Alwin Kienle

In vivo determination of the optical properties of muscle with time-resolved reflectance using a layered model

We have investigated the possibility of determining the optical coefficients of muscle in the extremities with in vivo time-resolved reflectance measurements using a layered model. A solution of the diffusion equation for two layers was fitted to three-layered Monte Carlo calculations simulating the skin, the subcutaneous fat and the muscle. Relative time-resolved reflectance data at two distances were used to derive the optical coefficients of the layers. We found for skin and subcutaneous fat layer thicknesses (l2) of up to 10 mm that the estimated absorption coefficients of the second layer of the diffusion model have differences of less than 20% compared with those of the muscle layer of the Monte Carlo simulations if the thickness of the first layer of the diffusion model is also fitted. If l2 is known, the differences are less than 5%, whereas the use of a semi-infinite model delivers differences of up to 55%. Even if l2 is only approximately known the absorption coefficient of the muscle can be determined accurately. Experimentally, the time-resolved reflectance was measured on the forearms of volunteers at two distances from the incident beam by means of a streak camera. The thicknesses of the tissues involved were determined by ultrasound. The optical coefficients were derived from these measurements by applying the two-layered diffusion model, and results in accordance with the theoretical studies were observed.

Multiple scattering of polarized light: influence of absorption

This work continues previous research about multiple scattering of polarized light propagation in turbid media, putting emphasis on the imaginary part of the scatterers complex refractive index. The whole angle-dependent Müller matrix is evaluated by comparing results of a polarization sensitive radiative transfer solution to Maxwell theory. Turbid media of defined scatterer concentrations are modelled in three dimensions by sphere ensembles kept inside a cubic or spherical simulation volume. This study addresses the impact of absorption on polarization characteristics for selected media from low to high absorption. Besides that, effects caused by multiple and dependent scattering are shown for increasing volume concentration. In this context some unique properties associated with multiple scattering and absorption are pointed out. Further, scattering results in two dimensions are compared for examples of infinite parallel cylinders of high absorption and perpendicularly incident plane waves.

Monte Carlo modeling of polarized light propagation: Stokes vs. Jones. Part I

This bipartite comparative study aims at inspecting the similarities and differences between the Jones and Stokes-Mueller formalisms when modeling polarized light propagation with numerical simulations of the Monte Carlo type. In this first part, we review the theoretical concepts that concern light propagation and detection with both pure and partially/totally unpolarized states. The latter case involving fluctuations, or depolarizing effects, is of special interest here: Jones and Stokes-Mueller are equally apt to model such effects and are expected to yield identical results. In a second, ensuing paper, empirical evidence is provided by means of numerical experiments, using both formalisms.

Polarization influence on reflectance measurements in the spatial frequency domain

In this work, we quantify the influence of crossed polarizers on reflectance measurements in the spatial frequency domain. The use of crossed polarizers is a very common approach for suppression of specular surface reflections. However, measurements are typically evaluated using a non-polarized scalar theory. The consequences of this discrepancy are the focus of our study, and we also quantify the related errors of the derived optical properties. We used polarized Monte Carlo simulations for forward calculation of the reflectance from different samples. The samples scatterers are assumed to be spherical, allowing for the calculation of the scattering functions by Mie theory. From the forward calculations, the reduced scattering coefficient [Formula: see text] and the absorption coefficient μa were derived by means of a scalar theory, as commonly used. Here, we use the analytical solution of the scalar radiative transfer equation. With this evaluation approach, which does not consider polarization, we found large errors in [Formula: see text] and μa in the range of 25% and above. Furthermore, we investigated the applicability of the use of a reference measurement to reduce these errors as suggested in literature. We found that this method is not able to generally improve the accuracy of measurements in the spatial frequency domain. Our general recommendation is to apply a polarized theory when using crossed polarizers.

An algorithm for simulating image formation in optical coherence tomography for cylinder scattering

An algorithm for the simulation of Fourier domain optical coherence tomography (OCT) images of a cylinder based on an analytical solution of Maxwells equations is presented. The characteristics of the simulated OCT signal are discussed and the whispering gallery modes as well as the geometrical optics signals from the cylinder are identified. An OCT scanner with an incident Gaussian beam is implemented to simulate two-dimensional B-scans.

Penetration depth of focused beams in highly scattering media investigated with a numerical solution of Maxwell's equations in two dimensions

Abstract. The propagation of different focused beams (e.g., Gaussian or quasi-Bessel beams) through scattering media is studied. The finite-difference time-domain method, a numerical solution of Maxwell’s equations, is applied to propagate the light beams in two dimensions. The focused beams are modeled by applying the angular spectrum of the plane waves method. The results show that weakly focused beams exhibit comparable performance to strongly focused beams in delivering focused light deep into scattering media.

Light propagation in structural anisotropic media in the steady-state and time domains.

The determination of the reduced scattering and absorption coefficients of structural anisotropic turbid semi-infinite media and slabs was investigated in the steady-state and time domains. Forward calculations were performed with a Monte Carlo model that considered both cylindrical scatterers aligned in different directions as well as scatterers that were described by a rotationally symmetric scattering function. Analytical solutions of the isotropic and anisotropic diffusion equations were applied to retrieve the optical properties. It was found in the steady-state and time domains that the solutions of the anisotropic diffusion equation have systematic errors compared to the Monte Carlo simulations not only for small distances from the source. However, it is shown that in the time domain it is possible to retrieve useful values for the optical properties using the isotropic and the anisotropic diffusion equations.

New method for correction of surface scattering in spatial frequency domain imaging for an accurate determination of volume scattering

Utilizing a modified analytical solution of the radiative transfer equation allows to fit both the spatial frequency domain reflectance and the phase. The additional information then allows to correct for surface reflections.

Resolving the depth of fluorescent light by structured illumination and shearing interferometry

A method for the depth-sensitive detection of fluorescent light is presented. It relies on a structured illumination restricting the excitation volume and on an interferometric detection of the wave front curvature. The illumination with two intersecting beams of a white-light laser separated in a Sagnac interferometer coupled to the microscope provides a coarse confinement in lateral and axial direction. The depth reconstruction is carried out by evaluating shearing interferograms produced with a Michelson interferometer. This setup can also be used with spatially and temporally incoherent light as emitted by fluorophores. A simulation workflow of the method was developed using a combination of a solution of Maxwells equations with the Monte Carlo method. These simulations showed the principal feasibility of the method. The method is validated by measurements at reference samples with characterized material properties, locations and sizes of fluorescent regions. It is demonstrated that sufficient signal quality can be obtained for materials with scattering properties comparable to dental enamel while maintaining moderate illumination powers in the milliwatt range. The depth reconstruction is demonstrated for a range of distances and penetration depths of several hundred micrometers.

Time-resolved diffuse spectroscopy measurements using a hybrid Green's function for the radiative transfer equation

Time-resolved diffuse optical spectroscopy measurements of phantoms at small source-detector separations yield good results for the retrieved coefficients of reduced scattering and absorption when a hybrid Green’s function of the radiative transfer equation for semi-infinite media is used.