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PubMedDate not supplied

Effect of finite phosphor thickness on detective quantum efficiency.

In this paper we describe theoretically the relationship between the finite thickness of a phosphor screen and its spatial-frequency-dependent detective quantum efficiency DQE(f-). The finite thickness of the screen causes a variation in both the total number of light quanta emitted from the screen in a burst from a given x-ray interaction and in the spatial distribution of the quanta within the light burst [i.e., shape or point spread function (PSF) of the light burst]. The variation in magnitude of the burst gives rise to a spatial-frequency-independent reduction in DQE, characterized by the scintillation efficiency As. The variation in PSF causes a roll off in DQE with increasing spatial frequency which we have characterized by the function Rc(f). Both As and Rc(f) can be determined from the moments of the distribution of the spatial Fourier spectrum of light bursts emitted from the phosphor and thus they are related: As is a scaling factor for Rc(f). Our theory predicts that it is necessary for all light bursts which appear at the output to have the same magnitude to maximize As and the same shape to maximize Rc(f). These requirements can lead to the result that the fluorescent screen with the highest modulation transfer function will not necessarily have the highest DQE(f) even at high spatial frequencies.

Fourier Analysis↗
PubMedDate not supplied

[Ultrasonography and computerized tomography in giant Baker's cyst].

The US study of the posterior compartment of the knee usually allows the clinical suspicion of popliteal cyst to be confirmed. Nevertheless, in case of giant Baker's cysts--which are, at any rate, less frequent--US diagnosis is more difficult. CT can help determine the benign nature of the lesion, and allow an accurate spatial evaluation. Moreover CT, when accurately performed, demonstrates the caudal or cranial cystic spread, together with its clear separation from adjacent muscular tissues. Direct coronal scans are more useful to obtain better spatial definition than reformatted images.

Humans↗
PubMed1993

Induction of cortical spreading depression with potassium chloride upregulates levels of messenger RNA for glial fibrillary acidic protein in cortex and hippocampus: inhibition by MK-801.

The present study evaluates the time course and spatial extent of changes in GFAP mRNA expression following the induction of spreading depression. Spreading depression was elicited by applying filterpaper pledgets soaked in KCl (3 M) to exposed parietal cortex for ten minutes. Animals were killed 1.5, 3, 6, 12, 24, 48, 96 and 192 h post-KCl application, and the forebrains were prepared for quantitative in situ hybridization. The KCl treatment led to a many-fold increase in GFAP mRNA content in the ipsilateral hippocampus and neocortex and, to a lesser extent, in the contralateral hippocampus, but did not affect GFAP mRNA levels in the contralateral cortex or in the thalamus. The time course of increased expression of GFAP mRNA in the hippocampus differed markedly from that of the cortex. In the hippocampus, GFAP mRNA levels rose rapidly to a maximum at 24 h post-exposure, then fell rapidly. In the cortex, levels rose more slowly and did not reach a maximum until 4 days post-exposure. Analysis of GFAP mRNA levels by dot blot hybridization using samples from a separate set of animals killed at one and 4 days following the KCl exposure confirmed both the upregulation in GFAP mRNA levels and the regional time course differences. Intraperitoneal injection of MK-801, a non-competitive NMDA antagonist which prevents spreading depression, blocked the upregulation of GFAP mRNA in both the hippocampus and the cortex, as demonstrated by both in situ and dot blot hybridization. The results suggest that the physiological changes accompanying spreading depression have a powerful influence on glial cell gene expression.

Animals↗
PubMed2005

Application of spatially and temporally apodized non-confocal acoustic transmission microscopy to imaging of directly bonded wafers.

Application of a line-shaped point spread function (PSF) to imaging of void defects in directly bonded wafers is considered. Two non-confocally adjusted spherical transducers are employed to implement an acoustic microscope operating in transmission with a time dependent point spread function, whose shape is optimized by both temporal apodization of the received signal and spatial apodization of the transducer aperture. Strong imaging artifacts resulting from the generation and detection of edge waves are eliminated in this way. It is shown by several examples that only a broadband system can be utilized in order to obtain a line-shaped PSF suitable for imaging.

