Diffuse reflectance spectroscopy is a non-invasive technique for spectral analysis of light after its interaction with turbid samples. Biological tissues have, according to the chemical composition and structure, certain optical properties that affect the way in which the light is absorbed and scattered. A result of those interactions is a diffuse reflectance spectrum, from which tissue properties can be extracted by using appropriate methods. In order to facilitate easy access to a range of different biomedical samples we often use small and lightweight optical probes. These allow remote acquisition of spectra, which is especially useful in operating rooms, where the entry of diagnostic and research equipment is limited due to hygienic reasons. In the process of acquiring diffuse reflectance spectra, the operator usually presses the probe against the tissue to improve the optical coupling and to eliminate specular reflections from the tissue. The light contact pressure applied by the operator may lead to structural changes in the tissue and thus can alter the acquired diffuse reflectance spectra. The repeatability of diffuse reflectance measurements therefore largely depend on the operator's ability to apply the same level of contact pressure. To evaluate the measurement repeatability and the influence of the contact pressure on diffuse reflectance spectra, it is crucial to measure the intensity of the applied contact pressure. In vivo assessment of the contact pressure repeatability is a difficult task. Consequently, previous studies have used synthetic or natural phantoms instead of the tissue. The main drawback of these studies is the lack of critical comparison between the mechanical properties of phantoms and the studied tissue. In this thesis, we found that the mechanical properties of tissues and phantoms significantly affect the intensity of contact pressure applied by the operator. Therefore, to assess the repeatability of the conducted measurements, the mechanical properties of phantoms must reflect the properties of the studied tissue. Besides the assessment of the light contact pressure intensity during the measurements, it
is highly important to assess how the applied contact pressure affects diffuse reflectance spectra. Previous studies have developed different techniques to measure the contact pressure during the acquisition of diffuse reflectance spectra. In general, the employed techniques provide inaccurate contact pressure measurements that are inadequate for a detailed study on the effects of the contact pressure on diffuse reflectance spectra. Consequently, we propose a novel fully automated system for controlled application of the contact pressure and acquisition of diffuse reflectance spectra. A high sampling rate of the automated system offers additional insight into the time lapse of the changes. In this thesis, we studied the influence of the contact pressure on diffuse reflectance spectra of skin in vivo. In biomedicine, skin is frequently studied by diffuse reflectance spectroscopy as it is easily accessible and complex enough to contain all important tissue chromophores
(oxyhemoglobin, deoksyhemoglobin, water, lipids and melanin).
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