TIME-DEPENDENT OPTICAL SPECTROSCOPY AND IMAGING FOR BIOMEDICAL APPLICATIONS

While optical spectroscopy and imaging are essential tools in science and engineering, their application in living tissue is complicated by multiple scattering of light. In spectroscopy, this scattering causes uncertainty in the pathlength traveled by photons in the tissue, while images suffer reduced resolution and contrast. Picosecond light sources and fast detectors have made it possible to address these Problems by direct measurement of the photon time-of-flight. Diffusion models of light propagation can be used to relate the measured distribution of Photon transit times to the scattering and absorption coefficients of the tissue. A separate measurement of absorption and scattering adds a new dimension to the quantification of functionally related optical parameters in tissue. The advantages of absolute absorption measurement are demonstrated for two problems: determination of hemoglobin oxygenation in tissue and in vivo measurement of the uptake of an exogenous chromophore such as a photosensitizer. Optical imaging may also be improved by the elimination of multiply scattered photons (based on their longer time-of-flight) or by selective detection of photons arriving from a given region of the tissue (based on mean time-of-flight). The potential advantages of these techniques are discussed and illustrated with experimental data. Spectroscopy and imaging can also be performed in the frequency domain by using intensity modulated light and measuring the phase and modulation of the detected light. The principle of this technique and its relation to time-domain methods are explained.