1,745 research outputs found
A model for MRI contrast enhancement using T_1 agents
Contrast in MRI relies on differences in the local environment of water and is often enhanced by using contrast agents. We present a simple model for evaluating the minimal contrast agent concentration required to produce âsatisfactoryâ contrast enhancement in magnetic resonance images. Previous strategies have been based largely on empirical results for specific systems. The present tissue contrast model (TCM) can be applied to âconventional,â targeted, or biochemically responsive agents. The model results are formulated so that only a small number of parameters are required to analyze a given scenario. The TCM is a particularly useful tool in the development of new classes of magnetic resonance contrast media. These agents will have the ability to target specific cells or tissue, and perhaps be able to report on their physiological status. As an example of the applicability of the TCM, we test it against in vivo magnetic resonance microscopy results in frog embryos that have focal cell populations labeled with contrast agent by using calibrated single-cell microinjection techniques
Upgrade of the Glasgow photon tagging spectrometer for Mainz MAMI-C
The Glasgow photon tagging spectrometer at Mainz has been upgraded so that it
can be used with the 1500 MeV electron beam now available from the Mainz
microtron MAMI-C. The changes made and the resulting properties of the
spectrometer are discussed.Comment: 20 pages, 12 figure
RG-improved single-particle inclusive cross sections and forward-backward asymmetry in production at hadron colliders
We use techniques from soft-collinear effective theory (SCET) to derive
renormalization-group improved predictions for single-particle inclusive (1PI)
observables in top-quark pair production at hadron colliders. In particular, we
study the top-quark transverse-momentum and rapidity distributions, the
forward-backward asymmetry at the Tevatron, and the total cross section at
NLO+NNLL order in resummed perturbation theory and at approximate NNLO in fixed
order. We also perform a detailed analysis of power corrections to the leading
terms in the threshold expansion of the partonic hard-scattering kernels. We
conclude that, although the threshold expansion in 1PI kinematics is
susceptible to numerically significant power corrections, its predictions for
the total cross section are in good agreement with those obtained by
integrating the top-pair invariant-mass distribution in pair invariant-mass
kinematics, as long as a certain set of subleading terms appearing naturally
within the SCET formalism is included.Comment: 55 pages, 14 figures, 6 table
Evaluating satellite-based diurnal cycles of precipitation in the African tropics
Precipitation plays a major role in the energy and water cycles of the earth. Because of its variable nature, consistent observations of global precipitation are challenging. Satellite-based precipitation datasets present an alternative to in situâbased datasets in areas sparsely covered by ground stations. These datasets are a unique tool for model evaluations, but the value of satellite-based precipitation datasets depends on their application and scale. Numerous validation studies considered monthly or daily time scales, while less attention is given to subdaily scales. In this study subdaily satellite-based rainfall data are analyzed in West Africa, a region with strong diurnal variability. Several satellite-based precipitation datasets are validated, including Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA), TRMM 3G68 products, Precipitation Estimation from Remotely Sensed Information Using Artificial Neural Networks (PERSIANN), and Climate Prediction Center (CPC) morphing technique (CMORPH) data. As a reference, highly resolved in situ data from the African Monsoon Multidisciplinary AnalysisâCouplage de lâAtmosphere Tropical et du Cycle Hydrologique (AMMA-CATCH) are used. As a result, overall the satellite products capture the diurnal cycles of precipitation and its variability as observed on the ground reasonably well. CMORPH and TMPA data show overall good results. For locally induced convective rainfall in the evening most satellite data show slight delays in peak precipitation of up to 2 h
NNLO corrections to top-pair production at hadron colliders: the all-fermionic scattering channels
This is a second paper in our ongoing calculation of the
next-to-next-to-leading order (NNLO) QCD correction to the total inclusive
top-pair production cross-section at hadron colliders. In this paper we
calculate the reaction which was not considered
in our previous work on due to its phenomenologically
negligible size. We also calculate all remaining fermion-pair-initiated
partonic channels and that contribute to top-pair
production starting from NNLO. The contributions of these reactions to the
total cross-section for top-pair production at the Tevatron and LHC are small,
at the permil level. The most interesting feature of these reactions is their
characteristic logarithmic rise in the high energy limit. We compute the
constant term in the leading power behavior in this limit, and achieve
precision that is an order of magnitude better than the precision of a recent
theoretical prediction for this constant. All four partonic reactions computed
in this paper are included in our numerical program Top++. The calculation of
the NNLO corrections to the two remaining partonic reactions,
and , is ongoing.Comment: 1+16 pages; 3 figure
Exclusive measurement of coherent eta photoproduction from the deuteron
Coherent photoproduction of eta mesons from the deuteron has been measured
from threshold up to incident photon energies of 750 MeV using the photon
spectrometer TAPS at the tagged photon facility at the Mainz microtron MAMI.
