<p> The rapid development of the use of lasers in therapeutic and diagnostic medicine in the past few years has generated interest in measuring the optical properties of tissue. In particular, the development of photodynamic therapy (PDT) has necessitated studies of the optical properties of tissue at wavelengths around 630nm, this being the wavelength at which the photosensitizer commonly used in PDT, namely dihematoporphyrin ether (DHE), is normally activated.</p> <p> The control of the volume of tissue from which information about the interaction coefficients of the tissue is obtained is an important problem in diffuse
reflectance spectroscopy and other applications of light, because it is critical to understanding which tissue volumes are sampled by the injected photons that eventually are re-emitted. This report describes a simple model that predicts the parameters that control the volume of tissue interrogated by photons during reflectance spectroscopy.</p> <p> In optical fiber based diffuse reflectance spectroscopy, incident radiation is applied at one point on a tissue surface and collected at another point, a radial distance, r, away. Information about the light multiply scattered by the tissue is used to deduce optical scattering and absorption coefficients of the tissue. In this report both steady state and pulse techniques are studied. In the steady state method, the spatial dependence of the backscattered light is the measured
quantity, while the pulse technique uses the temporal broadening of a picosecond
(ps) pulse to determine the interaction coefficients.</p> <p> The relative contribution of a volume element of tissue to the observed signal depends on its location, the measurement geometry and the optical properties of the tissue. Knowledge of this dependence would allow some control of the volume interrogated by reflectance spectroscopy, and would provide insight into the influence of inhomogeneities.</p> <p> In the work reported here a simple diffusion model of light propagation in tissue based on the Boltzmann radiative transfer equation has been used to derive mathematical expressions for the relative time spent by photons in a given tissue volume element. Using optical interaction coefficients typical of mammalian soft tissues, results are presented for both steady state and pulse irradiation in
both semi-infinite and infinite media.</p> <p> The residency time depth profile calculated by this model for index matched and zero fluence boundary conditions has the same shape as that predicted by Weiss (1989), who used a somewhat different model based on a 3-dimensional random walk theory. These profiles are characterized by a build-up region near
the surface and exponential fall far away from the surface in the 'diffusion region'. The influence of the absorption coefficient μa and the fiber separation on the residency time as predicted by this model is in good agreement to that predicted by the random-walk theory (i.e the depth-profile of the residency time tends to sharpen as the absorption coefficient increases. This is attributed to the fact that long trajectories are less likely with large absorption probabilities. As well, the greater the fiber separation, the wider and flatter the depth distribution of the residency time. This is because photons that reach the surface at greater r values have, in general, migrated farther from the immediate vicinity of the source
and detector and hence have sampled a larger volume of tissue). All the integrations in this report were performed numerically using the IMSL/LIB on the Microvax computer system in the Hamilton Regional Cancer Center. The adequacy of this numerical integration was tested and was found to be good.</p> <p> This model therefore suggests that during diffuse reflectance spectroscopy, the volume sampled by re-emitted photons can be controlled by changing parameters such as the fiber separation (in both steady state and time-resolved
techniques) and the detection time (in the time-resolved method).</p> / Thesis / Master of Science (MSc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/22624 |
| Date | 08 1900 |
| Creators | Osei, Ernest Kwaku |
| Contributors | Patterson, M. S., Health and Radiation Physics |
| Source Sets | McMaster University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
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