We implemented a two-dimensional finite-difference time-domain (FDTD) method for the calculation of the
scattering by turbid slabs containing cylindrical scatterers. We present validation results of the FDTD method
used for the calculation of the scattering by an infinite dielectric cylinder. In particular the error caused by
numerical dispersion due to an expansion of the simulation grid is discussed. Finally, an analytical solution of
the scattering by an infinite cylinder has been used to analyze the error caused by the discrete near- to far-field
transformation.
We present a two-axis goniometer for measuring the phase function of scattering media with an angular resolution of about 0.2 deg having 12 decades of dynamic range and covering almost the full solid angle. The setup is evaluated with polystyrene spheres and with perpendicularly and obliquely illuminated thin glass cylinders. The scattering pattern and its intensity distribution are in excellent agreement with analytical theory. A multiple scattering configuration composed of two parallel cylinders is also examined. Finally, the phase function of dentin slabs is measured and its dependence on the dental microstructure is discussed.
Fat emulsions like Intralipid are frequently used in research of light propagation in turbid media as tissue phantoms. We investigated the phase functions of different major brands and concentrations (10% and 20%) of these fat emulsions at 633nm and 543nm. A theoretical phase function for fat emulsions was calculated using Mie theory. Only small differences of the phase function of the investigated fat emulsions were found.
A way to determine the depth of an embedded fluorescent object, for example deep-lying tumors marked with a fluorescent probe, is to detect fluorescent light that has propagated through the medium at two different wavelength bands. A ratio can then be calculated between the corresponding intensities. The wavelength regions should be chosen such that there is a difference in the absorption in the medium. This spectral information could be used as a complement in other methods, for example in tomography, due to its straightforward implementation. In this study we have performed phantom measurements to determine the depth of a fluorescent object, filled with fluorophores. The transmission of yellow and red fluorescence was measured and a ratio of yellow to red fluorescence was calculated for several depths in tissue with a thickness of 2 cm. The ratio showed a clear dependence on the depth of the object.
A time-correlated single photon counting (TCSPC) module (SPC-730, Becker & Hickl, Germany) was connected to a laser scanning microscope (Zeiss, Germany) equipped with an ultrafast photomultiplier. Short pulse excitation was achieved with two laser diodes emitting at 398nm and 434nm with a pulse duration of 70ps and 60 ps (PicoQuant, Germany) to allow intracellular fluorescence lifetime imaging (FLIM).
With this setup, fluorescence lifetime of the mitochondrial marker Rhodamine 123 could be studied in solution under the same instrumental conditions as used for fluorescence lifetime imaging of cell monolayers. With the same set of parameters, fluorescence lifetime of Rhodamine 123 was calculated with good reproducibility in mitochondria of living cells.
We present here a comparison of different fitting routines, including a multiexponential fitting based on the method of Laplace transformation. Fluorescence lifetimes calculated with the multiexponential fitting routine proved to be particularly useful to study the distribution of 5-ALA metabolites in cell monolayers.
The knowledge of the scattering phase function, which describes the angular intensity distribution of the scattered light, is important for modeling the light propagation in turbid media. This is especially true for structured media, where light propagation is anisotropic, leading to direction-dependent reflected or transmitted intensity profiles. We investigated scattering by combining analytical solutions and finite difference time domain (FDTD) simulations of the Maxwell equations with two-axes goniometric experiments. Using polystyrene spheres and cylindrical phantoms the methods were successfully validated. The phase functions of dentin slabs were measured and it is shown that the scattering of dentin tubuli resembles cylinder scattering.
Measurements of the absolute spatially resolved reflectance and transmittance from dentin slabs were performed using a HeNe laser at (lambda) equals633 mm as source and a CCD camera as detector. Solid phantom slabs were used to test the experimental setup and to enable absolute measurements. The reduced scattering and absorption coefficients were derived by applying a solution of the diffusion theory for a slab geometry as theoretical model. Monte Carlo simulations were used to investigate the validity of the diffusion solution. Compared to other biological tissues large values were found for the reduced scattering coefficient of dry dentin slabs, in which the tubuli were parallel to the slab extension. In addition, we measured a strong dependence of the optical properties on the water content of the dentin slabs. We also found that the anisotropic structure of dentin strongly influences the light propagation and, thus, the derived optical properties.
We investigated the influence of the phase function on the determination of the optical properties of turbid media by measuring the spatially resolved reflectance, R((rho) ). Monte Carlo simulations of R((rho) ) using different experimental and theoretical phase functions served as experimental data. We show that the errors in the optical properties are as high as 100%, if these data are fitted with a standard solution of the diffusion equation. In addition, Monte Carlo simulations are applied to fit the `experimental data' using the scattering coefficient, the absorption coefficient and an additional parameter, that characterizes the phase function, as fitting parameters. It is shown that the errors in the optical parameters are < 20%.
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