical Applications

Sammanfattning: Tunable diode laser absorption spectroscopy can be utilised for gas detection in biological and medical applications. The common approach is to analyse transmitted light according to the Beer-Lambertian law. Normally, the light has a well-defined path length through the gas sample since it can be assumed that it has negligible scattering properties. Samples which, by contrast, have strong scattering properties, meaning that light is extensively scattered, are investigated in this work. In such materials, the light travels an undefined path through the sample. Thus, light propagation in scattering media must be understood. The aim of this work has been to combine knowledge from two fields, namely, tunable diode laser absorption spectroscopy and light propagation in scattering media, in order to extract information of gas contents within porous samples for biological and medical applications. Porous materials containing free gas are common in our everyday environment. Examples are wood and other construction materials, and fruit and many other food stuffs. The human paranasal sinuses located within the human skull are air-filled cavities surrounded by strongly scattering tissue. Thus, light transport through the cavities becomes complicated. Measurements of molecular oxygen at 760 nm and water vapour at 935 nm were successfully performed within the sinuses on volunteers, using tunable diode laser absorption spectroscopy combined with wavelength modulation techniques. To fully understand the signals recorded, measurements on phantoms, representing human sinuses, and Monte Carlo simulations, were first carried out. Gas measurements based on the same technique have also been demonstrated in pharmaceutical tablets to investigate the potential for extracting information on the porosity of the tablets. This is of importance since the porosity is directly related to the drug delivery time to the body after administration. To understand the drying process of wood, the same type of measurements were carried out on samples of wood. The results showed that it is possible to extract additional information on the drying process, not provided by the commercial techniques available. In horticultural produce different gases have a significant role in the respiration process. Promising gas measurements have been performed on fruit, in particular apples, to study the potential of developing a system based on tunable diode lasers to measure the gas content within the product when different packaging techniques are employed. From the experience with the setups and the various measurements performed in the laboratory, a mobile system based on two pigtailed diode lasers was developed, making measurements outside the laboratory possible. Based on the previous promising results, a clinical trial at the Lund University Hospital in cooperation with the Ear, Nose and Throat Clinic, the Radiology Clinic, and the Oncology Clinic has been initiated to investigate the human sinuses in healthy and diseased sinuses. Preliminary results are presented in this thesis.