Options
Effects of laser radiation on surface tissues
Author
Poh, Deh Tong
Supervisor
Chia, Teck Chee
Gouw, Huat Suan
Abstract
Distribution of light in biological media has interested many researchers especially with the advancement in medical applications of laser. It is well known that light distribution in tissues is governed by the absorption coefficient, μa, scattering coefficient, μa, and the anisotropy factor g. Many researchers have already developed models to map the light distribution in tissues in order to understand its behaviour better.
The models that are studied here are Beer's Law, Kubelka-MunkTheory, Wave Theory, Radiative Transport Theory and the Diffusion Approximation. The shortcomings and conditions of each theory is presented. Due to the nature of surface tissues, i.e. being very thin and multilayered, coupled with the fact that a perfectly diffused state is not achievable in such thin layer, direct application of these theories on the surface tissues is not possible.
A method of measuring the lateral distribution profile on the epidermis is developed. Detailed theoretical considerations and calibrations of the system for both collimated and diffused light sources are conducted. Simultaneous measurements of lateral distribution profile for both the upper and lower surfaces are achieved. Samples used are from chickens, pigs, dogs and humans. The effects of beam diameter variations of incident light on lateral distribution is studied. It is found that when a collimated light source with a small beam diameter (-1.0 mm) is used, the distribution profiIe is closest to a normal distribution. With a larger beam diameter (-2.5 mm), the profile broadened at the centre but fell off more rapidly at larger lateral distances.
Comparison between a "high absorption" skin (Black Chicken) and "high scattering* skin (Normal Chicken) is conducted. The lateral distribution profile is also studied using a diffused light source. Across various beam diameters of incident light, the distribution profiles showed a lack of distinct shape when diffused light was used. In this case, the result show that the beam diameter has little effect in the profile whereas the intensity of the diffused light does. Experiments were also conducted with normal and carcinoma human rectal tissues. The difference in profile between normal and carcinoma tissues was observed. The results indicate a lower amount of absorption and a higher amount of scattering in carcinoma tissues.
A fundamental optical parameter, the refractive index is often used in light distribution studies. Its value, however, is usually assumed to be that of water. A simple yet accurate method for the measurement of refractive index on surface tissues is developed. The mismatch in refractive index between glass and tissue is exploited to achieve total internal reflection. Detailed theoretical studies of the measurement of the refractive index on skin is carried out. In-vitro experiments are conducted on porcine, chicken, and human abdominal epidermis. The refractive index of chicken and porcine epidermis at 633 nm are found to be 1.418 and 1.444 respectively.
Lasers of different wavelengths are used and correlations of refractive indices of abdominal epidermis between male and female, and across different age groups are studied. The results show that across all wavelengths studied, the refractive index of human abdominal epidermis is independent of age and sex.
In-vivo experiments are conducted on human palm epidermis at various wavelengths. Comparison of refractive index between Chinese and Indian palm epidermis is also performed. There is no significant difference between their refractive indices. Results also showed that the refractive indices of the palm are higher than the abdominal epidermis across all three wavelengths examined.
The models that are studied here are Beer's Law, Kubelka-MunkTheory, Wave Theory, Radiative Transport Theory and the Diffusion Approximation. The shortcomings and conditions of each theory is presented. Due to the nature of surface tissues, i.e. being very thin and multilayered, coupled with the fact that a perfectly diffused state is not achievable in such thin layer, direct application of these theories on the surface tissues is not possible.
A method of measuring the lateral distribution profile on the epidermis is developed. Detailed theoretical considerations and calibrations of the system for both collimated and diffused light sources are conducted. Simultaneous measurements of lateral distribution profile for both the upper and lower surfaces are achieved. Samples used are from chickens, pigs, dogs and humans. The effects of beam diameter variations of incident light on lateral distribution is studied. It is found that when a collimated light source with a small beam diameter (-1.0 mm) is used, the distribution profiIe is closest to a normal distribution. With a larger beam diameter (-2.5 mm), the profile broadened at the centre but fell off more rapidly at larger lateral distances.
Comparison between a "high absorption" skin (Black Chicken) and "high scattering* skin (Normal Chicken) is conducted. The lateral distribution profile is also studied using a diffused light source. Across various beam diameters of incident light, the distribution profiles showed a lack of distinct shape when diffused light was used. In this case, the result show that the beam diameter has little effect in the profile whereas the intensity of the diffused light does. Experiments were also conducted with normal and carcinoma human rectal tissues. The difference in profile between normal and carcinoma tissues was observed. The results indicate a lower amount of absorption and a higher amount of scattering in carcinoma tissues.
A fundamental optical parameter, the refractive index is often used in light distribution studies. Its value, however, is usually assumed to be that of water. A simple yet accurate method for the measurement of refractive index on surface tissues is developed. The mismatch in refractive index between glass and tissue is exploited to achieve total internal reflection. Detailed theoretical studies of the measurement of the refractive index on skin is carried out. In-vitro experiments are conducted on porcine, chicken, and human abdominal epidermis. The refractive index of chicken and porcine epidermis at 633 nm are found to be 1.418 and 1.444 respectively.
Lasers of different wavelengths are used and correlations of refractive indices of abdominal epidermis between male and female, and across different age groups are studied. The results show that across all wavelengths studied, the refractive index of human abdominal epidermis is independent of age and sex.
In-vivo experiments are conducted on human palm epidermis at various wavelengths. Comparison of refractive index between Chinese and Indian palm epidermis is also performed. There is no significant difference between their refractive indices. Results also showed that the refractive indices of the palm are higher than the abdominal epidermis across all three wavelengths examined.
Date Issued
1996
Call Number
QH324.9.L37 Poh
Date Submitted
1996