[Literature Review]

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Acknowledgement

I would take this opportunity to thank my research supervisor, family and friends for their support and guidance without which this research would not have been possible (Bakker, 2006,, 95).

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Literature Review

The optical properties of the skin are of interest because of their effect on non-invasive optical measurements of deeper tissue, and also because of the possibility of using the skin as an accessible organ for determining the constituents of blood in vivo. The difficulties involved in determining the optical properties of tissue in vivo are well known: advanced instrumentation is required (time or phase resolved spectroscopy) and the volume of tissue probed is hard to define. However, it has been shown that the differential pathlength of light in tissue measured post-mortem is similar to that measured in vivo. Tissue excision and storage may produce changes in the optical properties due to blood drainage but these effects are predictable as a loss of absorption coefficient, and ex vivo measurements remain a good method of isolating particular tissue types. (Weitzenblum & Fraisse 2010 148-152)

The purpose of the method described here is to obtain the average optical properties of small volumes of tissue (less than 0.5 ml). These optical properties are intended to be used in mathematical models of light transport in larger heterogeneous tissue volumes such as the head, limbs or abdomen for optical imaging and spectroscopy applications. Mathematical models of large complex tissue volumes tend to use finite element methods and approximations to the radiation transport equation. An accuracy of 10% in optical properties is typically sufficient to evaluate the effects of light transport across tissue boundaries. (Vassa & Campbell 2010 878-881) This degree of accuracy is also consistent with 'normal' variations of absorption coefficient caused by blood volume variations and what appears to be the natural variance of scattering coefficient seen in tissues such as the brain.

Tissue optical properties are usually described in terms of three parameters: the scattering coefficient _s , absorption coefficient _a and scattering anisotropy factor g. Optical measurements on tissue, such as reflectance, are the result of the combined effect of these properties making it necessary to understand light transport. The light transport model compares its measurement predictions with experimental temporal, frequency and/or spatial measurements in order to recover these optical properties. Of these, steady-state intensity measurements are the simplest to perform. The radiative transport equation is perhaps the most successful for predicting the flux for a particular geometry and set of optical coefficients. (Tibbles & Edelsberg2008 1642-1648) However, complications can occur at boundaries and not all geometries are trivial to compute analytically. Alternatively, stochastic models such as the Monte Carlo and random walk models may be ...