Sébastien Bourguignon
logoECN logoLS2N

Assistant professor
École Centrale de Nantes (ECN) / Laboratoire des Sciences du Numérique de Nantes (LS2N)
Research Team SIMS (Signal, Image and Sound)
Office S-207
Phone (33) 2 40 37 69 15
Sebastien dot Bourguignon at ec-nantes.fr

Current research activity

My research activity focuses on the development of signal and image processing methods, especially in the field of inverse problems, sparse approximation, optimization and MCMC algorithms. The main application fields concerned are ultrasonic nondestructive testing and imaging, microwave imaging, structured illumination microscopy, hyperspectral imaging, and astronomical time series.

Global optimization for l0-norm-based sparse approximation

with Ramzi Ben Mhenni (PhD student), Jordan Ninin (Lab-STICC, Brest), Hervé Carfantan (IRAP, Toulouse), Marcel Mongeau (MAIAA lab, Toulouse).

This project lies at the interface between signal processing and operations research. It aims to propose new optimization strategies based on mixed integer programming (MIP) methods, in order to solve exactly some moderate size, yet difficult, l0-norm sparse approximation problems encountered in various signal processing applications such as spike train deconvolution, hyperspectral unmixing or time series analysis. First results have shown that:

However, efficiency of such MIP methods remains limited by the problem size, the sparsity degree of the solution and the level of noise. This work's perspective is to study various levers in order to overpass the limits of using generic branch-and-bound MIP solvers and then tackle larger and more complex problems, by exploiting prior knowledge from the sparse approximation perspective.

Link to dedicated ANR MIMOSA project website: Mixed Integer Programming Methods for Optimization of Sparse Approximation Criteria

Main publications

Download corresponding programs and test examples

Inversion methods for nondetructive testing

Ultrasonic characterization, ultrasonic imaging

with Nizar Bouhlel (IETR, Rennes), Jérôme Idier (LS2N, Nantes), Aroune Duclos, Jean-Philippe Groby, Laurent Simon (LAUM, Le Mans), Nans Laroche (PhD student) and Ewen Carcreff (DB-SAS company, Nantes)

Ultrasonic inverse problems such as spike train deconvolution, synthetic aperture focusing or tomography aim to reconstruct spatial properties of an object from noisy and incomplete measurements. Such problems rely on both an accurate direct model describing the acquisition process and appropriate prior information constraining the solution. We are interested in modelling acoustic propagation in different kinds of materials, and then in developing appropriate estimation algorithms in order, either to detect discontinuities, delaminations, flaws. etc. (non-destructive testing), or to characterize the inspected material (non-destructive evaluation). First results concerned the deconvolution of spike trains (sparse sequences) in homogeneous, yet attenuative, materials. We are currently working with more complex materials and heterogeneous or anisotropic materials, such as porous and multilayered materials.

Main publications

Resistivity and capacity tomography for concrete monitoring

with Marie-Antoinette AlHajj (PhD student), Géraldine Villain and Sérgio Palma Lopes (IFSTTAR, Nantes)

Microwave imaging

with Corentin Friedrich (former PhD student) and Jérôme Idier (IRCCyN, Nantes), Yves Goussard (Ecole Polytechnique de Montréal, Québec, Canada)

The development of microwave imaging algorithms has received a lot of interest in the last few years with applications such as biomedical imaging and geoscience. Nonlinear inversion methods are required, which usually rely on iterative algorithms to reconstruct the dielectric properties of the unknown object. Solving the inverse scattering problem under realistic conditions presents several difficulties, a critical one being the computational cost associated with three-dimensional problems. Our work studies new algorithmic strategies in order to reduce the computational cost associated with the optimization problem.

Main publications

Blind Structured Illumination Microscopy

with Penghuan Liu (former PhD student) and Jérôme Idier (IRCCyN, Nantes), Simon Labouesse, Marc Allain and Anne Sentenac (Institut Fresnel, Marseille)

Fluorescence microscopy is now a classical biological imaging technique. In order to overpass the resolution limit due to the optical system, the structured illumination microscopy (SIM) technique uses modulated illuminations in order to access higher frequencies, that are not directly available through optical observations. By combining numerically several images obtained with different illumination patterns, a super-resolved image can be reconstructed. Recent works have shown that such an image can be obtained, even if the illumination patterns are not known. The so-called blind-SIM approach is then easier to implement, since monitoring illuminations is a difficult task. In this project, we investigate statistical properties of the blind-SIM solution in order to better understand the conditions that guarantee super-resolution. Then, we propose new estimation strategies with more robust statistical properties than the previously proposed method, and we build associated computation algorithms in order to address large 2D (or, in the future, 3D) problems.

Main publications

Main past research projects

Hyperspectral imaging in Astronomy (ANR DAHLIA project, 2009-2013)

with David Mary and Éric Slezak (Fizeau laboratory), Hervé Carfantan (IRAP, Toulouse)

My main research topic within the ANR DAHLIA project (Dedicated Algorithms for HyperspectraL Imaging in Astronomy) was to develop methods for detection and characterization of astrophysical sources from hyperspectral observations, as those obtained by the MUSE instrument (Multi-Unit Spectroscopic Explorer). This is a difficult issue, mainly because of the size of the data (typically, 300x300 pixels x 4000 wavelengths), the strong level of noise and its spectrally-varying statistics, and high diversity of the searched objects.

Main publications

Spectral analysis of irregularly sampled data (2002-2006)

with Hervé Carfantan (IRAP, Toulouse)

My first research topic concerned the spectral analysis of astronomical time series. The study of many astrophysical phenomena is based on the search for periodicities in light or radial velocity curves. Such time series generally suffer from highly irregular sampling and periodic missing data, due in particular to the alternation of day and night or that of seasons impacting ground-based observations. Thus, Fourier-based spectral analysis may not be satisfactory, and widespread heuristic deconvolution algorithms based on the CLEAN method may lack accuracy. This project addressed spectral analysis as an inverse problem, where the spectrum is discretized on an arbitrarily thin frequency grid. This formulates a deconvolution problem in the Fourier domain. Regularization was addressed by enforcing the sparsity of the solution, as periodic oscillations are searched. Sparsity was first studied through l1-norm regularization, for which specific algorithms were proposed that allow a very high spectral resolution. A probabilistic modeling of sparsity by a Bernoulli-Gaussian model, combined with Markov Chain Monte-Carlo algorithms, was also studied, which leads to more comprehensive results through a fully unsupervised procedure. In particular, detection probabilities and uncertainties in the estimated parameters are information of prime importance to the astrophysicist, which can be estimated at the price of a much higher computational cost.

Main publications