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Polarization gating imaging

Polarization gating imaging

Probing the tissues in depth with elliptically polarized light

Intrinsic Optical Imaging (image of the intrinsic contrasts, absorption and scattering) is a precious tool for the non invasive functional exploration of biological tissues.
Based on a wide field illumination of the tissues and a macroscopic detection with a CCD/CMOS camera of the signal backscattered from the surface, this technique is widely used in Neurosciences (imaging of the exposed cortex in response to stimuli), in ophthalmology (images of the retina) or in oncology (detection and longitudinal monitoring of cancers of the superficial tissues such as the skin, uterus…). The reasons of this popularity rely on the fact that it allows a direct visualization of the local variations of the optical properties, especially the absorption coefficient.
However, due to the strong scattering of the biological tissues, the images collected are two-dimensional projections of signals coming from both the surface and the volume examined : any information on the location of the source producing the variations in the signal is thus lost.

We propose a technique of polarimetric filtering which makes it possible to retrieve the information on the depth of probed tissues according to the ellipticity of the polarization of the illumination [1].
Intuitively, through a scattering medium, the polarized light will more or less maintain its initial polarization state according to the number of scattering events.
A theoretical study [2] carried out on a wide range of optical properties (Figure 1) has shown that the greater the ellipticity of the polarization of light, the greater the probed volume, with a directly controllable penetration depth by simple adjustment of the optical elements of the imaging system.

Figure 1 : Mean Visitation Depth Zc as a function of the characteristic radius Rc of the backscattered signal, for different polarization channels.

The experimental set-up is schematized on Fig. 2.

Figure 2 : Schema of the experimental set-up.

The technique has been tested in vivo on the skin for examination of a burn scar [3] (Fig. 3) and on the exposed cortex of a rat (Fig. 4).

Figure 3 : Test on a burn scar of a volunteer.

Figure 4 : Test on the exposed cortex of a rat. Collaboration with Dr. I. Vanzetta (Institut des Neurosciences de la Timone, INT, Marseille).

[1] Anabela Da Silva, Carole Deumié, and Ivo Vanzetta, “Elliptically polarized light for depth resolved optical imaging”, Biomedical Optics Express 3(11), 2907-2915, 2012.
[2] Simon Rehn, Anne Planat-Chrétien, Michel Berger, Jean-Marc Dinten, Carole Deumié, and Anabela da Silva," Depth Probing of Diffuse Tissues Controlled with Elliptically Polarized Light,” J. Biomedical Optics 18(1), 016007, 2013.
[3] Susmita Sridhar and Anabela Da Silva, “Enhanced contrast and depth resolution in polarization imaging using elliptically polarized light,” J. Biomed. Opt. 21(7), 071107 (2016). doi : 10.1117/1.JBO.21.7.071107.

This project has received the support of :
- CEA-LETI (collaboration contract and scholarship)
- Erasmus-Mundus PhD programs (C.P. Valdes-Escobar, S. Sridhar)
- ANR PRCE L-iOS (2017-2020)