Xavier Orlik

Recent studies have proposed improving Laser Speckle Contrast Imaging (LSCI) by using polarimetric filtering to isolate multiply scattered photons from moving red blood cells (RBCs), an approach referred to as Laser Speckle Orthogonal Contrast Imaging (LSOCI). This reliance on multiple scattering enables the development of a calibration method based on a moving reference sample, chosen to generate dynamic circular Gaussian speckle fields that replicate the statistical properties of RBC scattering in both intensity and the distribution of polarization states. Assuming that multiply scattered photons from both RBCs and the reference sample exhibit a homogeneous distribution of polarization states over the Poincaré sphere, the proposed calibration links in vivo speckle contrast reduction bijectively to an equivalent speed of the reference sample. We demonstrate that this equivalent-velocity metric yields consistent in vivo measurements across distinct instruments despite the use of different laser spectral widths, thereby providing a standardized and transferable means to quantify microcirculatory activity.

Recent studies have proposed improving Laser Speckle Contrast Imaging (LSCI) by using polarimetric filtering to isolate multiply scattered photons from moving red blood cells (RBCs), an approach referred to as Laser Speckle Orthogonal Contrast Imaging (LSOCI). This reliance on multiple scattering enables the development of a calibration method based on a moving reference sample, chosen to generate dynamic circular Gaussian speckle fields that replicate the statistical properties of RBC scattering in both intensity and the distribution of polarization states. Assuming that multiply scattered photons from both RBCs and the reference sample exhibit a homogeneous distribution of polarization states over the Poincaré sphere, the proposed calibration links in vivo speckle contrast reduction in a bijective manner to an equivalent speed of the reference sample. We demonstrate that this equivalent-velocity metric yields consistent in vivo measurements across distinct instruments despite the use of different laser spectral widths, thereby providing a standardized and transferable means to quantify microcirculation activity.

Sap flow within a leaf is a critical indicator of plant vitality and health. This paper introduces an easy-to-use, non-invasive and real-time imaging method for sap microcirculation imaging. From the coherent backscattering of light on a leaf, we show that the acquisition frequency of dynamic speckle can be linked to the microcirculation speed inside the leaf. Unlike conventional methods based on speckle contrast, which use integration times long enough to observe temporal decorrelation within a single image, our approach operates in a regime where speckle patterns appear ‘frozen’ in each frame of a given sequence. This ‘frozen’ state implies that any decorrelation of the speckle pattern within a frame is negligible. However, between successive frames, decorrelation becomes substantial, and it is this inter-frame decorrelation that enables the extraction of dynamic information. In this context, the integration time primarily influences the radiometric levels, while the frame acquisition rate emerges as the key parameter for generating activity index maps. Thus, by accessing different ranges of sap flow activity levels by varying the frame acquisition rate, we reveal, in a non-invasive way, the anatomy of the leaf’s circulatory network with unprecedented richness. We experimentally validate the ability of the method to characterize the vitality of a fig leaf in real time by observing the continuous decrease in sap circulation, first in the smaller vessels and then in the larger ones, following the cutting of the leaf over a 48 h period.

Using two independent speckle imaging systems based on the recently published LSOCI method operating in contact mode on the skin, we assess the reproducibility of in vivo measurements and conduct an inter-instrument comparison. To this aim, we propose a calibration method to handle each imaging system as a comprehensive unit, which includes the laser source, optics, and camera. Key to our method is the introduction of a new index that quantifies the departure of the temporal contrast observed in vivo from the spatial contrast scattered from a reference static element generating a circular Gaussian speckle field. Using such near-real-time calibration method, we demonstrate that the microcirculation images produced by 2 different instruments exhibit high accuracy and stability, with microcirculation activity values in excellent agreement, thereby paving the way for clinical applications.

Using two independent speckle imaging systems based on the recently published LSOCI method operating in contact mode on the skin, we assess the reproducibility of in vivo measurements and conduct an inter-instrument comparison. To this aim, we propose a calibration method to handle each imaging system as a comprehensive unit, which includes the laser source, optics, and camera. Our experimental results demonstrate that the microcirculation images produced by both instruments exhibit high accuracy and stability, with microcirculation activity values in excellent agreement, thereby paving the way for clinical applications.

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.

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.

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.

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.