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The opportunity to monitor the onset and progression of disease may be enabled by the smart implementation of biomedical optical engineering approaches to monitor tissue biochemistry. Uncorrected, these biochemical disturbances manifest themselves in microscopic structural changes which lead to gross pathophysiology and symptoms by which the disease is outwardly identified. Biomedical optical engineering techniques offer the opportunity to detect these biochemical changes and provide diagnostic information at earlier stages in the disease process, enabling greater efficacy of therapeutic intervention. In this paper, we concentrate on the fluorescence and phosphorescence lifetime spectroscopy due to advantages these techniques have to offer in tissues. The development of fluorescent and phosphorescent dyes which excite and re-emit in the near-infrared wavelength region promises the capacity for non- invasive biochemical sensing in tissues. Fluorescence intensity and fluorescence lifetime spectroscopies are established methods by which a fluorophore can provide sensing in dilute, non-scattering samples. However, fluorescence intensity or fluorescence lifetime spectroscopy in tissues or other scattering media is a complex problem. In order to extract the intrinsic fluorescence intensity for identification of the fluorophore concentration and yield, a priori information about tissue absorption and scattering must be obtained or assumed. Yet in tissues, the optical properties of absorption and scattering are highly variable. Nonetheless when successful, fluorescence intensity spectroscopy enables determination of the product of fluorescent yield and fluorophore concentration. In contrast to fluorescence intensity spectroscopy, fluorescence-lifetime tissue spectroscopy offers the ability to directly determine metabolite concentration independently of the concentration of fluorophore, whether it is endogenous or exogenous. Instead of monitoring the fluorescent intensity due to the re- emission process, the 'lifetime' or stability of the photon-activated fluorophore is measured. The lifetime of the activated state is defined as the mean time between absorption of the excitation photon and re-emission of a fluorescent photon. Typically, endogenous fluorophores have lifetimes on the order of nanoseconds while exogenous compounds have lifetimes ranging from sub nanosecond to milliseconds.
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