66 research outputs found
Spatial stratification of acoustic oscillations in the solar photosphere
Space-time variations of solar atmosphere parameters are derived by solving non-equilibrium radiation transfer problem. Acoustic oscillations were extracted using k − ω filtration of variations. In the lower photosphere there are evanescent remnants of underphotosphere oscillations; in the middle and high photosphere there are discrete sources of oscillations, which are excited by granule decay and formation of a new intergranule. The photosphere is penetrated by narrow “channels”, by which energy of fluctuations tunnels with minimal losses into the higher atmosphere layers; such “channels” arise mostly between ascending and descending flows. Particularities of the wave propagation in the solar atmosphere are determined by relationship between wavelength and the effective size of inhomogeneities
Spatial variations in the field of velocities and real solar granulation
In this paper, the physical conditions within the inhomogeneous solar
atmosphere have been reconstructed by means of solving the inverse problem of
Non Local Thermodynamic Equilibrium (NLTE) radiative transfer. The profiles of
nm FeI spectral line of high spatial and time resolution were
used as observational data. The velocity field has been studied for the real
solar granulation in superadiabatic layer and overshooting convection region.
Also, we investigate the vertical structure of inhomogeneous solar photosphere
and consider penetration of granules from convective region into upper layers
of stable atmosphere. The microturbulent velocity appears to be minimal at the
bottom of overshooting convection region and increases sharply through
superadiabatic layer and upper photosphere. High-turbulent layers emerge either
in the central part of a flow or at the boundary of an incipient flow with
following drift toward the centre of the flow. Wide descending flows tend to
disintegrate into structures having turbulence augmented, these structures
correspond to the flows of matter. High microturbulence of the intensive flows
provokes steep temperature depression in upper photosphere leading to the
second inversion of temperature for the intergranules. The inversion of
vertical velocities is observed to be frequent in the solar granulation. Some
of the convective flows reach the minimum temperature region. Vertical
convective velocities of the matter flows were found to be smaller in the
middle and upper photosphere. Also, the effect of finite resolution on the
spacial variations of the velocities in solar photosphere has been estimated.Comment: 8 pages, 7 figure
Comparison of the myocardial clearance of endothelial progenitor cells injected early versus late into reperfused or sustained occlusion myocardial infarction
Stem cell transplantation following AMI has shown promise for the repair or reduction of the amount of myocardial injury. There is some evidence that these treatment effects appear to be directly correlated to cell residence time. This study aims to assess the effects of (a) the timing of stem cell injection following myocardial infarction, and (b) flow milieu, on cell residence times at the site of transplantation by comparing three time points (day of infarction, week 1 and week 4-5), and two models of acute myocardial infarction (sustained occlusion or reperfusion). Twenty-one dogs received 2 injections of 30 million endothelial progenitor cells. The first injections were administered by epicardial (n = 8) or endocardial injection (n = 13) either on the day of infarction (n = 15) or at 1 week (n = 6). The second injections were administered by only endocardial injection (n = 18) 4 weeks following the first injection. Cell clearance half-lives were comparable between early and late injections. However, transplants into sustained occlusion infarcts resulted in slower cell clearance 77.1 ± 6.1 (n = 18) versus reperfused 59.4 ± 2.9 h (n = 21) p = 0.009. Sustained occlusion infarcts had longer cell retention in comparison to reperfusion whereas the timing of injection did not affect clearance rates. If the potential for myocardial regeneration associated with cell transplantation is, at least in part, linked to cell residence times, then greater benefit may be observed with transplants into infarcts associated with persistent coronary artery occlusion. © 2012 The Author(s)
Solar granulation from photosphere to low chromosphere observed in BaII 4554 A line
The purpose of this paper is to characterize the statistical properties of
solar granulation in the photosphere and low chromosphere up to 650 km. We use
velocity and intensity variations obtained at different atmospheric heights
from observations in BaII 4554 A. The observations were done during good seeing
conditions at the VTT at the Observatorio del Teide on Tenerife. The line core
forms rather high in the atmosphere and allows granulation properties to be
studied at heights that have been not accessed before in similar studies. In
addition, we analyze the synthetic profiles of the BaII 4554 A line by the same
method computed taking NLTE effects into account in the 3D hydrodynamical model
atmosphere. We suggest a 16-column model of solar granulation depending on the
direction of motion and on the intensity contrast measured in the continuum and
in the uppermost layer. We calculate the heights of intensity contrast sign
reversal and velocity sign reversal. We show that both parameters depend
strongly on the granulation velocity and intensity at the bottom photosphere.
