6,577 research outputs found
Sparsity driven ultrasound imaging
An image formation framework for ultrasound imaging from synthetic transducer arrays based on sparsity-driven regularization functionals using single-frequency Fourier domain data is proposed. The framework involves the use of a physics-based forward model of the ultrasound observation process, the formulation of image formation as the solution of an associated optimization problem, and the solution of that problem through efficient numerical algorithms. The sparsity-driven, model-based approach estimates a complex-valued reflectivity field and preserves physical features in the scene while suppressing spurious artifacts. It also provides robust reconstructions in the case of sparse and reduced observation apertures. The effectiveness of the proposed imaging strategy is demonstrated using experimental data
Optimal Estimates for the Electric Field in Two-Dimensions
The purpose of this paper is to set out optimal gradient estimates for
solutions to the isotropic conductivity problem in the presence of adjacent
conductivity inclusions as the distance between the inclusions goes to zero and
their conductivities degenerate. This difficult question arises in the study of
composite media. Frequently in composites, the inclusions are very closely
spaced and may even touch. It is quite important from a practical point of view
to know whether the electric field (the gradient of the potential) can be
arbitrarily large as the inclusions get closer to each other or to the boundary
of the background medium.
In this paper, we establish both upper and lower bounds on the electric field
in the case where two circular conductivity inclusions are very close but not
touching. We also obtain such bounds when a circular inclusion is very close to
the boundary of a circular domain which contains the inclusion. The novelty of
these estimates, which improve and make complete our earlier results published
in Math. Ann., is that they give an optimal information about the blow-up of
the electric field as the conductivities of the inclusions degenerate.Comment: 26 page
Pathogen-host reorganization during Chlamydia invasion revealed by cryo-electron tomography
Invasion of host cells is a key early event during bacterial infection, but the underlying pathogen-host interactions are yet to be fully visualised in three-dimensional detail. We have captured snapshots of the early stages of bacterial-mediated endocytosis in situ by exploiting the small size of chlamydial elementary bodies (EBs) for whole cell cryo-electron tomography. Chlamydiae are obligate intracellular bacteria that infect eukaryotic cells and cause sexually transmitted infections and trachoma, the leading cause of preventable blindness. We demonstrate that Chlamydia trachomatis LGV2 EBs are intrinsically polarised. One pole is characterised by a tubular inner membrane invagination, while the other exhibits asymmetric periplasmic expansion to accommodate an array of type III secretion systems (T3SSs). Strikingly, EBs orient with their T3SS-containing pole facing target cells, enabling the T3SSs to directly contact the cellular plasma membrane. This contact induces enveloping macropinosomes, actin-rich filopodia and phagocytic cups to zipper tightly around the internalising bacteria. Once encapsulated into tight early vacuoles, EB polarity and the T3SSs are lost. Our findings reveal previously undescribed structural transitions in both pathogen and host during the initial steps of chlamydial invasion
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