1,871 research outputs found
An observable for vacancy characterization and diffusion in crystals
To locate the position and characterize the dynamics of a vacancy in a
crystal, we propose to represent it by the ground state density of a quantum
probe quasi-particle for the Hamiltonian associated to the potential energy
field generated by the atoms in the sample. In this description, the h^2/2mu
coefficient of the kinetic energy term is a tunable parameter controlling the
density localization in the regions of relevant minima of the potential energy
field. Based on this description, we derive a set of collective variables that
we use in rare event simulations to identify some of the vacancy diffusion
paths in a 2D crystal. Our simulations reveal, in addition to the simple and
expected nearest neighbor hopping path, a collective migration mechanism of the
vacancy. This mechanism involves several lattice sites and produces a long
range migration of the vacancy. Finally, we also observed a vacancy induced
crystal reorientation process
Propagation dynamics on networks featuring complex topologies
Analytical description of propagation phenomena on random networks has
flourished in recent years, yet more complex systems have mainly been studied
through numerical means. In this paper, a mean-field description is used to
coherently couple the dynamics of the network elements (nodes, vertices,
individuals...) on the one hand and their recurrent topological patterns
(subgraphs, groups...) on the other hand. In a SIS model of epidemic spread on
social networks with community structure, this approach yields a set of ODEs
for the time evolution of the system, as well as analytical solutions for the
epidemic threshold and equilibria. The results obtained are in good agreement
with numerical simulations and reproduce random networks behavior in the
appropriate limits which highlights the influence of topology on the processes.
Finally, it is demonstrated that our model predicts higher epidemic thresholds
for clustered structures than for equivalent random topologies in the case of
networks with zero degree correlation.Comment: 10 pages, 5 figures, 1 Appendix. Published in Phys. Rev. E (mistakes
in the PRE version are corrected here
Establishing on-site reference values for 123I-FP-CIT SPECT (DaTSCAN®) using a cohort of individuals with non-degenerative conditions
To overcome the issue of reference values for DaTSCAN® requiring healthy controls, we propose an original approach using scans from individuals with non- degenerative conditions performed at one single center following the same acquisition protocol. Procedures: From a cohort of 970 consecutive patients, we identified 182 patients with a clinical diagnosis of non-degenerative parkinsonism or tremor and a visually normal DATSCAN®. Caudate nucleus (C), putamen (P), and striatum (S) uptake values, C/P ratios, and asymmetry indexes (AI) were calculated using semi-quantitative methods. Outcomes were assessed according to age and gender, and reference limits were established using the percentile approach. Results: A significant negative linear effect of age was found upon striatal nuclei uptake of 0.21–0.22 per decade (6.8 %/decade for striatum), whereas a potential gender influence proved unclear. Inferior reference limits were established at the 5th percentile. C/P ratios and AIs were not influenced by age or gender, and superior reference limits were set at the 95th percentile. Conclusions: We here propose a convenient approach to calculate site-specific reference limits for DaTSCAN® outcomes not requiring scanning healthy controls. The method appears to yield robust values that range within nearly identical limits as those obtained in healthy subjects
Synthesis and electronic properties of polycyclic aromatic hydrocarbons doped with phosphorus and sulfur
International audienc
Intensity-Based Registration of Freehand 3D Ultrasound and CT-scan Images of the Kidney
This paper presents a method to register a pre-operative Computed-Tomography
(CT) volume to a sparse set of intra-operative Ultra-Sound (US) slices. In the
context of percutaneous renal puncture, the aim is to transfer planning
information to an intra-operative coordinate system. The spatial position of
the US slices is measured by optically localizing a calibrated probe. Assuming
the reproducibility of kidney motion during breathing, and no deformation of
the organ, the method consists in optimizing a rigid 6 Degree Of Freedom (DOF)
transform by evaluating at each step the similarity between the set of US
images and the CT volume. The correlation between CT and US images being
naturally rather poor, the images have been preprocessed in order to increase
their similarity. Among the similarity measures formerly studied in the context
of medical image registration, Correlation Ratio (CR) turned out to be one of
the most accurate and appropriate, particularly with the chosen non-derivative
minimization scheme, namely Powell-Brent's. The resulting matching transforms
are compared to a standard rigid surface registration involving segmentation,
regarding both accuracy and repeatability. The obtained results are presented
and discussed
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