3,151 research outputs found
Gap Filling of 3-D Microvascular Networks by Tensor Voting
We present a new algorithm which merges discontinuities in 3-D images of tubular structures presenting undesirable gaps. The application of the proposed method is mainly associated to large 3-D images of microvascular networks. In order to recover the real network topology, we need to ïŹll the gaps between the closest discontinuous vessels. The algorithm presented in this paper aims at achieving this goal. This algorithm is based on the skeletonization of the segmented network followed by a tensor voting method. It permits to merge the most common kinds of discontinuities found in microvascular networks. It is robust, easy to use, and relatively fast. The microvascular network images were obtained using synchrotron tomography imaging at the European Synchrotron Radiation Facility. These images exhibit samples of intracortical networks. Representative results are illustrated
Coupling and robustness of intra-cortical vascular territories
Vascular domains have been described as being coupled to neuronal functional units enabling dynamic blood supply to the cerebral cyto-architecture. Recent experiments have shown that penetrating arterioles of the grey matter are the building blocks for such units. Nevertheless, vascular territories are still poorly known, as the collection and analysis of large three-dimensional micro-vascular networks are difïŹcult. By using an exhaustive reconstruction of the micro-vascular network in an 18 mm 3 volume of marmoset cerebral cortex, we numerically computed the blood ïŹow in each blood vessel. We thus deïŹned arterial and venular territories and examined their overlap. A large part of the intracortical vascular network was found to be supplied by several arteries and drained by several venules. We quantiïŹed this multiple potential to compensate for deïŹciencies by introducing a new robustness parameter. Robustness proved to be positively correlated with cortical depth and a systematic investigation of coupling maps indicated local patterns of overlap between neighbouring arteries and neighbouring venules. However, arterio-venular coupling did not have a spatial pattern of overlap but showed locally preferential functional coupling, especially of one artery with two venules, supporting the notion of vascular units. We concluded that intra-cortical perfusion in the primate was characterised by both very narrow functional beds and a large capacity for compensatory redistribution, far beyond the nearest neighbour collaterals
Understanding the atomic-scale contrast in Kelvin Probe Force Microscopy
A numerical analysis of the origin of the atomic-scale contrast in Kelvin
probe force microscopy (KPFM) is presented. Atomistic simulations of the
tip-sample interaction force field have been combined with a non-contact Atomic
Force Microscope/KPFM simulator. The implementation mimics recent experimental
results on the (001) surface of a bulk alkali halide crystal for which
simultaneous atomic-scale topographical and Contact Potential Difference (CPD)
contrasts were reported. The local CPD does reflect the periodicity of the
ionic crystal, but not the magnitude of its Madelung surface potential. The
imaging mechanism relies on the induced polarization of the ions at the
tip-surface interface owing to the modulation of the applied bias voltage. Our
findings are in excellent agreement with previous theoretical expectations and
experimental observations
A simple laser system for atom interferometry
We present here a simple laser system for a laser cooled atom interferometer,
where all functions (laser cooling, interferometry and detection) are realized
using only two extended cavity laser diodes, amplified by a common tapered
amplifier. One laser is locked by frequency modulation transfer spectroscopy,
the other being phase locked with an offset frequency determined by an
Field-Programmable Gate Array (FPGA) controlled Direct Digital Synthesizer
(DDS), which allows for efficient and versatile tuning of the laser frequency.
Raman lasers are obtained with a double pass acousto-optic modulator. We
demonstrate a gravimeter using this laser system, with performances close to
the state of the art
High-order implicit palindromic discontinuous Galerkin method for kinetic-relaxation approximation
We construct a high order discontinuous Galerkin method for solving general
hyperbolic systems of conservation laws. The method is CFL-less, matrix-free,
has the complexity of an explicit scheme and can be of arbitrary order in space
and time. The construction is based on: (a) the representation of the system of
conservation laws by a kinetic vectorial representation with a stiff relaxation
term; (b) a matrix-free, CFL-less implicit discontinuous Galerkin transport
solver; and (c) a stiffly accurate composition method for time integration. The
method is validated on several one-dimensional test cases. It is then applied
on two-dimensional and three-dimensional test cases: flow past a cylinder,
magnetohydrodynamics and multifluid sedimentation
Microcontact printing process for the patterned growth of individual CNTs
We report an original approach to pattern a substrate with isolated carbon nanotubes. Through the improvement of the microcontact printing technique by the use of a new composite stamp, we were able to produce on flat substrates micrometric features of a catalyst suitable for the localised growth of single-walled carbon nanotubes by catalytic chemical vapour deposition. This catalyst material is for the first time prepared via an original solâgel process. The
growth of straight carbon nanotubes between the patterns was observed and a method to promote the controlled growth of such isolated nanoobjects is thus conceivable
A DTN routing scheme for quasi-deterministic networks with application to LEO satellites topology
We propose a novel DTN routing algorithm, called DQN, specifically designed for quasi-deterministic networks with an application to satellite constellations. We demonstrate that our proposal efficiently forwards the information over a satellite network derived from the Orbcomm topology while keeping a low replication overhead. We compare our algorithm against other well-known DTN routing schemes and show that we obtain the lowest replication ratio without the knowledge of the topology and with a delivery ratio of the same order of magnitude than a reference theoretical optimal routing
Evolution of dispersion of carbon nanotubes in Polyamide 11 matrix composites as determined by DC conductivity
Double-walled Carbon NanoTubes (DWCNTs) have been dispersed in a Polyamide 11 (PA11) matrix by two routes: in the solvent way, Polyamide 11 was first dissolved in its solvent to ensure a liquid state dispersion of carbon nanotubes by ultrasonic way; in the melt mixing way, an optimization of the extrusion parameters, such as mixing time, mixing speed, mixing temperature and screw rotation direction allow to reach satisfactory dispersion. Dispersion and percolation threshold have been compared thanks to the evolution of DC conductivity with carbon nanotubes weight fraction in Polyamide 11. An electrical percolation threshold of 1% in weight was found by the solvent way while the melt mixing way offers one of the lower percolation thresholds for a semi-crystalline matrix (0.93% in weight). An interpretation of the limitation of the electrical percolation threshold value in a semi-crystalline matrix
will be proposed
From homogeneous to fractal normal and tumorous microvascular networks in the brain
We studied normal and tumorous three-dimensional (3D) microvascular networks in primate and rat
brain. Tissues were prepared following a new preparation technique intended for high-resolution
synchrotron tomography of microvascular networks. The resulting 3D images with a spatial
resolution of less than the minimum capillary diameter permit a complete description of the entire
vascular network for volumes as large as tens of cubic millimeters. The structural properties of the
vascular networks were investigated by several multiscale methods such as fractal and power-
spectrum analysis. These investigations gave a new coherent picture of normal and pathological
complex vascular structures. They showed that normal cortical vascular networks have scale-
invariant fractal properties on a small scale from 1.4 lm up to 40 to 65 lm. Above this threshold,
vascular networks can be considered as homogeneous. Tumor vascular networks show similar
characteristics, but the validity range of the fractal regime extend to much larger spatial dimensions.
These 3D results shed new light on previous two dimensional analyses giving for the first time a
direct measurement of vascular modules associated with vessel-tissue surface exchange
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