Measuring the Optical Properties of Human Muscle Tissue using Time-of-Flight Spectroscopy in the Near Infrared

Abstract: Optical spectroscopy is commonly used in technology and science today. The presence and concentration of a substance can be determined by its spectral signature, the typical wavelengths that are absorbed (or emitted) by the atoms or molecules. Standard absorption spectroscopy requires that the substance is clear and that the optical path-length is known to obtain quantitative information. Unfortunately in many materials, such as human tissue or pharmaceutical tablets, there are also a strong scattering of light which complicates measurements. The pathlength of the light is now no longer known and the intensity of the detected light can in many cases be more affected by high scattering than by the absorption values. One method to separate these two values are photon Time-of-Flight Spectroscopy (TOFS). By sending many short light pulses through a sample, and recording the time for a single photon to arrive at our detector for each pulse, we can build a histogram that represents the broadening of the light pulse that is determined by both the scattering and absorption. By tting computer generated theoretical curves against the recorded histogram, we can extract the values for absorption and scattering from the curve with the best fit. The Biophotonics group at the department of Physics at Lund University has implemented a system that can deliver continuous absorption/ scattering spectra from 500 nm to 1400 nm. It uses a broadband laser as source and tunable optical filters to select narrow wavelength bands for measurements. In this thesis we expanded the set-up with a new laser source and new filters. We performed tests comparing measurements using the new filter with results from the old system. We could show that the new lter gave better results, due to the sharper line width of the output light pulse. Study were also conducted on the absorption and scattering spectra of muscle tissue in the near infrared, between 650 nm and 1350 nm, probing the lower left arm of a volunteer. This was performed by positioning two optical fibres against the skin, sending light in with one and measuring the scattered light arriving at the second. The results are shown to be comparable to other studies done for wavelengths up to 1000 nm, and give new data up to 1350 nm that is consistent with the properties of the main absorbing components in this range, lipids and water. One uncertainty that appear in the results are due to the compression of the tissue by the fibres, this is something that should be addressed in repeat measurement that where not possible to perform in this study.