Measurement and modelling of the optical properties of human tissue in the near infrared

There is an increasing interest in the use of optical methods in medicine. Areas of interest are optical methods for patient monitoring, screening, photodynamic therapy and laser surgery. The main area of clinical application for the work described in this thesis lies in the monitoring of metabolism in the brain using optical spectroscopy, and the localisation of areas of metabolic deficiency. The objective of this study was firstly to determine the optical properties of brain tissue that govern the transport of light through tissue and secondly to develop a model for light transport in tissue that makes it possible to predict various parameters of clinical interest. These parameters are the effective optical pathlength for light transmitted through tissue, needed for the purpose of quantative spectroscopy, and the light distribution in tissue produced by an external light source in order to determine the deposited dose and for the study of imaging through tissue. To this end, a number of instruments have been designed and developed to measure the absorption and scattering properties of brain tissue as a function of wavelength. The effect of gestation on these properties is investigated. To study light transport in tissue, a Monte Carlo model has been developed. This model gives a full 3-D simulation of light transport, and takes into account specular reflection and refraction at the tissue boundaries. Objects of differing absorption and scattering properties and of arbitrary shape can be included. To validate the model, predictions have been tested against reliable analytical data. A number of applications of the model, together with results from experiments are presented