1,779 research outputs found
Test-retest reliability of structural brain networks from diffusion MRI
Structural brain networks constructed from diffusion MRI (dMRI) and tractography have been demonstrated in healthy volunteers and more recently in various disorders affecting brain connectivity. However, few studies have addressed the reproducibility of the resulting networks. We measured the testâretest properties of such networks by varying several factors affecting network construction using ten healthy volunteers who underwent a dMRI protocol at 1.5 T on two separate occasions. Each T1-weighted brain was parcellated into 84 regions-of-interest and network connections were identified using dMRI and two alternative tractography algorithms, two alternative seeding strategies, a white matter waypoint constraint and three alternative network weightings. In each case, four common graph-theoretic measures were obtained. Network properties were assessed both node-wise and per network in terms of the intraclass correlation coefficient (ICC) and by comparing within- and between-subject differences. Our findings suggest that testâretest performance was improved when: 1) seeding from white matter, rather than grey; and 2) using probabilistic tractography with a two-fibre model and sufficient streamlines, rather than deterministic tensor tractography. In terms of network weighting, a measure of streamline density produced better testâretest performance than tract-averaged diffusion anisotropy, although it remains unclear which is a more accurate representation of the underlying connectivity. For the best performing configuration, the global within-subject differences were between 3.2% and 11.9% with ICCs between 0.62 and 0.76. The mean nodal within-subject differences were between 5.2% and 24.2% with mean ICCs between 0.46 and 0.62. For 83.3% (70/84) of nodes, the within-subject differences were smaller than between-subject differences. Overall, these findings suggest that whilst current techniques produce networks capable of characterising the genuine between-subject differences in connectivity, future work must be undertaken to improve network reliability
Development of a multibody systems model for investigation of the effects of hybrid electric vehicle powertrains on vehicle dynamics
With ever increasing numbers of Hybrid Electric Vehicles (HEVâs) being developed, come new challenges in the field of automotive engineering. Whilst there has been considerable work conducted on HEVâs from a powertrain, efficiency, and control systems perspective, very little work has been instigated in the field of how the introduction of such hybrid systems effect passive vehicle dynamics. One of the possible obstacles in the way of such studies is the multitude of powertrain architectures that are present or possible in HEVâs. This obstacle can make investigations very application specific, and leads to inefficiencies in the modelling process. This paper discusses the development of a model constructed in Dymola in order to investigate the effects of hybrid powertrains on ride and handling. The modelling methodology is presented, along with model based testing and validation of component and the full vehicle models. Whilst the development of the model is introduced for a specific study, it is shown that the way in which the model has been developed lends itself easily to use in other fields. It is shown that the modular construction of the model, and the physical, object orientated modelling approach facilitated by Dymola, allow varying numbers and complexities of component models to be utilised within the same basic model. Such an approach means that one base model can be utilised for differing hybrid architectures for ride, handling and drivability studies thus reducing modelling time and complexity
Chemical abundances for Hf 2-2, a planetary nebula with the strongest known heavy element recombination lines
We present high quality optical spectroscopic observations of the planetary
nebula (PN) Hf 2-2. The spectrum exhibits many prominent optical recombination
lines (ORLs) from heavy element ions. Analysis of the H {\sc i} and He {\sc i}
recombination spectrum yields an electron temperature of K, a factor
of ten lower than given by the collisionally excited [O {\sc iii}] forbidden
lines. The ionic abundances of heavy elements relative to hydrogen derived from
ORLs are about a factor of 70 higher than those deduced from collisionally
excited lines (CELs) from the same ions, the largest abundance discrepancy
factor (adf) ever measured for a PN. By comparing the observed O {\sc ii}
4089/4649 ORL ratio to theoretical value as a function of
electron temperature, we show that the O {\sc ii} ORLs arise from ionized
regions with an electron temperature of only K. The current
observations thus provide the strongest evidence that the nebula contains
another previously unknown component of cold, high metallicity gas, which is
too cool to excite any significant optical or UV CELs and is thus invisible via
such lines. The existence of such a plasma component in PNe provides a natural
solution to the long-standing dichotomy between nebular plasma diagnostics and
abundance determinations using CELs on the one hand and ORLs on the other.Comment: 12 pages, 5 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
Ion crystals in anharmonic traps
There is currently intensive research into creating a large-scale quantum computer with trapped ions. It is well known that for a linear ion crystal in a harmonic potential, the ions near the center are more closely spaced compared to the ions near the ends. This is problematic as the number of ions increases. Here, we consider a linear ion crystal in an anharmonic potential that is purely quartic in position. We find that the ions are more evenly spaced compared to the harmonic case. We develop a variational approach to calculate the properties of the ground state. We also characterize the zigzag transition in an anharmonic potential
Detuning effects in the one-photon mazer
The quantum theory of the mazer in the non-resonant case (a detuning between
the cavity mode and the atomic transition frequencies is present) is written.
