Multi-modal assessment of light transport through biological tissue

The advent of biomedical optics and understanding of light transport through tissues has gathered enormous popularity since the works of Britton Chance, Steven Jacques and Valery Tuchin. Understanding light transport has allowed diagnoses, and in vivo and noncontact assessment of tissue. This thesis provides an interdisciplinary approach to using various modalities for optically interacting with biological tissue. The methods focus on lowcost, non-contact, non-invasive and/or simple methods to assess biological tissue. Ultrasound imaging, which is a key radiological imaging tool in today’s hospitals, is combined with powerful ray tracing tools to provide a quantitative assessment of tissue. The thesis discusses four individual studies, linked through either their methods or applications. Specifically, the biological tissues that light interacts within this thesis are skin and its layers, muscle, bone and blood. Skin-safe lasers are used in the studies to interact with participants through simulations and experiments. Through the course of this research, I investigate the optical compatibility of human skin with synthetic skin samples known as human skin equivalents (HSEs)1 . The result is a novel assessment combining tissue engineering and biomedical optics. Secondly, a simulated analysis of light interaction with a two-layer model was subsequently analysed in a study to look for anaemic blood condition markers in the reflectance and fluence of photons. This study resulted in a unique assessment of light transport through two-layer models. The models accommodate melanin and haemoglobin concentrations in the layers of the skin, thereby accounting for all skin types and healthy and anaemic blood perfusion in the dermal layer. In a third study, the understanding and consideration of the influence of melanin and haemoglobin in the skin layers are extended to developing full-finger models. The full-finger models are based on high-frequency ultrasound image data2 . The optical models were assessed in visible and near-infrared wavelengths using Monte Carlo simulations. This provides a method to assess tissue damage before treatments such as photodynamic therapy. Finally, an image processing exercise to identify and monitor vascular activity was undertaken in the fourth study of this thesis. The vascular activity was imaged and monitored using a simple transmission-based experimental strategy and off-theshelf equipment. Vascular activity analogous to heart rate was successfully monitored for the participants of the study, accounting for motion of the finger in a non-contact experimental and processing workflow