The transport properties of highly scattering media including biotissues with expressed fibrillar structure are studied by
measuring coherent backscattering. Fitting of the experimental data with the coated coherent potential approximation led
to evaluation of the transport parameters without a-priori knowledge of the optical properties of scattering particles for
strongly scattering dense media. The peculiarities of light diffusion in multiple scattering media characterized by
macroscopic anisotropy of scattering properties (such as, the collagen-containing biological tissue with partially oriented
fibrillar structure) are introduced. The experimentally obtained ratio of the reduced scattering coefficients in the
directions of parallel and perpendicular to the fibers is equal to 0.37 which is in a good agreement with the same value
obtained with laser videoreflectometry method.
Diffuse reflectance of polarized light is studied in application to characterization of multiple scattering anisotropic
media (such as, e.g., the fibrous tissues) with the use of focused probe laser beam. The theoretical model for description
of the transport properties (the values of transport scattering coefficients and effective refractive index for different
polarization states of probe light) of closely packed systems of partially disordered dielectric cylinders is considered on
the base of coherent potential approximation. The influence of structural and optical characteristics of fibrous systems
(such as the volume fraction of cylindrical scatters, their average diameter, the degree of disorder, refractive indices of
scatters and surrounding medium) on diagnostical parameters determined with the polarization videoreflectometry can
be analyzed in the framework of the considered theoretical model. The obtained theoretical results satisfactorily agree
with the experimental data on polarization videoreflectometry and spectral-polarization measurements in the
transmittance mode of in-vitro samples of demineralized bone, muscular tissue and phantom samples (partially oriented
polymer films).
The transport properties of densely packed layers of titanium dioxide (rutile) powder are studied in visible and near infrared region by measuring coherent backscattering (CBS). Fitting of the experimental data with the coated coherent potential approximation led to evaluation of the transport parameters (such as transport mean free path and effective refractive index) of studied samples without a-priori knowledge of the optical properties of scattering particles.
Application of polarized probe light and polarization discrimination of backscattered light give the additional possibilities for characterization and imaging of various diseases localized in superficial layers of tissues because of the high sensitivity of polarization state of multiply scattered light, which is detected in the backscattering mode, to alterations in optical parameters of the probed tissue. In this work we compare two methods of analysis of multiple scattering anisotropic media (demineralized bone), such as laser videoreflectometry and method of coherent backscattering. The agreement between results obtained with these two techniques is satisfactory.
The potential for monitoring of the laser-mediated thermal modification of tissue by means dynamic laser speckles methods is examined. The specific case of delivering of speckle-modulated light backscattered by cartilage with the use of a fiber-optic bundle probe is considered. The proposed technique is experimentally compared with the conventional laser speckle contrast analysis. Both techniques similarly display the basic features of laser-mediated alterations in tissue but the bundle-based technique provides the monitoring of tissue modification with more details and more pronounced correlation between the speckle contrast and the tissue temperature. The possibility of evaluation of the fundamental physical-chemical parameters of tissue modification (in particular, the activation energy) from the temperature-dependent dynamics of the speckle contrast is demonstrated.
The possibilities of characterization of the relaxation processes in thermally treated collagenous tissues with the use of the cumulant analysis of speckle intensity fluctuations are discussed. This method deals with the correlation analysis of spatial-temporal fluctuations of laser light multiply scattered by thermally modified tissue under the condition of polarization discrimination of detected speckle-modulated optical signals. Experimental results, which were obtained
with ex-vivo cartilage samples, are presented.
Possible approach to monitoring of the thermal-mediated cartilage reshaping can be based on application of the polarization-sensitive speckle-correlometry. This method deals with correlation analysis of spatial-temporal fluctuations of laser light scattered by modified tissue under the condition of polarization discrimination of detected speckle-modulated scattered optical signals. In the presented paper, various techniques of correlation analysis of polarization-dependent speckle intensity fluctuations are discussed. Experimental results obtained for tissue phantoms and partially denaturated in-vitro tissue samples are presented. The sensitivity of the polarization-dependent correlation functions of the intensity fluctuations of scattered light to the content of denaturated tissue fraction ("amorphous phase") is demonstrated.
Two modalities for speckle contrast monitoring of tissue structure modification in the case of diffuse scattered probe light detection is discussed. The first modality is based on the detection of dynamic speckle patterns in the image plane of the image-transferring optical system while the other one is based on speckle detection in the diffraction zone. In the latter case the combination of the image-transferring lens and fiber-optical bundle is used in order to collect the scattered light and to deliver it to the detection unit. Experimental results obtained using both speckle techniques are presented.
Thermal modification of collagenous tissue (e.g. cartilage) used for its reshaping is a modern approach in laser medicine. The necessity to provide conditions for proper tissue modification and to avoid the dramatic changes in tissue structure due to collagen denaturation requires to provide the real-time monitoring of the process. One of the possibilities for such monitoring procedure is related to the analysis of the dynamic scattering of a probe coherent light by treated tissue. In this work we studied some peculiarities of the small-angle dynamic scattering of probe laser light by thermally treated collagenous tissues.
Results of the contrast analysis of time-averaged dynamic speckle patterns in application to monitoring of the structure modification of the thermally treated collagenous tissue such as cartilage are presented. The modification presumably induced by the bound to free water phase transition in the matrix of the treated tissue cause the specific feature of evolution of the time-averaged speckle contrast with the change of the current temperature of modified collagen tissue. This evolution appears as hysteresis associated with irreversible changes in tissue structure.
Analysis of multiple interactions of probe light with moving scattering particles indicates the manner in which scattering occurs. The study of statistical properties of Doppler shifted components of scattered light allows one to diagnose the structure and dynamic properties of scattering medium. In this paper, we consider an influence of the statistical properties of pathlength distributions that characterize the probe light transfer in scattering medium on analyzed dynamic speckle patterns. The convenient way to analyze dynamic speckle and to obtain in such manner the information about dynamic properties of scattering medium is to apply the contrast analysis of time-averaged speckle images known as Laser Speckle Contrast Analysis (LASCA) technique. Possibility to analyze the non-stationary layered media with use of the LASCA technique is discussed. Some manifestations of the statistical properties of the pathlength distributions of scattered field partial components in Doppler shift frequency distributions for these components are studied with use of the Monte-Carlo simulation.
Imaging of absorbing inhomogeneity hidden in the multiply scattering media by means of the CW transillumination technique is considered. The effect of inversion of spatial distribution of transilluminated intensity in the region of inhomogeneity shadow was demonstrated in the experiments with phantom scattering object such as rectangular Teflon plate with cylindrical channel filled with water solution of black ink as absorbing substance. Qualitative analysis of the observed shadow inversion effect described as the manifestation of the image projection function peculiarity is carried out by means of Monte-Carlo simulation.
Method for inhomogeneous media probing ny means of the contrast measurements for multiply scattered speckles is considered. Value of the speckle contrast is obtained on the basis of the discrete scatter model as the integral transform of probability density function of differences of effective optical paths characterizing the scattering system. Pathlength distributions were simulated for different detection conditions and optical properties of scattering medium by using Monte-Carlo technique. Comparison of the discussed approach with traditional method of diagnostics and visualization of turbid media such as direct measurements of the scattered light intensity is made.
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