Optical spectroscopy of turbid media: time-domain measurements and accelerated Monte Carlo modelling

Abstract: Optical spectroscopy is a versatile and powerful tool to probe translucent materials. In this work, the focus is on characterization of strongly scattering (turbid) materials by means of time-of-flight spectroscopy (TOFS). Instrumentation and modelling aspects of TOFS were investigated and improved, enabling significantly more accurate spectroscopic measurements. It was shown that the commonly used diffusion theory fails to accurately describe time-domain light propagation in e.g. tissue. A fully scalable Monte Carlo (MC) scheme (WMC) was developed, enabling MC to replace diffusion models in TOFS data evaluation. Consequently, the accuracy and capabilities of TOFS were significantly improved. Graphics processing units (GPUs) were introduced for acceleration of MC simulations in general, resulting in three orders of magnitude speedup. It was shown that proper utilization of the capabilities of modern GPUs allow similar performance, even for more complex problems. TOFS in combination with WMC was used in in vivo interstitial spectroscopy of the human prostate, demonstrating the need for better modelling in many clinical applications. To aid future interstitial in vivo measurements, a single-fibre TOFS system was developed and demonstrated in phantom experiments. Turning to investigations of pharmaceutical samples, a time-of-flight spectrometer, covering the 650-1400 nm spectral range, was developed, enabling TOFS for vibrational spectroscopy of solids. In spatially resolved TOFS measurements, compaction induced anisotropic light diffusion was observed. This is of great importance for the application of model-based optical spectroscopic tech- niques and may, in addition, provide important information about the sample microstructure. Furthermore, TOFS was used together with laser-based gas sensing to probe porous solids. Although a need for better models was revealed, excellent correlation between optical and actual porosity was demonstrated.