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Interaction of laser light with biological specimens
Author
Ng, Stanley Mong Seng
Supervisor
Chia, Teck Chee
Diong, Cheong Hoong
Abstract
Light distribution in tissues is a problem that has baffled researchers of laser therapy. With the increased popularity of photodynamic therapy to treat certain tumours, the understanding of the light distribution in tissues becomes increasingly important. It is known that the light distribution in a tissue is governed by the absorption coefficient, p,, scattering coefficient, p,, and the anisotropy coefficient, g. Work has been done by many researchers to develop models to map light distribution in tissues in order to better understand its behaviour. The methods studied in this thesis were the Beer's Law, Transport Theory, Diffusion Approximation and the Monte Carlo simulations. The conditions and shortcomings of each method were presented. Recent developments have also indicated that the Monte Carlo method is effective in simulating light transport in tissues. As light impinges on a biological media, it undergoes a region of anisotropic distribution near the surface characterised by an increased fluence due to the backscattering of the incident light. This incident light, after passing through a region of tissue, causes the light distribution to go into diffusion characterised by a gradual drop in light intensity as the tissue depth is increased.
The 2-dimensional light distribution in a tissue was compared with a Monte Carlo simulation based on tissue optical parameters of p=, p, and g. This light distribution study was extended to 3-dimensions in bovine, chicken and porcine tissues. It was found that light distribution in the tissues depends considerably on the scattering and absorption in the tissue giving rise to distinct distribution patterns with the spread of the light intensity increasing with beam diameter. Chicken tissues having the greatest scattering, showed the least drop in intensity with increasing lateral displacement from the incident beam at a fixed tissue thickness, while bovine tissues, having low scattering, showed the greatest drop. The degree of beam spread in the tissue is also found to depend on the abeldo of the light distribution. The dependence of light transport on tissue refractive index, n, is always ignored in light distribution studies. An investigation on refractive index was carried out together with a simple method of determining its value in tissues. Values of the refractive index in simple biological materials, such as coagulated egg white, as well as in selected human tissues were found and compared. In selected human tissues, the refractive index of tissues from different patients as well as different wavelengths were compared.
The 2-dimensional light distribution in a tissue was compared with a Monte Carlo simulation based on tissue optical parameters of p=, p, and g. This light distribution study was extended to 3-dimensions in bovine, chicken and porcine tissues. It was found that light distribution in the tissues depends considerably on the scattering and absorption in the tissue giving rise to distinct distribution patterns with the spread of the light intensity increasing with beam diameter. Chicken tissues having the greatest scattering, showed the least drop in intensity with increasing lateral displacement from the incident beam at a fixed tissue thickness, while bovine tissues, having low scattering, showed the greatest drop. The degree of beam spread in the tissue is also found to depend on the abeldo of the light distribution. The dependence of light transport on tissue refractive index, n, is always ignored in light distribution studies. An investigation on refractive index was carried out together with a simple method of determining its value in tissues. Values of the refractive index in simple biological materials, such as coagulated egg white, as well as in selected human tissues were found and compared. In selected human tissues, the refractive index of tissues from different patients as well as different wavelengths were compared.
Date Issued
1996
Call Number
QP82.2.L3 Ng
Date Submitted
1996