100 research outputs found
Mathematik entdecken und erforschen â SchĂŒler/innen entwickeln selbststĂ€ndig mathematische Konzepte
<br
Spirometryâbased reconstruction of realâtime cardiac MRI: Motion control and quantification of heartâlung interactions
Comparison of cardiac volumetry using real-time MRI during free-breathing with standard cine MRI during breath-hold in children
Background Cardiac real-time magnetic resonance imaging (RT-MRI) provides high-quality images even during free- breathing. Difficulties in post-processing impede its use in clinical routine. Objective To demonstrate the feasibility of quantitative analysis of cardiac free-breathing RT-MRI and to compare image quality and volumetry during free-breathing RT-MRI in pediatric patients to standard breath-hold cine MRI. Materials and methods Pediatric patients (n= 22) received cardiac RT-MRI volumetry during free breathing (1.5 T; short axis; 30 frames per s) in addition to standard breath-hold cine imaging in end-expiration. Real-time images were binned retrospec- tively based on electrocardiography and respiratory bellows. Image quality and volumetry were compared using the European Cardiovascular Magnetic Resonance registry score, structure visibility rating, linear regression and BlandâAltman analyses. Results Additional time for binning of real-time images was 2 min. For both techniques, image quality was rated good to excellent. RT-MRI was significantly more robust against artifacts (P< 0.01). Linear regression revealed good correlations for the ventricular volumes. BlandâAltman plots showed a good limit of agreement (LoA) for end-diastolic volume (left ventricle [LV]: LoA -0.1 ± 2.7 ml/m2, right ventricle [RV]: LoA -1.9 ± 3.4 ml/m2), end-systolic volume (LV: LoA 0.4 ± 1.9 ml/m2, RV: LoA 0.6 ± 2.0 ml/m2), stroke volume (LV: LoA -0.5± 2.3 ml/m2, RV: LoA -2.6± 3.3 ml/m2) and ejection fraction (LV: LoA -0.5 ± 1.6%, RV: LoA -2.1 ± 2.8%). Conclusion Compared to standard cine MRI with breath hold, RT-MRI during free breathing with retrospective respiratory binning offers good image quality, reduced image artifacts enabling fast quantitative evaluations of ventricular volumes in clinical practice under physiological conditions
Svep1 stabilises developmental vascular anastomosis in reduced flow conditions
Molecular mechanisms controlling the formation, stabilization and maintenance of blood vessel connections remain poorly defined. Here we identify blood flow and the large extracellular protein Svep1 as co-modulators of vessel anastomosis during developmental angiogenesis in zebrafish embryos. Both loss of Svep1 and blood flow reduction contribute to defective anastomosis of intersegmental vessels. The reduced formation and lumenisation of the dorsal longitudinal anastomotic vessel (DLAV) is associated with a compensatory increase in Vegfa/Vegfr pERK signalling, concomittant expansion of apelin-positive tip cells, but reduced expression of klf2. Experimentally, further increasing Vegfa/Vegfr signalling can rescue the DLAV formation and lumenisation defects, while its inhibition dramatically exacerbates the loss of connectivity. Mechanistically, our results suggest that flow and Svep1 co-regulate the stabilization of vascular connections, in part by modulating the Vegfa/Vegfr signalling pathway
Ages of Young Star Clusters, Massive Blue Stragglers, and the Upper Mass Limit of Stars: Analyzing Age-dependent Stellar Mass Functions
Massive stars rapidly change their masses through strong stellar winds and mass transfer in binary systems. The latter aspect is important for populations of massive stars as more than 70% of all O stars are expected to interact with a binary companion during their lifetime. We show that such mass changes leave characteristic signatures in stellar mass functions of young star clusters that can be used to infer their ages and to identify products of binary evolution. We model the observed present-day mass functions of the young Galactic Arches and Quintuplet star clusters using our rapid binary evolution code. We find that the shaping of the mass function by stellar wind mass loss allows us to determine the cluster ages as 3.5 ± 0.7 Myr and 4.8 ± 1.1 Myr, respectively. Exploiting the effects of binary mass exchange on the cluster mass function, we find that the most massive stars in both clusters are rejuvenated products of binary mass transfer, i.e., the massive counterpart of classical blue straggler stars. This resolves the problem of an apparent age spread among the most luminous stars exceeding the expected duration of star formation in these clusters. We perform Monte Carlo simulations to probe stochastic sampling, which support the idea of the most massive stars being rejuvenated binary products. We find that the most massive star is expected to be a binary product after 1.0 ± 0.7 Myr in Arches and after 1.7 ± 1.0 Myr in Quintuplet. Today, the most massive 9 ± 3 stars in Arches and 8 ± 3 in Quintuplet are expected to be such objects. Our findings have strong implications for the stellar upper mass limit and solve the discrepancy between the claimed 150 M â limit and observations of four stars with initial masses of 165-320 M_â in R136 and of supernova 2007bi, which is thought to be a pair-instability supernova from an initial 250 M_â star. Using the stellar population of R136, we revise the upper mass limit to values in the range 200-500 M_â
The impact of magnetic fields on the IMF in star-forming clouds near a supermassive black hole
Star formation in the centers of galaxies is thought to yield massive stars
with a possibly top-heavy stellar mass distribution. It is likely that magnetic
fields play a crucial role in the distribution of stellar masses inside
star-forming molecular clouds. In this context, we explore the effects of
magnetic fields, with a typical field strength of 38 microG, such as in RCW 38,
and a field strength of 135 microG, similar to NGC 2024 and the infrared dark
cloud G28.34+0.06, on the initial mass function (IMF) near (< 10 pc) a 10^7
solar mass black hole. Using these conditions, we perform a series of numerical
simulations with the hydrodynamical code FLASH to elucidate the impact of
magnetic fields on the IMF and the star-formation efficiency (SFE) emerging
from an 800 solar mass cloud. We find that the collapse of a gravitationally
unstable molecular cloud is slowed down with increasing magnetic field strength
and that stars form along the field lines. The total number of stars formed
during the simulations increases by a factor of 1.5-2 with magnetic fields. The
main component of the IMF has a lognormal shape, with its peak shifted to
sub-solar (< 0.3 M_sun) masses in the presence of magnetic fields, due to a
decrease in the accretion rates from the gas reservoir. In addition, we see a
top-heavy, nearly flat IMF above ~2 solar masses, from regions that were
supported by magnetic pressure until high masses are reached. We also consider
the effects of X-ray irradiation if the central black hole is active. X-ray
feedback inhibits the formation of sub-solar masses and decreases the SFEs even
further. Thus, the second contribution is no longer visible. We conclude that
magnetic fields potentially change the SFE and the IMF both in active and
inactive galaxies, and need to be taken into account in such calculations.Comment: 10 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysics. 2 more references adde
Vergleich des Outcome von Hemiprothesen und inversen Schulterprothesen nach Humeruskopffrakturen beim Àlteren Menschen
Stellt nickelfreier Stahl eine Alternative zu 316L-Stahl und Titan fĂŒr Plattenosteosynthesen dar?
Behandlung von Claviculafrakturen mit winkelstabiler Plattenosteosynthese (LCP-SynthesÂź) - eine retrospective Singlecenterstudie
- âŠ