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This project had two main objectives. The first of these was to design and construct a spectroscopic instrument to monitor small changes in optical transmission across an infant's head at several near infrared wavelengths resulting from changes in the cerebral oxygenation status. The overall attenuation of light by brain tissue is very high and is dominated by the scattering properties of the tissue. Hence a major requirement of the instrument was the ability to measure spectral changes at very low light levels. Once the instrument was available, the second objective was to convert the measured changes in optical transmission into changes in the concentration of the naturally occurring chromophores oxyhaemoglobin, deoxyhaemoglobin and oxidised and reduced cytochrome c oxidase. An important aspect of the work was that the chromophore concentration measurements should be quantified in non-arbitrary units. Medical Physics is, by its nature, highly interdisciplinary and this is reflected in the introductory chapter which briefly covers the clinical problems, the medical science background and the technical aspects of monitoring the cerebral oxygenation status of newborn infants. The second and third chapters examine those constituents of brain tissue which absorb and scatter light and how the complication of multiple scattering can be dealt with in performing quantitative spectroscopy. The fourth and fifth chapters describe the technical details of the instrument design and construction from the initial step of setting its design specifications to the final testing of its performance. The sixth chapter examines the absorption characteristics of the main chromophores of interest namely oxyhaemoglobin, deoxyhaemoglobin and the cytochrome enzymes of the respiratory chain within the brain cells. A discussion on the interpretation of the redox state of the respiratory enzymes in terms of the metabolic state of the brain is also included. The final chapter describes the data analysis methods, the measurement of optical pathlengths in scattering media and introduces a non-linear modification to the Beer-Lambert law which improves the accuracy of the spectroscopic measurements in highly scattering media. |
