Images of light distribution in biological soft tissue we used to study the optical characteristics of tissue. The light distribution image was taken under a microscope with light injected through a pinhole close to the edge of the top surface. Images taken on skin, fat, and muscle tissues were compared to study the effect of cellular structure and temperature on the light intensity distribution. Monte Carlo simulation with the same conditions was also performed to simulate the light intensity distribution in tissue for comparison. The anisotropy scattering of light in tissue is affected by the tissue microscopic structure, such as the direction of muscle tissue fibers. The change in optical properties of fat and muscle tissue with temperature was observed. The two-dimensional light distribution images offer more information than general reflectance and transmission measurements. By matching the simulated light intensity distribution with the light distribution image, the optical properties of biological tissue could be estimated. This method might be applied in tissue engineering as an economic way for evaluating the microscopic structure of tissue.
Light propagation and the intensity distribution in a biological tissue are characterized by its optical properties. The purpose of this study is to find out how the optical properties affect the profile of skin reflectance spectrum. A Monte Carlo simulation was performed to simulate the skin near-infrared reflectance spectrum in the spectral region between 900 nm and 1300 nm. The absorption coefficient of tissue was determined from the absorption properties and volume fractions of tissue constituents. The simulated reflectance spectra were compared with the skin reflectance spectra measured by a bifurcated fiber bundle. The simulation results show that most of the detected reflectance was scattered back by the dermis tissue. The scattering coefficient and anisotropy factor can be adjusted to modify the simulation result to match the measured spectrum. This inversion technique could be used to determine the scattering and anisotropy properties of skin tissue.
Light in the near-IR (NIR) spectral region can penetrate relatively deep into soft tissue. In this region, the light absorption property is determined by tissue constituents, especially water, fat, and collagen, and their combination ratio. If the light absorption spectra of tissue constituents were known, the combination ratio could be determined by quantifying the light path length in different tissue constituents. In order to obtain the accurate absorption property, the absorption spectra were measured by a Shimadzu 3101-PC spectrophotometer. Since animal fat contains many kinds of fatty acid, five kinds of major fatty acid found in human fat were mixed with proper ratio as a standard sample. The results show that temperature has a stronger effect on the absorption property of water than on that of fatty acid mixture. The absorption spectrum of hog eye lens was measured to obtain the absorption property of collagen. Its absorption spectrum is quite similar to that of dry bovine gelatin. NIR spectroscopy might be useful to characterize or identify different types of soft tissue based on their major chemical composition, such as detecting a fat plaque in a muscular tissue or a tumor in a high fat content tissue.
The diffusive reflectance intensity of skin in the near-infrared range is shown to be greatly influenced by its water content, similar to the way in which hemoglobin content affects the skin color in the visible range. The different behavior of water and hemoglobin at selected wavelengths might allow us to distinguish changes in fluid balance. The simplicity and high sensitivity of an optical method show a promising way of identifying a slight change in water content in soft tissue. The understanding of how the skin reflectance spectrum responds to the change in blood and water content will be the basis of a simplified multi-wavelength monitoring instrument.
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