Dr Denis Grebenkov - Localisation regime in diffusion NMR: anomalous or normal?
Upcoming webinar: Friday 24th July 6:00 am Brisbane. Register online
Abstract: High diffusion-sensitising magnetic field gradients have been more and more often applied nowadays to achieve a better characterisation of the microstructure. As the resulting spin-echo signal significantly deviates from the conventional Gaussian form, various models have been employed to interpret these deviations and to relate them with the microstructural properties of a sample. In this talk, we argue that the non-Gaussian behaviour of the signal is a generic universal feature of the Bloch-Torrey equation. We provide a simple yet rigorous description of the localisation regime that exhibits "anomalous" stretched-exponential dependence on the gradient strength, despite the fact that diffusion process remains a "normal" Brownian motion. We identify its origin as a symmetry breaking at the reflecting boundary. We compare the consequent non-Gaussian signal decay to other diffusion NMR regimes such as slow-diffusion, motional-narrowing and diffusion-diffraction regimes. We emphasise limitations of conventional perturbative techniques and advocate for non-perturbative approaches which should pave a way to new imaging modalities in this field.
Dr Denis Grebenkov graduated from Saint Petersburg State University (Russia) and Ecole Polytechnique (France) after defending two PhD theses, one in statistical and the other in theoretical physics. Since 2006, he is a permanent CNRS researcher at Laboratory of Condensed Matter Physics of Ecole Polytechnique. He is interested in understanding and modeling transport phenomena in complex systems. He developed various models, theoretical approaches and numerical tools for studying Laplacian transport towards irregular interfaces, anomalous diffusions in living cells, first passage times of intermittent surface-mediated processes in porous media, diffusion magnetic resonance imaging of lungs and brain, convective and diffusive transport in the placenta, to name a few.