The generalization from the resonant case is far from being direct. Interesting
effects of the mazer physics are pointed out. In particular, it is shown that
the cavity may slow down or speed up the atoms according to the sign of the
detuning and that the induced emission process may be completely blocked by use
of a positive detuning. It is also shown that the detuning adds a potential
step effect not present at resonance and that the use of positive detunings
defines a well-controlled cooling mechanism. In the special case of a mesa
cavity mode function, generalized expressions for the reflection and
transmission coefficients have been obtained. The general properties of the
induced emission probability are finally discussed in the hot, intermediate and
cold atom regimes. Comparison with the resonant case is given.Comment: 9 pages, 8 figure
Recombination processes in ionised plasmas.
The observational analysis of astrophysical plasmas relies on accurate calculations of the atomic processes involved. The recombination spectra of singly ionised oxygen (O il) and carbon (C il) present excellent tools for investigating regions such as planetary nebulae and H II regions. In this thesis, detailed treatments of the recombination processes of both O II and C II are presented. Using the R-matrix solution to the close coupling equations, I present the results of accurate photoionisation calculations. Bound state energy levels are determined and oscillator strengths calculated for both species. Recombination coefficients were evalu ated for low n and 1, for C II in LS-coupling, and 0 II in intermediate coupling, taking particular care to treat resonances effectively. Sample photoionisation cross-sections are presented for both species, and compared to previous work. A complete radiative-cascade model is treated for both species, in order to determine line emissivities under nebular conditions at a wide range of temperatures and densities. Collisional effects are treated for C II, along with, for the first time, the effects of high temperature dielectronic recombination, allowing the modelling of regions of much higher electron temperature than previous work. The O II calculations were performed under intermediate coupling for the first time, allowing the effects of non-statistical popula tions of the parent ion fine-structure levels and dielectronic recombination onto bound states within this fine-structure to be taken into account in line emissivities. Detailed comparison with previous theoretical work was made for both species. The application of the C II and 0 n recombination spectra to determining tempera ture and densities from the observed spectra of a number of ionised nebulae is considered. The potential for using the new recombination spectra as diagnostic tools to solve some of the key problems in the study of ionised nebulae is demonstrated
Obtention d'alliages réfractaires SiCxNy(O) par dépÎt chimique à oartir d'une gazeuse Si(CH3)4-NH3
By pure thermal CVD from Si(CH/sub 3/)/sub 4/NH/sub 3/ mixtures SiC/sub x/N/sub y/(O) films have been prepared at 1475 K. Using electron microprobe analysis a continuous composition variation between SiC and Si/sub 3/N/sub 4/ is shown. The difficulty of quantitative analysis due to the conductivity of the samples is underlined. By selecting the right conditions, precise measurements were obtained. These films have an optical energy band gap in the range 2.2-4.1 eV and a constant photoluminescence peak. The film structure is discussed and these materials may be probably ascribed to an alloy phas
Development of modern analysis techniques for characterization and testing of coatings
Adaptive thresholding for reliable topological inference in single subject fMRI analysis
Single subject fMRI has proved to be a useful tool for mapping functional areas in clinical procedures such as tumour resection. Using fMRI data, clinicians assess the risk, plan and execute such procedures based on thresholded statistical maps. However, because current thresholding methods were developed mainly in the context of cognitive neuroscience group studies, most single subject fMRI maps are thresholded manually to satisfy specific criteria related to single subject analyses. Here, we propose a new adaptive thresholding method which combines Gamma-Gaussian mixture modelling with topological thresholding to improve cluster delineation. In a series of simulations we show that by adapting to the signal and noise properties, the new method performs well in terms of the trade-off between false negative and positive cluster error rates as well as in terms of over and underestimation of the true activation border. We also show through simulations and a motor test-retest study on ten volunteer subjects that adaptive thresholding improves reliability, mainly by accounting for the global signal variance. This in turn increases the likelihood that the true activation pattern can be determined
On the texture in diffusion-grown layers of silicides and germanides with the FeB structure, MeX (Me = Ti,Zr; X = Si,Ge) or the ZrSi2 structure (ZrSi2, HfSi2, ZrGe2)
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