Dmitry Y. Churmakov

Influence of refractive index matching on the photon diffuse reflectance.

Photon migration in a randomly inhomogeneous, highly scattering and absorbing semi-infinite medium with a plane boundary is considered by a Monte Carlo (MC) technique. The employed MC technique combines the statistical weight scheme and real photon paths simulation, allowing the exclusion of the energy conservation problem. The internal reflection of the scattered radiation on the medium interface is taken into account by allowing the trajectories of photon packets to be split into reflected and transmitted parts. The spatial photon sensitivity profile (SPSP), spatially resolved diffuse reflectance and angular and spatial photon detector weight distributions are considered in terms of Fresnels reflection/refraction on the boundary of the medium. The effect of the refractive index match is predicted correctly by the MC method and by the diffusion approximation. The results demonstrate that matching of the refractive index of the medium significantly improves the contrast and spatial resolution of the spatial photon sensitivity profile (SPSP). The results of simulation of the spatially resolved diffuse reflectance agree well with the results predicted by the diffusion approximation and the experimental results reported earlier.

Amending of fluorescence sensor signal localization in human skin by matching of the refractive index.

Fluorescence diagnostic techniques are notable amongst many other optical methods because they offer high sensitivity and noninvasive measurement of tissue properties. However, a combination of multiple scattering and physical heterogeneity of biological tissues hampers interpretation of the fluorescence measurements. Analyses of the spatial distribution of endogenous and exogenous fluorophores excitation within tissues and their contribution to the detected signal localization are essential for many applications. We have developed a novel Monte Carlo technique that gives a graphical perception of how the excitation and fluorescence detected signal are localized in tissues. Our model takes into account the spatial distribution of fluorophores, the variation of concentrations and quantum yield. We demonstrate that matching the refractive indices of the ambient medium and topical skin layer improves spatial localization of the detected fluorescence signal within the tissues.

Monte Carlo simulation of coherent effects in multiple scattering

Using a combination of the stochastic Monte Carlo technique and the iteration procedure of the solution to the Bethe–Salpeter equation, it has been shown that the simulation of the optical path of a photon packet undergoing an nth scattering event directly corresponds to the nth–order ladder diagram contribution. In this paper, the Monte Carlo technique is generalized for the simulation of the coherent back–scattering and temporal correlation function of optical radiation scattered within the randomly inhomogeneous turbid medium. The results of simulation demonstrate a good agreement with the diffusing wave theory and experimental results.

Crossed source–detector geometry for a novel spray diagnostic: Monte Carlo simulation and analytical results

Sprays and other industrially relevant turbid media can be quantitatively characterized by light scattering. However, current optical diagnostic techniques generate errors in the intermediate scattering regime where the average number of light scattering is too great for the single scattering to be assumed, but too few for the diffusion approximation to be applied. Within this transitional single-to-multiple scattering regime, we consider a novel crossed source-detector geometry that allows the intensity of single scattering to be measured separately from the higher scattering orders. We verify Monte Carlo calculations that include the imperfections of the experiment against analytical results. We show quantitatively the influence of the detector numerical aperture and the angle between the source and the detector on the relative intensity of the scattering orders in the intermediate single-to-multiple scattering regime. Monte Carlo and analytical calculations of double light-scattering intensity are made with small particles that exhibit isotropic scattering. The agreement between Monte Carlo and analytical techniques validates use of the Monte Carlo approach in the intermediate scattering regime. Monte Carlo calculations are then performed for typical parameters of sprays and aerosols with anisotropic (Mie) scattering in the intermediate single-to-multiple scattering regime.

Investigation of glucose-hemoglobin interaction by optical coherence tomography

In this study, the refractive index of glucose-hemoglobin solutions at different glucose concentrations was measured. Measurements were performed using Abbe refractometer at 589 nm and OCT system at 1300 nm. The different amount of glucose was added to hemoglobin solution. Theoretical values of refractive index of the glucose-hemoglobin solutions were calculated in assumption that hemoglobin and glucose molecules do not interact. The difference between the measured and calculated values of refractive index can be connected with glucose binding to hemoglobin. It is shown that the refractive index measurements can be applied to the evaluation of glycated hemoglobin amount.

