PROBING BIOLOGICAL TISSUES WITH ELLIPTICALLY POLARIZED LIGHT

International audience2D Intrinsic Optical Imaging (IOI) is an essential tool for non invasive screening of biological tissues. It allows obtaining high resolution 2D images, with functional in so far as spatial variations of the absorption of elementary chromophores (e.g. oxy- and deoxy- hemoglobyn, glucose). The technique is widely used in neurosciences (cortex imaging), ophtalmology (retina), or oncology (detection/monitoring of skin, uterus cancers…). Conventional IOI systems also offer the possibility of a dynamic monitoring, by monitoring the time variations of the absorption of the chromophores of interest in response to stimuli (visual, physical, etc.). However, the images obtained are a combination of backscattered signals coming from both surface and different sub-surface layers, and do not carry information in depth. These imaging systems suffer from i) a blurring due to specular reflections; ii) a loss of information on the origin of the measured signal, because light is collected without any discrimination on its path. Getting rid of the first problem can be obtained by using linear polarizers under cross-linear configuration. Light collected is then supposed to come from deep layers, without any further information in depth though. Polarization gating is a unique tool that allows a filtering of light with respect to its state of polarization along its path through the probed tissue. Intuitively, polarized light will keep its original polarization up to a certain pathlength depending on the scattering mean free path that is the number of scattering events met during its travel through the tissue. The approach we use is based on the fact that circularly polarized light is maintained for a larger number of scattering events in scattering media composed of Mie scatterers (large particles), which is the case for most of biological tissues. This maximal depth depends on the optical properties of the tissues (absorption and scattering). Furthermore, it has been shown that this depth can be tuned, between surface up to this maximum depth, as a function of the ellipticity of the polarization gate. This principle will be illustrated with theoretical [4] and experimental studies