Imaging the vasculature of a beating heart by dynamic speckle: the challenge of a quasiperiodic motion.

The spatial and temporal evolution of the field backscattered by a beating heart while illuminated with a coherent light reveals its macro- and microvascularization in real time. To perform these vascularization images, we use a recently published method of laser speckle imaging, based on the selective detection of spatially depolarized speckle field that is mainly generated by multiple scattering. We consider the calculation of the speckle contrast, by a spatial or temporal estimation. We show that the signal-to-noise ratio of the observed vascular structure can be noticeably increased by a postprocessing method implying the calculation of a motion field that allows the selection of similar frames extracted from different heartbeat periods. This later optimization reveals vascular microstructures with a spatial resolution of the order of 100 µ m .

Dynamic speckle imaging of human skin vasculature with a high-speed camera.

We demonstrate the ability of high-speed acquisition (up to 30 kHz) of dynamic speckle to provide images of the human vascularization at various scales. A comparative study involving the speckle contrast, the first term of the intensity autocorrelation function, and the zero-crossings of the field intensity is proposed, together with a proper preprocessing scheme based on image registration and filtering. Experimental results show the potential of the first term of the autocorrelation function to provide efficient model-free mapping of the microvascular activity (i.e. small-scale random motion associated with the presence of a vessel). With the help of this parameter, various scales of vascularization including large vessels in the wrist, microvessels in the ear and fingers, and thinner inflammatory structures are observed, which suggests the imaging abilities of this parameter are broad. The minimum acquisition time is shown to be of the order of 50 ms, demonstrating video imaging capabilities.

Imaging of the skin microvascularization using spatially depolarized dynamic speckle.

SIGNIFICANCE: We propose a technique devoted to real-time high-resolution imaging of skin microvascularization. AIM: The process utilizes the temporal variation of the spatially depolarized optical speckle field generated by moving red blood cells when illuminated with fully polarized coherent light. APPROACH: Polarimetric filtering prevents the contribution of surface scattering from reaching the camera and thus favors the detection of multiscattered photons from the deeper layers of the skin. RESULTS: Full-field images reveal the microvasculature with a spatial resolution of 80 µm. The acquisition speed allows for real-time applications. CONCLUSIONS: We demonstrate the ability of this method to determine in 1 s a stable and reliable microvascular activity, enabling numerous clinical applications that require quantitative measurements.

Spectralon spatial depolarization: towards an intrinsic characterization using a novel phase shift distribution analysis.

Spectralons are reference radiometric samples which exhibit a calibrated reflectance. However, in case of low reflectance samples, the degree of polarization (DOP) of scattered light is hard to characterize. Here, an accurate determination of spectralon spatial depolarization is proposed. Based on a spatially resolved polarimetric imaging system, the polarization state of the scattered light is characterized for every pixel. A statistic distribution analysis is carried out over the entire image. The relative phase shift distribution between two orthogonal components of the scattered electric field clearly exhibits a high sensitivity to the reflectance, the phase statistics following a circular Voigt profile. The intrinsic part of the spatial depolarization is demonstrated to be linked to the circular Cauchy contribution of that phase dispersion. An analytic equation is proposed to estimate the monochromatic spatially integrated DOP, as a function of the reflectance.

Simulation of polarized optical speckle fields: effects of the observation scale on polarimetry.

In this paper, we propose the simulation of polarized speckle fields using the Stokes formalism, which allows the description of partially polarized electromagnetic waves. We define a unique parameter which determines the partial decorrelation of the involved fields, allowing to simulate the polarized speckles produced by all types of scatterers, from simple to multiple scatterers. We validate this model by comparison with experimental measurements. We use that simulation model to study the impact of the imaging device parameters on polarimetric measurements: first we emphasize a limit of resolution on retardance measurements, then we study the spatial depolarization, which appears when an observer is measuring any space-variant polarization map.

Spectral degree of linear polarization of light from healthy skin and melanoma.

A non-invasive optical technique, based on a supercontinuum laser source and hyperspectral sensors, is established to measure the spectral degree of linear polarization (DOLP) in a broad spectral range from 525 nm to 1000 nm. Several biomaterials of interest, such as healthy and cancerous skins, are considered. The spectral DOLP of melanoma, from 5 mm to 9 mm diameter, are measured and analyzed. An increase of the spectral DOLP is reported for 100% of the melanoma samples compared to healthy skin samples. The spectral DOLP of a given melanoma appears to be correlated to the stage of its development: the larger the melanoma, the higher the DOLP. Such trend could be explained by a decrease of the surface roughness along the evolution of the disease. In addition, a significant spectral dependence of the DOLP is reported for melanoma samples as it exhibits a decrease in the near infrared from 750 nm to 1000 nm.

Polarized vortices in optical speckle field: observation of rare polarization singularities.

Using a recent method able to characterize the polarimetry of a random field with high polarimetric and spatial accuracy even near places of destructive interference, we study polarized optical vortices at a scale below the transverse correlation width of a speckle field. We perform high accuracy polarimetric measurements of known singularities described with an half-integer topological index and we study rare integer index singularities which have, to our knowledge, never been observed in a speckle field.

Polarization analysis of speckle field below its transverse correlation width : application to surface and bulk scattering.

An experimental method for accurate polarimetric characterization of speckle field below its transverse correlation width is proposed. Using a polarimetric analyzer, the speckle field under investigation is probed by a set of polarimetric projections describing the full Poincaré sphere surface. Spatial polarimetric variations of the speckle field are thus observed with an accuracy of 1% for each Stokes parameter. Moreover, all the experimental data can be guaranteed by a validity criterion. Using white paper sheet and rough metal samples, the method exhibits strong potential to analyze and differentiate speckle fields generated by bulk and surface scattering.

Effect of speckle on APSCI method and Mueller Imaging.

The principle of the polarimetric imaging method called APSCI (Adapted Polarization State Contrast Imaging) is to maximize the polarimetric contrast between an object and its background using specific polarization states of illumination and detection. We perform here a comparative study of the APSCI method with existing Classical Mueller Imaging(CMI) associated with polar decomposition in the presence of fully and partially polarized circular Gaussian speckle. The results show a noticeable increase of the Bhattacharyya distance used as our contrast parameter for the APSCI method, especially when the object and background exhibit several polarimetric properties simultaneously.

Full analytical solution of Adapted Polarisation State Contrast Imaging.

We have earlier proposed a 2-channel imaging technique: Adapted Polarisation State Contrast Imaging (APSCI), which noticeably enhances the polarimetric contrast between an object and its background using fully polarised incident state adapted to the scene, such that the polarimetric responses of those regions are located as far as possible on the Poincaré sphere. We address here the full analytical and graphical analysis of the ensemble of solutions of specific incident states, by introducing 3-Distance Eigen Space and explain the underlying physical structure of APSCI and the effect of noise over the measurements.

Adapted polarization state contrast image.

We propose a general method to maximize the polarimetric contrast between an object and its background using a predetermined illumination polarization state. After a first estimation of the polarimetric properties of the scene by classical Mueller imaging, we evaluate the incident polarized field that induces scattered polarization states by the object and background, as opposite as possible on the Poincar e sphere. With a detection method optimized for a 2-channel imaging system, Monte Carlo simulations of low flux coherent imaging are performed with various objects and backgrounds having different properties of retardance, dichroism and depolarization. With respect to classical Mueller imaging, possibly associated to the polar decomposition, our results show a noticeable increase in the Bhattacharyya distance used as our contrast parameter.