Time-resolved reflectance spectroscopy in turbid tissues

Monte Carlo simulations illustrate how various absorption and scattering coefficients ( mu _a and mu _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 _a in cm ^-1 can be obtained from measurements of R(t) after the first 20-200 ps (depending on mu _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) ¹2/, where t is the time of measurement and D is the optical diffusion constant. At 50 ps and typical values of mu _s=100 cm ^-1 and anisotropy equal to 0.9, r equals 5 mm.