The larger the two parameters, the higher the reversal takes place in the
atmosphere. On average, this happens at about 200-300 km. We suggest that this
number also depends on the line depth of the spectral line used in
observations. Despite the intensity and velocity reversal, about 40% of the
column structure of granulation is preserved up to heights around 650 km.Comment: accepted by Astronomy and Astrophysic
Properties of convective motions in facular regions
In this paper, we study the properties of solar granulation in a facular
region from the photosphere up to the lower chromosphere. Our aim is to
investigate the dependence of granular structure on magnetic field strength. We
use observations obtained at the German Vacuum Tower Telescope (Observatorio
del Teide, Tenerife) using two different instruments: Triple Etalon SOlar
Spectrometer (TESOS), in the BaII 4554 A line to measure velocity and intensity
variations along the photosphere; and, simultaneously, Tenerife Infrared
Polarimeter (TIP-II), in the FeI 1.56 m lines to the measure Stokes
parameters and the magnetic field strength at the lower photosphere. We obtain
that the convective velocities of granules in the facular area decrease with
magnetic field while the convective velocities of intergranular lanes increase
with the field strength. Similar to the quiet areas, there is a contrast and
velocity sign reversal taking place in the middle photosphere. The reversal
heights depend on the magnetic field strength and are, on average, about 100 km
higher than in the quiet regions. The correlation between convective velocity
and intensity decreases with magnetic field at the bottom photosphere, but
increases in the upper photosphere. The contrast of intergranular lanes
observed close to the disc center is almost independent of the magnetic field
strength. The strong magnetic field of facular area seems to stabilize the
convection and to promote more effective energy transfer in the upper layers of
the solar atmosphere, since the convective elements reach larger heights.Comment: accepted by Astronomy and Astrophysic
Technical note: development of a 3D printed subresolution sandwich phantom for validation of brain SPECT analysis
Purpose: To make an adaptable, head shaped radionuclide phantom to simulate molecular imaging of the brain using clinical acquisition and reconstruction protocols. This will allow the characterization and correction of scanner characteristics, and improve the accuracy of clinical image analysis, including the application of databases of normal subjects. Methods: A fused deposition modeling 3D printer was used to create a head shaped phantom made up of transaxial slabs, derived from a simulated MRI dataset. The attenuation of the printed polylactide (PLA), measured by means of the Hounsfield unit on CT scanning, was set to match that of the brain by adjusting the proportion of plastic filament and air (fill ratio). Transmission measurements were made to verify the attenuation of the printed slabs. The radionuclide distribution within the phantom was created by adding 99mTc pertechnetate to the ink cartridge of a paper printer and printing images of gray and white matter anatomy, segmented from the same MRI data. The complete subresolution sandwich phantom was assembled from alternate 3D printed slabs and radioactive paper sheets, and then imaged on a dual headed gamma camera to simulate an HMPAO SPECT scan. Results: Reconstructions of phantom scans successfully used automated ellipse fitting to apply attenuation correction. This removed the variability inherent in manual application of attenuation correction and registration inherent in existing cylindrical phantom designs. The resulting images were assessed visually and by count profiles and found to be similar to those from an existing elliptical PMMA phantom. Conclusions: The authors have demonstrated the ability to create physically realistic HMPAO SPECT simulations using a novel head-shaped 3D printed subresolution sandwich method phantom. The phantom can be used to validate all neurological SPECT imaging applications. A simple modification of the phantom design to use thinner slabs would make it suitable for use in PET
Small-animal SPECT and SPECT/CT: application in cardiovascular research
Preclinical cardiovascular research using noninvasive radionuclide and hybrid imaging systems has been extensively developed in recent years. Single photon emission computed tomography (SPECT) is based on the molecular tracer principle and is an established tool in noninvasive imaging. SPECT uses gamma cameras and collimators to form projection data that are used to estimate (dynamic) 3-D tracer distributions in vivo. Recent developments in multipinhole collimation and advanced image reconstruction have led to sub-millimetre and sub-half-millimetre resolution SPECT in rats and mice, respectively. In this article we review applications of microSPECT in cardiovascular research in which information about the function and pathology of the myocardium, vessels and neurons is obtained. We give examples on how diagnostic tracers, new therapeutic interventions, pre- and postcardiovascular event prognosis, and functional and pathophysiological heart conditions can be explored by microSPECT, using small-animal models of cardiovascular disease
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