Time-resolved reflectance spectroscopy in turbid tissues

Monte Carlo simulations illustrate how various absorption and scattering coefficients ( mu /sub a/ and mu /sub s/, respectively) influence time-dependent reflectance R(t) from a semi-infinite homogeneous turbid tissue following a pulse of narrow-beam irradiation. The value of mu /sub a/ in cm/sup -1/ can be obtained from measurements of R(t) after the first 20-200 ps (depending on mu /sub s/) following a pulse, using an expression that contains n, the tissue refractive index and c, the in vacuo speed of light. Data obtained during the first 20-200 ps do not conform to this expression or to diffusion theory. Monte Carlo simulations allow study of the early R(t) behavior. The volume of tissue involved in a measurement is specified by a volume radius r that approximately equals (6Dtc/n)/sup 1/2/, where t is the time of measurement and D is the optical diffusion constant. At 50 ps and typical values of mu /sub s/=100 cm/sup -1/ and anisotropy equal to 0.9, r equals 5 mm.<
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