Yu. P. Sinichkin

In vivo investigation of the immersion-liquid-induced human skin clearing dynamics

The results of an in vivo investigation of human skin clearing caused by the immersion-liquid-induced matching of the refractive indices are reported for the first time. It was established that subcutaneous injections of a glucose solution produce a significant long-term suppression of the light scattering in the skin dermis, which is an important factor ensuring an increase in efficacy of the optical tomography, photodynamic therapy, and photodestruction of deep objects.

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.

Optical clearing of skin under action of glycerol: Ex vivo and in vivo investigations

The behavior of optical parameters of the skin of a laboratory rat under the action of an aqueous solution of glycerol is studied ex vivo and in vivo. It is found that the collimated transmission coefficient of ex vivo skin samples increases by a factor of 20–40-fold depending on the wavelength in the studied spectral range, and the diffuse reflection coefficient of skin in vivo decreases on the average by 16%. The results presented can be useful for many methods of laser therapy and optical diagnostics of skin diseases and localization of subcutaneous neoplasms.

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.

The effect of the optical anisotropy of scattering media on the polarization state of scattered light

The effect of the optical anisotropy of scattering media on the polarization state of scattered light is studied. The study is performed using a simple polarization method based on the comparison of the spectral composition of the co-and cross-polarized components of transmitted light measured for samples differently oriented with respect to the plane of polarization of probe linearly polarized light. The experimental results obtained are interpreted theoretically in terms of ordinary methods used in optics of birefringent media. Using rat skin as an example, it is shown that surface tissues can be characterized by a high degree of orientational order of the local optical axis of a medium within large areas (with a size of 5 mm or more), which manifests itself in macroscopic optical measurements. In such measurements in the spectral range 550–700 nm, whole rat skin behaves as a partially depolarized phase plate with a difference between the principal refractive indices Δn ≈ 0.00023.

Acoustooptic visualization of scattering media

Application of coherent interaction of laser light with a focused ultrasonic wave to the technique of acoustooptic visualization in multiple-scattering media is discussed. By analyzing spatial distribution of the optical radiation modulated by ultrasound (the photocurrent at ultrasonic frequency), images of large-sized inhomogeneities embedded into the scattering medium have been obtained. A light-absorbing half-plane and a square with sides of 5 mm were used as the inhomogeneities. The visualization was performed under optimal conditions for measuring the alternating photocurrent calculated for the proposed model of coherent interaction between the laser and acoustic beams (the Raman-Nath diffraction). The alternating current at the ultrasonic frequency was obtained as a result of mixing the waves of the diffraction fields on the detector’s photocathode. All experimental values were obtained from a single measurement without averaging the alternating photocurrent at the ultrasonic frequency of 3 MHz, with the scattering parameter varying up to μL≤37.5, where μ is the extinction coefficient and L is the thickness of the scattering medium along the laser beam axis. The measured quantities varied in the course of the measurements by more than 10 orders of magnitude.

Features of applying fiber-optic sensors in spectral measurements of biological tissues

The fiber-optic sensor is a key element for in vivo spectral measurements of the diffusion reflection of biotissues. Its construction and geometry of the detection of light emerging from the biotissue have a significant effect upon the spectral composition of the detected light. This work presents the results of theoretical and experimental studies of the effect of the detection area on the spectral composition of diffusion reflection of skin. It was found that significant losses took place when detecting the light of the red and near-infrared light ranges if the dimensions of the detection area were comparable with the dimensions of the illuminated skin. The losses can be minimized with an increase in the detection area up to dimensions exceeding those of the illuminated skin by a factor of 3–4. In this case, losses during detection of the long-wavelength spectral range of light diffusely reflected by the skin are less than 5%.

Ultimate degree of residual polarization of incoherently backscattered light for multiple scattering of linearly polarized light

By invoking ideas about the distribution of the optical paths of partial components of the scattered field, we obtain an expression for estimating the degree of residual polarization of light that is incoherently backscattered from a disordered multiply scattering semi-infinite medium illuminated by linearly polarized light. In the backscattering regime, the depolarization length of the linearly polarized light in the disordered medium becomes smaller with the passage from the isotropic to anisotropic scattering. Experiments with model media featuring substantially anisotropic scattering (the anisotropy parameter of 0.90 ≤ g ≤ 0.95) demonstrated that for backscattering of linearly polarized light, the depolarization length is close to the transport length of the scattering medium.

The effect of an external mechanical compression on in vivo optical properties of human skin

We have studied the influence of an external mechanical compression on diffuse reflection spectra of skin tissue under in vivo conditions. An analysis of these spectra based on the diffusion approximation of the radiation transfer theory has allowed us to find that the application of the external compression weakens absorbing and scattering properties of skin tissue. After the removal of the compression, the recovery time of the skin tissue (on the order of 1 h) considerably exceeds the stabilization time of its parameters after application of external mechanical compression (several minutes). In this case, at the initial moment of time after the removal of the compression, the fullness of blood vessels and the degree of oxygenation of blood hemoglobin in the skin tissue increase considerably compared to normal skin.

Simple Parametric Representations of the Polarization Optical Properties of Birefringent Biological Tissues in Reflection Polarization Spectroscopy

The use of polarization reflection spectroscopy for investigating complex scattering media, including orientationally ordered optically anisotropic elements, requires a special choice of output parameters (represented by spectra) that characterize polarized light scattering by objects under study because the standard characteristics, such as the Mueller matrices, depend on the azimuthal orientation of the medium. We propose compact and convenient sets of experimentally determined output parameters that involve separate detection of co- and cross-polarized components of light scattered by a sample irradiated by a normally incident linearly polarized beam and include characteristics invariant with respect to the azimuthal rotation of the sample.