1,403 research outputs found
Improving Fiber Alignment in HARDI by Combining Contextual PDE Flow with Constrained Spherical Deconvolution
We propose two strategies to improve the quality of tractography results
computed from diffusion weighted magnetic resonance imaging (DW-MRI) data. Both
methods are based on the same PDE framework, defined in the coupled space of
positions and orientations, associated with a stochastic process describing the
enhancement of elongated structures while preserving crossing structures. In
the first method we use the enhancement PDE for contextual regularization of a
fiber orientation distribution (FOD) that is obtained on individual voxels from
high angular resolution diffusion imaging (HARDI) data via constrained
spherical deconvolution (CSD). Thereby we improve the FOD as input for
subsequent tractography. Secondly, we introduce the fiber to bundle coherence
(FBC), a measure for quantification of fiber alignment. The FBC is computed
from a tractography result using the same PDE framework and provides a
criterion for removing the spurious fibers. We validate the proposed
combination of CSD and enhancement on phantom data and on human data, acquired
with different scanning protocols. On the phantom data we find that PDE
enhancements improve both local metrics and global metrics of tractography
results, compared to CSD without enhancements. On the human data we show that
the enhancements allow for a better reconstruction of crossing fiber bundles
and they reduce the variability of the tractography output with respect to the
acquisition parameters. Finally, we show that both the enhancement of the FODs
and the use of the FBC measure on the tractography improve the stability with
respect to different stochastic realizations of probabilistic tractography.
This is shown in a clinical application: the reconstruction of the optic
radiation for epilepsy surgery planning
Mjerenje vremena poluraspada 8Li
The β-decay half-life of 8Li has been measured along with in a β-NMR experiment, using thermal 8Li atoms adsorbed in ultra high vacuum on a silicon single crystal surface. A special hardware-based scaler electronics was used to circumvent part of the dead time corrections. The half-life is found to be (839.60 ± 1.06) ms, in accordance with previous experiments.Mjerili smo vrijeme poluraspada u β-raspadu 8Li (uz eksperiment β-NMR) rabeći termičke 8Li atome koji su bili adsorbirani na površini monokristala Si u ultravisokom vakuumu. Primijenili smo poseban elektronički brojački sustav radi izbjegavanja dijela popravki za mrtvo vrijeme. Ishod za vrijeme poluraspada je (839.60 ± 1.06) ms, i on je u skladu s ranijim mjerenjima
Mjerenje vremena poluraspada 8Li
The β-decay half-life of 8Li has been measured along with in a β-NMR experiment, using thermal 8Li atoms adsorbed in ultra high vacuum on a silicon single crystal surface. A special hardware-based scaler electronics was used to circumvent part of the dead time corrections. The half-life is found to be (839.60 ± 1.06) ms, in accordance with previous experiments.Mjerili smo vrijeme poluraspada u β-raspadu 8Li (uz eksperiment β-NMR) rabeći termičke 8Li atome koji su bili adsorbirani na površini monokristala Si u ultravisokom vakuumu. Primijenili smo poseban elektronički brojački sustav radi izbjegavanja dijela popravki za mrtvo vrijeme. Ishod za vrijeme poluraspada je (839.60 ± 1.06) ms, i on je u skladu s ranijim mjerenjima
Expansion of the Gibbs potential for quantum many-body systems: General formalism with applications to the spin glass and the weakly non-ideal Bose gas
For general quantum systems the power expansion of the Gibbs potential and
consequently the power expansion of the self energy is derived in terms of the
interaction strength. Employing a generalization of the projector technique a
compact representation of the general terms of the expansion results. The
general aspects of the approach are discussed with special emphasis on the
effects characteristic for quantum systems. The expansion is systematic and
leads directly to contributions beyond mean-field of all thermodynamic
quantities. These features are explicitly demonstrated and illustrated for two
non-trivial systems, the infinite range quantum spin glass and the weakly
interacting Bose gas. The Onsager terms of both systems are calculated, which
represent the first beyond mean-field contributions. For the spin glass new
TAP-like equations are presented and discussed in the paramagnetic region. The
investigation of the Bose gas leads to a beyond mean-field thermodynamic
description. At the Bose-Einstein condensation temperature complete agreement
is found with the results presented recently by alternative techniques.Comment: 17 pages, 0 figures; revised version accepted by Phys Rev
Matched case-control studies: a review of reported statistical methodology
Background: Case-control studies are a common and efficient means of studying rare diseases or illnesses with long latency periods. Matching of cases and controls is frequently employed to control the effects of known potential confounding variables. The analysis of matched data requires specific statistical methods. Methods: The objective of this study was to determine the proportion of published, peer-reviewed matched case-control studies that used statistical methods appropriate for matched data. Using a comprehensive set of search criteria we identified 37 matched case-control studies for detailed analysis. Results: Among these 37 articles, only 16 studies were analyzed with proper statistical techniques (43%). Studies that were properly analyzed were more likely to have included case patients with cancer and cardiovascular disease compared to those that did not use proper statistics (10/16 or 63%, versus 5/21 or 24%, P = 0.02). They were also more likely to have matched multiple controls for each case (14/16 or 88%, versus 13/21 or 62%, P = 0.08). In addition, studies with properly analyzed data were more likely to have been published in a journal with an impact factor listed in the top 100 according to the Journal Citation Reports index (12/16 or 69%, versus 1/21 or 5%, P Conclusion: The findings of this study raise concern that the majority of matched case-control studies report results that are derived from improper statistical analyses. This may lead to errors in estimating the relationship between a disease and exposure, as well as the incorrect adaptation of emerging medical literature.</p
Density-operator approaches to transport through interacting quantum dots: Simplifications in fourth-order perturbation theory
Various theoretical methods address transport effects in quantum dots beyond
single-electron tunneling while accounting for the strong interactions in such
systems. In this paper we report a detailed comparison between three prominent
approaches to quantum transport: the fourth-order Bloch-Redfield quantum master
equation (BR), the real-time diagrammatic technique (RT), and the scattering
rate approach based on the T-matrix (TM). Central to the BR and RT is the
generalized master equation for the reduced density matrix. We demonstrate the
exact equivalence of these two techniques. By accounting for coherences
(nondiagonal elements of the density matrix) between nonsecular states, we show
how contributions to the transport kernels can be grouped in a physically
meaningful way. This not only significantly reduces the numerical cost of
evaluating the kernels but also yields expressions similar to those obtained in
the TM approach, allowing for a detailed comparison. However, in the TM
approach an ad hoc regularization procedure is required to cure spurious
divergences in the expressions for the transition rates in the stationary
(zero-frequency) limit. We show that these problems derive from incomplete
cancellation of reducible contributions and do not occur in the BR and RT
techniques, resulting in well-behaved expressions in the latter two cases.
