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.

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 Poincaré 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.

Human microvascularization provides nutrient and waste exchange at the cellular level. It exhibits a remarkable potential to modulate blood flow inside microvessels to adapt permanently to metabolic needs. However, we know that this capacity is altered in many diseases, such as skin cancer,¹ diabetes,^2,3 hypertension,⁴ Alzheimer’s disease,⁵ and even very recently SARS-COV-2.⁶ Thus, we expect in vivo observation of the human skin microvascularization to allow for early and reliable diagnosis. Moreover, a real-time monitoring ability could broaden the application field to flap surgery and early detection of sepsis.⁷ However, despite the strong therapeutic potential that could result in such observations, a real-time noninvasive, high-resolution imaging technique to visualize the skin microvascularization of the whole body remains unfulfilled. Among the main existing techniques for in vivo imaging of the vasculature, ultra-high resolution computed tomography angiography has recently reached the spatial resolution of 0.2 mm,⁸ but it requires ionizing radiation and contrast agents. Magnetic resonance angiography (MRA), eventually optimized with contrast agents, is typically limited to a millimeter spatial resolution.⁹ Photoacoustic imaging uses an optical excitation by light pulses having energy that is converted into an acoustic wave by moving scatterers. The principle relies on acoustic detection and thus limits the spatial resolution to some hundreds of microns.¹⁰ Super-resolution ultrasound imaging^{11 – 14} has allowed for overcoming the acoustic diffraction limit using gas-filled microbubbles providing a high echogenic contrast agent. It exhibits one of the best spatial resolutions, with a value of $40\mu \mathrm{m}$ being recently demonstrated,¹⁴ and an achievable penetration depth of 10 mm,¹² but the use of a contrast agent is inherent to the technique and is invasive. In general, purely optical imaging techniques benefit from short wavelengths that favor better spatial resolution but at the expense of smaller penetration depths. Among these techniques, optical coherence tomography¹⁵ is a depth resolved scanning imaging modality that uses low coherence interferometry. It benefits from a spatial resolution on the order of few microns and is typically limited to submillimeter penetration depth^16,17 due to the inherent use of a wide optical spectrum inside which some wavelengths have reduced penetration depths. Recently, using an optical spectrum shifted to the infrared region,¹⁸ penetration depth was increased by roughly 1 mm, but spatial resolution appears to decrease rapidly with depths greater than 1 mm. OCT has broadened its application field to the imaging of blood vessels.¹⁹ This later technique, called OCTA, uses the temporal variation of the OCT signal caused mainly by moving red blood cells. OCTA is now a well-established technique in the field of ophthalmology^{20 – 22} and seems promising for skin characterization.²³ Among the numerous modalities that emerged from OCTA, Doppler OCT²⁴ can image both blood flow dynamic and tissue structure by combining Doppler velocimetry with OCT. In addition, speckle variance OCT²⁵ is sensitive to flow perpendicular to the scanning beam by measuring the intensity speckle change in the OCT signal. Taking into account the vector nature of the electric field, polarization-sensitive OCT (PS-OCT) can detect the birefringent properties of the sample.²⁶ However, the interferometric signal of the OCT is subjected to polarization-induced artifacts due to polarization mismatch between the sample and reference beams.²⁷ This difficulty has recently been bypassed by a PS-OCT angiography system at the price of a dual-channel OCT setup.²⁸

We propose a numerical approach to study optical speckle fields generated by various conditions of surface roughness and illumination, without any a priori assumption concerning the properties of the scattered field. By overcoming the limitations imposed by the use of the central limit theorem, we perform the study of the whole circular Gaussian transition undergone by the speckle field when varying the surface roughness and the size of the laser illumination. We focus our attention on the speckle contrast, the degree of circularity and the degree of asymmetry, including their radial dependence.

In vivo characterization of heart diseases benefits from various imaging techniques that use the properties of ultrasounds, x-rays, radioactivity, and magnetic resonance. They exhibit complementary advantages and the ability to reconstruct a volumetric image. The noninvasive technique of three-dimensional (3D) echocardiography is particularly suited for real-time imaging of the cardiac volume chambers and valves and is a powerful tool for evaluating congenital abnormalities. X-rays are routinely applied for imaging of coronary artery disease (CAD), either by invasive coronary angiography (ICA) or by coronary computed tomography angiography.¹ Radioactivity is also used in the case of cardiac scintigraphy for myocardial perfusion imaging.² Magnetic resonance imaging provides noninvasive anatomical and functional features, including millimeter-scale analysis of cardiac valves, ventricular volumes, and myocardial perfusion.³ Donors with extended criteria (advanced age, and cardiovascular comorbidities) are routinely allocated for heart transplantation nowadays. Severe CAD is a major determinant of primary graft failure and early death after transplantation. ICA is recommended for diagnosis of CAD in organ donors older than 55 years or in those older than 45 years with cardiovascular risk factors. However, ICA is only performed in one-third of donors at risk for CAD, mainly due to logistic reasons. Ex situ heart perfusion (ESHP) is an innovative approach to improve the preservation of the donor heart before transplantation.^4,5 This method allows for continuous organ monitoring including metabolic assessment of myocardial viability. ICA is the gold standard method for CAD diagnosis because of its high contrast and spatially resolved imaging capabilities but has been poorly reported during ESHP because of potential iatrogenic concerns including myocardial edema due to accumulation of contrast agent, coronary dissection, and infection.^6,7 Dang Van et al. recently reported their preliminary experience of ex situ perfusion of the donor heart in high risk transplantation.⁴ In this clinical series, surgeons performed a coronary angiography during ESHP. This procedure resulted in a major myocardial edema and primary graft failure after transplant. The authors stated that contrast agent used for ex situ coronary imaging was deleterious for the heart in this setting. There is, therefore, a need for an alternative safe coronary imaging protocol to discriminate donor hearts with CAD that would not be eligible for transplantation.

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.