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Tissue optics is a rapidly expanding field of great interest to those involved in the development of optical medical technologies. In the present review both strongly (multiple) scattering tissues, such as skin, brain tissues, and vessel walls, and weakly scattering high-transparent tissues, such as eye tissues (cornea and lens), are discussed. For the former, radiation transport theory or Monte Carlo simulation are used to describe the propagation of light (laser beams). For weakly scattering ordered tissues, ensembles of close-packed Rayleigh or Mie scatterers are employed. Methods for solving the inverse problem of finding biotissue optical parameters are discussed. The propagation of photon-density diffusion waves in scattering and absorbing media is analyzed and the prospects of these waves for optical tomography are discussed. Polarization phenomena in both strongly and weakly scattering biotissues are discussed.

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We modified the diffusion approximation of the time-dependent radiative transfer equation to account for a finite scattering delay time. Under the usual assumptions of the diffusion approximation, the effect of the scattering delay leads to a simple renormalization of the light velocity that appears in the diffusion equation. Accuracy of the model was evaluated by comparison with Monte Carlo simulations in the frequency domain for a semi-infinite geometry. A good agreement is demonstrated for both matched and mismatched boundary conditions when the distance from the source is sufficiently large. The modified diffusion model predicts that the neglect of the scattering delay when the optical properties of the turbid material are derived from normalized frequency- or time-domain measurements should result in an underestimation of the absorption coefficient and an overestimation of the transport coefficient. These observations are consistent with the published experimental data.

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