Additionally, we show that a standard regularization procedure of the TM rates
employed in the literature does not correctly reproduce the BR and RT
expressions. All the results apply to general quantum dot models and we present
explicit rules for the simplified calculation of the zero-frequency kernels.
Although we focus on fourth-order perturbation theory only, the results and
implications generalize to higher orders. We illustrate our findings for the
single impurity Anderson model with finite Coulomb interaction in a magnetic
field.Comment: 29 pages, 12 figures; revised published versio
Removal of pharmaceuticals in WWTP effluents by ozone and hydrogen peroxide
Ozonation to achieve removal of pharmaceuticals from wastewater effluents, with pH values in the upper and lower regions of the typical range for Swedish wastewater, was investigated. The main aim was to study the effects of varying pH values (6.0 and 8.0), and if small additions of H2O2 prior to ozone treatment could improve the removal and lower the reaction time. The effluents studied differed in their chemical characteristics, particularly in terms of alkalinity (65.3-427 mg center dot l(-1) HCO3-), COD (18.2-41.8 mg center dot l(-1)), DOC (6.9-12.5 mg center dot l(-1)), ammonium content (0.02-3.6 mg center dot l(-1)) and specific UV absorbance (1.78-2.76 l center dot mg(-1)center dot m(-1)). As expected, lower ozone decomposition rates were observed in the effluents at pH 6.0 compared to pH 8.0. When pH 8.0 effluents were ozonated, a higher degree of pharmaceutical removal occurred in the effluent with low specific UV absorbance. For pH 6.0 effluents, the removal of pharmaceuticals was most efficient in the effluent with the lowest organic content. The addition of H2O2 had no significant effect on the quantitative removal of pharmaceuticals but enhanced the ozone decomposition rate. Thus, H2O2 addition increased the reaction rate. In practice, this will mean that the reactor volume needed for the ozonation of wastewater effluents can be reduced
Magnetization reversal driven by spin-injection : a mesoscopic spin-transfer effect
A mesoscopic description of spin-transfer effect is proposed, based on the
spin-injection mechanism occurring at the junction with a ferromagnet. The
effect of spin-injection is to modify locally, in the ferromagnetic
configuration space, the density of magnetic moments. The corresponding
gradient leads to a current-dependent diffusion process of the magnetization.
In order to describe this effect, the dynamics of the magnetization of a
ferromagnetic single domain is reconsidered in the framework of the
thermokinetic theory of mesoscopic systems. Assuming an Onsager
cross-coefficient that couples the currents, it is shown that spin-dependent
electric transport leads to a correction of the Landau-Lifshitz-Gilbert
equation of the ferromagnetic order parameter with supplementary diffusion
terms. The consequence of spin-injection in terms of activation process of the
ferromagnet is deduced, and the expressions of the effective energy barrier and
of the critical current are derived. Magnetic fluctuations are calculated: the
correction to the fluctuations is similar to that predicted for the activation.
These predictions are consistent with the measurements of spin-transfer
obtained in the activation regime and for ferromagnetic resonance under
spin-injection.Comment: 20 pages, 2 figure
Ab-initio calculation of optical absorption in semiconductors: A density-matrix description
We show how to describe Coulomb renormalization effects and dielectric
screening in semiconductors and semiconductor nanostructures within a
first-principles density-matrix description. Those dynamic variables and
approximation schemes which are required for a proper description of dielectric
screening are identified. It is shown that within the random-phase
approximation the direct Coulomb interactions become screened, with static
screening being a good approximation, whereas the electron-hole exchange
interactions remain unscreened. Differences and similarities of our results
with those obtained from a corresponding GW approximation and Bethe-Salpeter
equation Green's function analysis are discussed.Comment: 10 pages, to be published in Physical Review
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