Dmitry A. Zimnyakov

Visualization of Blood Microcirculation Parameters in Human Tissues by Time‐Integrated Dynamic Speckles Analysis

Abstract: Statistical analysis of images of time‐integrated non‐stationary speckle patterns is a tool for diagnostics and imaging of in vivo dynamics of blood microcirculation in superficial layers of tissues and organs. The approach to monitoring blood microcirculation using the contrast analysis of time‐averaged speckle images is known as the laser speckle contrast analysis (LASCA) technique. This paper presents a modified version of LASCA, based on the application of a localized light source in combination with the speckle contrast analysis of time‐integrated dynamic speckle patterns. This method adds possibilities for the analysis of depth distributions of the blood microcirculation parameters. A theoretical background for the depth‐resolved analysis of blood microcirculation parameters is considered here on the basis of the concept of effective optical path distributions for a multiply scattered probe light.

Residual polarization of non-coherently backscattered linearly polarized light: the influence of the anisotropy parameter of the scattering medium

Abstract The effect of preservation of the residual polarization of backscattered light in the case of multiply scattered disordered media illumination by a linearly polarized plane wave is examined using the path-integral approach and Monte Carlo simulation. Disordered ensembles of non-interacting dielectric particles are considered as the model of scattering media. The influence of the anisotropy parameter of the scattering system on the degree of residual polarization is analysed. Experimental results obtained for various scattering systems at different wavelengths of illuminating light are in satisfactory agreement with the results of theoretical analysis and Monte Carlo simulation. The dependence of statistical properties of the polarization states of backscattered field partial components, such as probability distributions of ellipticity, on the anisotropy parameter is studied.

Spectropolarimetry of cancer change of biotissues

This work is directed to the investigation of the scope of the technique of laser polarimetry and polarization spectrometry of oncological changes of the human prostate tissue under the conditions of multiple scattering. It was shown that the third statistic moment in the intensity distribution proved to be the most sensitive to pathological changes in orientation structure. Its value in the intensity distribution of polarization image I (0 - 90) of oncologically changed tissue is 21 times higher if compared with the similar statistic parameter of the intensity distribution of the healthy tissue. The results of studies of size linear dichroism prostate gland, as healthy and affected by malignant tumor at different stages of its development was presented. Significant difference in the values of linear dichroism and its spectral dependence in the spectral range λ = 280 - 840 nm as between research facilities, and between biotissues - healthy (or affected by benign tumors) and cancer patients was shown. These results may have diagnostic value for detection and assessment of the development of cancer.

A study of polarization decay as applied to improved imaging in scattering media

Polarization decay of linearly polarized light propagating in a multiply scattering isotropic medium with embedded model inhomogeneity (absorbing half-plane) is studied experimentally and by means of Monte Carlo simulation. Possibilities of polarization visualization and the location of inhomogeneities masked by scattering media are discussed on the basis of results obtained after comparison with direct measurements of the spatial distribution of the intensity of the scattered light.

Fundamentals and applications of dynamic speckles induced by focused laser beam scattering

Speckle intensity first-order statistics for a large-scale scattering structure is investigated. Results of computer simulation and experimental studies of coherent light diffraction by large-scale scattering objects are presented. The correlation characteristics of dynamic speckles induced by focused Gaussian beam diffraction is theoretically and experimentally analyzed.

Polarization-sensitive speckle spectroscopy of scattering media beyond the diffusion limit

A scattering-media-characterization method that uses partially coherent radiation and polarization discrimination of multiply scattered light is described. The method is based on an analysis of the dependence of speckle contrast on the coherence length of the probe light. Polarization discrimination of detected speckles makes it possible to select scattered-light components that propagate in the probed medium at different distances. A theoretical analysis of the polarization-dependent speckle contrast as influenced by the probe-light coherence and parameters of the probed medium is presented. Experimental results obtained with various nondiffuse scattering samples are presented.

Fractality of speckle intensity fluctuations

Coherent-light diffraction on random phase screens with fractal properties leads to the formation of speckle patterns with peculiarities in correlation characteristics in the small-scale region. Such peculiarities are manifested in asymptotic behavior in intensity autocorrelation and structure functions in the vicinity of the zero values of their arguments. Intensity fluctuations in the far and the near diffraction zones are also characterized by values of fractal (Hausdorff-Besicovitch) dimensions D(HB), differing from the corresponding Euclidean dimension. Relationships between the exponential factors of the structure functions of boundary field phase and scattered-light intensity fluctuations as well as between values of D(HB) have been obtained as a result of speckle-formation analysis for different conditions. Their dependencies on the illumination and observation conditions obtained in experiments with fractallike scatterers (rough glass plates) are in satisfactory agreement with theoretical results.

Effect of absorption of multiply scattering media on the degree of residual polarization of backscattered light

The effect of absorption in a scattering medium on the degree of residual polarization of backscattered radiation is studied in the case of probing of multiply scattering media by a linearly polarized light. An approximate expression describing the dependence of the degree of residual linear polarization of the backscattered radiation on the optical characteristics of a multiply scattering medium is derived within the framework of the phenomenological approach, based on the concept of the distribution of the optical paths of partial components of the scattered optical field under the conditions of multiple scattering, and with the use of the ideas about the similarity of statistical moments of the multiply scattered optical fields. The cut-off of the partial components, characterized by a large value of the optical path, because of their absorption, results in a substantial increase of the degree of residual polarization for the bands of the selective absorption caused by the presence of chromophores in the scattering medium. The results of experiments with model scattering media (whole milk) and biological tissues (human skin in vivo) are presented.

Random media characterization using the analysis of diffusing light data on the basis of an effective medium model

The transport properties of dense random media such as rutile powder layers and polyball suspensions are analyzed in visible and near infrared on the basis of experimental data on coherent backscattering, diffuse transmittance, and low-coherence interferometry. The developed technique of retrieval of the transport parameters of examined scattering media allows the evaluation of the transport mean free path l* and the effective refractive index n(ef) of the medium without a priori knowledge of the optical properties of the scattering particles. It is found that with decreasing wavelength lambda(0) the value of localization parameter 2pin(ef)l*/lambda(0) of the studied rutile samples abruptly drops and approaches approximately 2.6 at 473 nm. This peculiarity is caused by the very large scattering efficiency of scatterers in the vicinity of the first Mie resonance.

Characteristic Scales of Optical Field Depolarization and Decorrelation for Multiple Scattering Media and Tissues

Decorrelation and depolarization properties of multiply scattering media and tissues in the case of propagation of coherent probe beams are analyzed in terms of photon path distribution. A specific correlation time determining the relationship between correlation and polarization states of scattered optical fields is introduced. Results of correlation and polarization experiments with phantom scatterers (such as water suspensions of polystyrene spheres) and tissues with controlled optical properties (such as the human sclera) are presented.