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Scattering Matrix Approach in Reflection applied to Turbid media

Periodic Reporting for period 1 - SMART (Scattering Matrix Approach in Reflection applied to Turbid media)

Reporting period: 2017-08-18 to 2019-08-17

The general theme of this project is an experimental study of the multiple scattering and distortion of classical waves, with the goal of improving imaging and characterization of complex biological materials. This theme is important for society to improve optical and acoustic medical imaging. In the world of classical wave imaging, multiple scattering and aberrations are often seen as unavoidable obstacles. Technologies such as echography, radar, and optical coherence tomography (OCT) rely on the assumption that detected waves have scattered only once from the target. When there are multiple scattering events between source and detector, the equivalence of time-of-flight and target depth is lost, so conventional imaging techniques no longer work. To achieve deeper imaging, more powerful sources (i.e. lasers for optics) can be used, but for in-vivo medical imaging the source power that can be used safetly is limited. In addition, these setups (such as conventional OCT for light) can be costly. The work proposed here is to develop an alternate approach to classical wave imaging which can go beyond all of these limitations.
We approach this theme with two related sub-topics:
(a) Deep imaging with classical waves (optics and acoustics) in biological (i.e. inhomogeneous) media.
(b) A fundamental study of extreme strong scattering of light.
The objectives are to develop superior imaging and characterization approaches based on (i) incoherent (low power) illumination and (ii) the measurement and manipulation of a matrix of responses between multiple inputs and outputs to the medium. The development of these sub-projects in parallel helps to advance the experimental and post-processing techniques of each. We have been able to demonstrate improved optical imaging with a SMART-OCT system in optics (an OCT-like imaging with low-coherence illumination, improved resolution and depth capabilities compared to OCT) and ultrasonic imaging (deeper imaging through strong aberration, and new contrasts for alternate characterization). We have developed post-processing techniques to image through areas of aberration and multiple scattering in biological media. We have also demonstrated that materials which scatter light exceptionally strongly can be investigated using a similar low coherence, multi-input-output imaging approach.
The applied sub-project consisted in the creation of (1) the creation of a SMART-OCT optical experimental setup, and (2) the development of acquisition and post-processing techniques for the two major limitations of classical wave imaging: aberration and multiple scattering. For each post-processing concept, mathematical foundations and proof of concepts were typically first developed and tested using data from ultrasound imaging, due to the rapidity of acquisition of large data sets and because this modality provides in general a more convenient testing platform than do experiments in optics. Then, analogous methods were developed and demonstrated for optical imaging. SMART-OCT, the second experimental setup for the direct measurement of wave-front distortion, and accompanying post-processing techniques, are the subject of a filed patent (2018), several conference presentations, and two articles in preparation. Two other patents have been filed (2018, 2019) and four other articles have been written (or in preparation) with regards to the parallel developments on ultrasound imaging.

The fundamental sub-project consisted of the development of an experiment to study materials in which light is extremely strongly scattered, resulting in very slow wave transport (and possibly Anderson localization). Such samples were created (compressed pastilles of TiO2 powder), and were studied with the experimental setup. Results were compared with theoretical predictions to determine whether Anderson localization had taken place and to measure the diffusion coefficient for each sample. One article is about to be submitted on this work.

Results were disseminated to the scientific community via nine conference presentations, and to the public via outreach activities.
We have demonstrated that SMART-OCT can back the limits of conventional optical imaging in biological tissues. With SMART-OCT, full-field imaging of an axial area of 1 mm^2 was demonstrated to 1 mm in depth in an opaque monkey cornea, which is equivalent to 10 scattering mean free paths (ls). This beats the capabilities of state-of-the art techniques of OCT, which are limited to only a few ls with a resolution of 1 um^3. We have submitted a patent on SMART-OCT, and the development of a marketable prototype is in progress. We expect that this approach to imaging will greatly advance the field of biological imaging, for, for example, the imaging of the human eye and skin. The application of these methods to ultrasound, which were developed in parallel, are also extremely promising for imaging in depth in human tissue. Notably, we have introduced novel imaging modalities, consisting of a focusing criteria enabling refractive index tomography, and multiple scattering quantification.
For the fundamental sub-project, we did not observe Anderson localization of light in the studied samples, despite them being some of the best candidates in which to observe this phenomenon. This finding, however, agrees with more recent theoretical predictions which are pessimistic about the very existence of this phenomenon for light in compressed powders. We have demonstrated the utility of this setup for the study of other such candidates, and for the quantification of light transport parameters in media, despite the eventual presence of absorption or non linear effects. Our experimental approach enables spatiotemporal observations of extremely slow light, measuring diffusion coefficients of D ~ 1 m^2/s.
Matrix approach for deep imaging from optical microscopy to acoustics