Near IR time resolved spectroscopy has been studied for quantitative determination of absorbance in highly scattering medium such as tissue. When a very narrow optical pulse is incedent into a scattering medium, the detected pulse through the medium broadens and the temporal profile is closely related to the optical property of the scattering medium. The photon migration in highly scattering medium can be described with the diffusion theory. Thus the optical property of the scattering medium can be determined by analyzing the shape of the detected tamporal profile with the diffusion equation. We have developed the time resolved spectroscopy (TRS) system based on a time correlated single photon counting technique for data acquisition and diffusion theory for data analysis. Pulsed laser diodes with two different wavelengths are used as light sources in the system. The system size is compact and it can be moved around a laboratory or hospital easily. We demonstrated its use in vivo experiments. As a result, we were able to accurately determine absorber concentrations in a highly scattering medium and the result of these in vivo experiments indicate possible use of the system for quantitative clinical studies.
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