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R Ferriere

Publications and source records attributed to R Ferriere.

3 recordsLinked to original sources
PubMed2004

Diffraction tomography: a geometrical distortion free procedure.

Ductal echography (DE) is a recent anatomically-led method of investigation of internal mammary structures that provides direct observation of ductolobular structures in mammary lobes. Indeed, breast cancer initiates from the epithelium (hypoechogenic) and develops first in ductolobular structures. Our research aims at developing an ultrasonic scanner (hemispherical antenna) that will allow three-dimensional measurement of the field diffracted by the gland. The present reconstruction procedure is an alternative solution to the standard elliptic back-projection (EBP) technique based on the Born approximation (weak scattering assumption). The latter implicitly assumes that the sound speed is (almost) constant within the breast. This unrealistic a priori is inconsistent with our heterogeneous media characterization purpose, and leads both to geometrical distortion in the reconstruction and to poor focusing of the backpropagated waves (low contrast imaging). However, the EBP technique retains a two-fold advantage: firstly it does not make any other assumption concerning the distribution of mechanical parameters apart from low gradients. Secondly, it shows great robustness, a high resolving power, and is easy to implement. Thus, in order to account for strong wavefront distortions, we integrate temporal compensation of the scattered signals acquired into the scattering EBP technique. The adjustment relies on a time of flight estimation based on a "layer stripping" approach. Numerical tests based on finite difference time domain simulations of data scattered by a random tissue-like phantom are proposed.

Algorithms↗
PubMed1995

Invading wave of cooperation in a spatial iterated prisoner's dilemma.

Explaining the emergence of cooperative behaviours in a selfish world remains a major challenge for sociobiology. The iterated prisoner's dilemma offers a well-studied metaphor with which to explore theoretically the evolution of cooperation by reciprocation. Our current understanding is that 'tit-for-tat' should be the very first step (if not the aim) of evolution towards cooperation, but that mobility of the players in space seems to raise a devastating obstacle to the spread of tit-for-tat, by allowing egoists to exploit cooperation and escape retaliation. The second point is based on models that represent mobility only implicitly (in terms of travelling costs) and assume random interactions. Here we develop a more explicit theory of spatial iterated games: individual mobility is represented in terms of a diffusion process and interactions--defined locally--are inherently non-random. Our model reveals the existence of critical levels of individual mobility allowing for the evolution of cooperation. In fact, tit-for-tat can spread and take over among mobile players even when originating from extreme rarity. The dynamics of invasion of tit-for-tat develop as a travelling wave which propagates the cooperative strategy through space. Significant mobility is required to make the pioneering moves of cooperators towards the front of invasion less hazardous; it also contributes to neutralizing those defectors who may intrude the core of a cluster of cooperative players.

Game Theory↗