Seminar – Friday 21 June 2019

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Waves & Imaging

Commonalities/complementarities of optical and ultrasound imaging

Venue: SV1717 – Poster Imaging Seminar

Free registration: Doodle

Chair: Prof. Michael Unser, Biomedical Imaging Group – EPFL


16:00 – Anne Sentenac

Institut Fresnel, Marseille, France

Super-resolution in diffraction microscopy: The interest of computational imaging

The resolution of diffraction (i.e. marker free) microscopy is fundamentally limited by the elastic light-matter interaction which states that, in the single scattering regime, the far-field scattered by an object illuminated under propagative waves conveys information on the object permittivity spatial frequencies up to 2/λ at most (where λ is the illumination wavelength in the background medium). In addition, this information is poorly restored with conventional analogical microscopes (such as brightfield or confocal) as the observed standard resolution limit, seen as the full width at half maximum of a point-like object is usually about 0.6 λ instead of the theoretical reachable diffraction limit of 0.3 λ. A widely explored solution for improving the resolution consists in taking advantage of evanescent waves for the illumination or the detection via near-field probes or metamaterial lenses. These techniques ameliorate the resolution, albeit seldom beyond 0.3 λ in practice, but at the expense of an increase in the experimental complexity and a restriction to surface imaging. In this talk, we present another research avenue which consists in extracting the most out of the sample scattered field using numerical reconstructions based on an accurate model of the sample-light interaction. This quantitative imaging approach has been shown experimentally to achieve the best possible resolution of 0.3 λ in the single scattering regime and even much better for certain samples in the multiple scattering regime for which the diffraction limit does not hold. In addition, quantitative imaging can take advantage of a priori information on the sample. The latter may be used to recover object spatial frequencies beyond the physically accessible domain. Hence, by imposing a binary behavior on the contrary permits a spectacular improvement of the resolution can be obtained on complex samples.

16:45 – Coffee break

17:00 – Mathias Fink

Mathias Fink, ESPCI, PSL Research University, CNRS, Paris, France

Reflection Matrix Approaches for Imaging: From Ultrasound to Optics

Noninvasive in vivo medical imaging with light or with ultrasound requires reflection-mode detection. As tissues are complex disordered media, containing random distribution of scatterers, these techniques suffer various limitations as distortion induced by aberrating layers as well as multiple scattering contributions. Multi illumination strategy is the solution to solve these problems. We will show that recording a time-gated reflection matrix can provided enough information both on the properties of aberrating layers and on the level of multiple scattering. We will show how to extract from the coherence properties of this reflection matrix enough information both to compensate the effects of aberrating layers and to overcome the contribution of multiple scattering. Various strategies to measure this refection matrix will be discussed and their applications will be presented both in ultrasonic imaging and in deep optical coherent tomography (OCT).

17:45 – Apéritif

Slides of Anne Sentenac: 

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 Slides of Mathias Fink: 

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