For the first time, differential coherent cross sections have been deduced from
the coincident detection of the eta meson and the recoil deuteron. A missing
energy analysis was used for the suppression of background events so that a
very clean identification of coherent eta-photoproduction was achieved. The
resulting cross sections agree with previous experimental results except for
angles around 90 deg in the photon-deuteron cm-system where they are smaller.
They are compared to various model calculations.Comment: 4 pages, 4 figure
Recommended from our members
Reliability of regional climate model simulations of extremes and of long-term climate
We present two case studies that demonstrate how a common evaluation methodology can be used to assess the reliability of regional climate model simulations from different fields of research. In Case I, we focused on the agricultural yield loss risk for maize in Northeastern Brazil during a drought linked to an El-Niño event. In Case II, the present-day regional climatic conditions in Europe for a 10-year period are simulated. To comprehensively evaluate the model results for both kinds of investigations, we developed a general methodology. On its basis, we elaborated and implemented modules to assess the quality of model results using both advanced visualization techniques and statistical algorithms. Besides univariate approaches for individual near-surface parameters, we used multivariate statistics to investigate multiple near-surface parameters of interest together. For the latter case, we defined generalized quality measures to quantify the model's accuracy. Furthermore, we elaborated a diagnosis tool applicable for atmospheric variables to assess the model's accuracy in representing the physical processes above the surface under various aspects. By means of this evaluation approach, it could be demonstrated in Case Study I that the accuracy of the applied regional climate model resides at the same level as that we found for another regional model and a global model. Excessive precipitation during the rainy season in coastal regions could be identified as a major contribution leading to this result. In Case Study II, we also identified the accuracy of the investigated mean characteristics for near-surface temperature and precipitation to be comparable to another regional model. In this case, an artificial modulation of the used initial and boundary data during preprocessing could be identified as the major source of error in the simulation. Altogether, the achieved results for the presented investigations indicate the potential of our methodology to be applied as a common test bed to different fields of research in regional climate modeling
Physical activity and clustered cardiovascular disease risk factors in young children: a cross-sectional study (the IDEFICS study)
<p>Background
The relevance of physical activity (PA) for combating cardiovascular disease (CVD) risk in children has been highlighted, but to date there has been no large-scale study analyzing that association in children aged ≤9 years of age. This study sought to evaluate the associations between objectively-measured PA and clustered CVD risk factors in a large sample of European children, and to provide evidence for gender-specific recommendations of PA.</p>
<p>Methods
Cross-sectional data from a longitudinal study in 16,224 children aged 2 to 9 were collected. Of these, 3,120 (1,016 between 2 to 6 years, 2,104 between 6 to 9 years) had sufficient data for inclusion in the current analyses. Two different age-specific and gender-specific clustered CVD risk scores associated with PA were determined. First, a CVD risk factor (CRF) continuous score was computed using the following variables: systolic blood pressure (SBP), total triglycerides (TG), total cholesterol (TC)/high-density lipoprotein cholesterol (HDL-c) ratio, homeostasis model assessment of insulin resistance (HOMA-IR), and sum of two skinfolds (score CRFs). Secondly, another CVD risk score was obtained for older children containing the score CRFs + the cardiorespiratory fitness variable (termed score CRFs + fit). Data used in the current analysis were derived from the IDEFICS (âIdentification and prevention of Dietary- and lifestyle-induced health EFfects In Children and infantSâ) study.</p>
<p>Results
In boys <6 years, the odds ratios (OR) for CVD risk were elevated in the least active quintile of PA (OR: 2.58) compared with the most active quintile as well as the second quintile for vigorous PA (OR: 2.91). Compared with the most active quintile, older children in the first, second and third quintiles had OR for CVD risk score CRFs + fit ranging from OR 2.69 to 5.40 in boys, and from OR 2.85 to 7.05 in girls.</p>
<p>Conclusions
PA is important to protect against clustering of CVD risk factors in young children, being more consistent in those older than 6 years. Healthcare professionals should recommend around 60 and 85 min/day of moderate-to-vigorous PA, including 20 min/day of vigorous PA.</p>
Photoproduction of pions and properties of baryon resonances from a Bonn-Gatchina partial wave analysis
Masses, widths and photocouplings of baryon resonances are determined in a
coupled-channel partial wave analysis of a large variety of data. The
Bonn-Gatchina partial wave formalism is extended to include a decomposition of
t- and u-exchange amplitudes into individual partial waves. The multipole
transition amplitudes for and are
given and compared to results from other analyses.Comment: 18 pages, 14 figure
- âŠ