Skin fluorescence model based on the Monte Carlo technique

The novel Monte Carlo technique of simulation of spatial fluorescence distribution within the human skin is presented. The computational model of skin takes into account spatial distribution of fluorophores following the collagen fibers packing, whereas in epidermis and stratum corneum the distribution of fluorophores assumed to be homogeneous. The results of simulation suggest that distribution of auto-fluorescence is significantly suppressed in the NIR spectral region, while fluorescence of sensor layer embedded in epidermis is localized at the adjusted depth. The model is also able to simulate the skin fluorescence spectra.

A novel Monte Carlo method for the optical diagnostics of skin

A novel Monte Carlo (MC) technique for photon migration through 3D media with the spatially varying optical properties is presented. The employed MC technique combines the statistical weighting variance reduction and real photon paths tracing schemes. The overview of the results of applications of the developed MC technique in optical/near-infrared reflectance spectroscopy, confocal microscopy, fluorescence spectroscopy, OCT, Diffusing Wave Spectroscopy (DWS) and Doppler flowmetry are presented.

Automatic enhancement of skin fluorescence localization due to refractive index matching

Fluorescence diagnostic techniques are notable amongst many other optical methods, as they offer high sensitivity and non-invasive measurements of tissue properties. However, a combination of multiple scattering and physical heterogeneity of biological tissues hampers the interpretation of the fluorescence measurements. The analyses of the spatial distribution of endogenous and exogenous fluorophores excitations within tissues and their contribution to the detected signal localization are essential for many applications. We have developed a novel Monte Carlo technique that gives a graphical perception of how the excitation and fluorescence detected signal are localized in tissues. Our model takes into account spatial distribution of fluorophores and their quantum yields. We demonstrate that matching of the refractive indices of ambient medium and topical skin layer improves spatial localization of the detected fluorescence signal within the tissue. This result is consistent with the recent conclusion that administering biocompatible agents results in higher image contrast.

Automatic amending of the tattoo sensor fluorescence localization by refractive index matching

Fluorescence diagnostic techniques are notable amongst many other optical methods, as they offer high sensitivity and non-invasive measurements of tissue properties. However, a combination of multiple scattering and physical heterogeneity of biological tissues hampers the interpretation of the fluorescence measurements. The analyses of the spatial distribution of endogenous and exogenous fluorophores excitations within tissues and their contribution to the detected signal localization are essential for many applications. We have developed a novel Monte Carlo technique that gives a graphical perception of how the excitation and fluorescence detected signal are localized in tissues. Our model takes into account spatial distribution of fluorophores and their quantum yields. We demonstrate that matching of the refractive indices of ambient medium and topical skin layer improves spatial localization of the detected fluorescence signal within the tissue. This result is consistent with the recent conclusion that administering biocompatible agents results in higher image contrast.

Imaging of flow velocity profiles within the complex geometry vessels by Doppler optical coherence tomography

Doppler Optical Coherence Tomography (DOCT) technique was applied to non-invasive monitoring of cross-sectional velocity profiles distributions within complex geometry vessels. A set of micro vessels of different diameters with T-shaped and Y-shaped bifurcation and vessels with aneurysm were built. The shape of the vessel was chosen to mimic human vessel shapes of similar characteristics. Intralipid, set in motion at constant input volume flow rate by a syringe pump, was used in the experiments. The influence of vessels geometry, including bifurcation (T- and Y- junctions) and the aneurysms, on the flow dynamics under different inlet flow rates was studied. We show that under constant input volume flow rate, the flow velocities distribution measured along a cross-sectional plane orthogonal to the inlet arm, located at 20 mm off the junction, exhibited stationary and laminar behaviour. A non-homogeneous distribution of flow velocity along a cross-sectional plane located at the junction was observed. The relation between the acquired velocity distribution and the vessel geometry is analyzed. The feasibility of DOCT for mapping the velocity profiles along the vessels junction with a spatial resolution of about 10×10×10 μm3 and a minimum detectable velocity of about 2 mm·s-1 is presented.