Journal Article↗
PubMed2002

Spatial mosaic and interfacial dynamics in a Müllerian mimicry system.

Uncovering why spatial mosaics of mimetic morphs are maintained in a Müllerian mimicry system has been a challenging issue in evolutionary biology. In this article, we analyze the reaction diffusion system that describes two-species Müllerian mimicry in one- and two-dimensional habitats. Due to positive frequency-dependent selection, a local population first approaches the state where one of the comimicking patterns predominates, which is followed by slow movement of boundaries where different patterns meet. We then analyze the interfacial dynamics of the boundaries to find whether a stable cline is maintained and to obtain the wave speed if the cline is unstable. The results are: (1) In a spatially uniform habitat the morph with greater base fitness spreads both in one and two species system. (2) The strength of cross-species interaction determines whether the mimetic morph clines of model and mimic species coalesce into the same geographical region or pass through each other. The joint wave speed of clines decreases by increasing the number of comimicking species in the mimicry ring. (3) In spatial heterogeneous habitats, stable clines can be maintained due to the balance between the base fitness gradient and the biased gene flow by negative curvature of boundary. This allows the persistence of a spatial mosaic even if one of the morphs is in every place advantageous over the other. A balanced cline is also maintained if there is a gradient in the population density. (4) A new advantageous morph occurring at a local region is doomed to go to extinction in a finite time if the "radius" of initial distribution is below a threshold. Possible applications to the heliconiine butterfly mimicry ring, heterozygous disadvantage systems of chromosomal rearrangement and hybrid zone, the third phase of Wright's Shifting Balance theory, and cytoplasmic incompatibility are discussed.

Animals↗
PubMed2005

Variation of microbiological parameters within planted soil filter for domestic wastewater treatment.

Microbial community structure was assessed in a horizontal subsurface flow planted sand filter treating domestic wastewater with molecular and culture-based methods. The diversity and spatial distribution of the microbial community was investigated using a PCR-DGGE (eubacterial and archaeal primers, ammonia-oxidizing bacteria, and ammonium monooxygenase specific primers), and spread plate and MPN counts. Significant differences were found in the spatial distribution of the microbial community structure. Data analysis revealed that different components of the microbial community possessed different spatial distribution patterns within the filter bed and depending on community type, relationships with soil chemical, and microbiological parameters varied. The most important spatial pattern in microbial community structure within the constructed wetland was related to the depth gradient, followed by differences between inflow and outflow. A comparison of a number of heterotrophic bacteria between inlet and outlet pipes as well as between two sampling depths showed no significant differences. In addition, the variation of the abundance of ammonia-oxidizing bacteria demonstrated no clear spatial pattern.

Bacteria↗
PubMed2005

Epidemic West Nile virus encephalomyelitis: a temperature-dependent, spatial model of disease dynamics.

Since first being detected in New York in 1999, West Nile virus (WNV) has spread throughout the United States and more than 20,000 cases of equine WNV encephalomyelitis have been reported. A spatial model of disease occurrence was developed, using data from an outbreak of serologically confirmed disease in an unvaccinated population of horses at 108 locations in northern Indiana between 3 August and 17 October 2002. Daily maximum temperature data were recorded at meteorological stations surrounding the study area. The distribution of the total number of degree-days elapsing between July 4 and the date of diagnosis of each case was best described by a normal distribution (mean=5243 degrees F, S.D.=1047). The days on which the average risk was >25, >50 and >75% were predicted (versus observed) to occur on August 23 (August 9), August 31 (September 2) and September 9 (September 9). The epidemic was predicted to occur 3 days earlier, or 4 days later, than observed if temperatures in the study area were uniformly increased, or decreased, by 5 degrees F, respectively. Maps indicated that WNV encephalomyelitis risk always remained greater in the northwest quadrant of the study area. Since WNV might exist at a hypoendemic level of infection, and occasionally re-emerge as a cause of epidemics in equine populations, by identifying factors that contributed to this epidemic, the potential impact of future epidemics can be reduced. Such studies rely on a GIS framework, availability of meteorological and possibly remotely sensed data and information on host and landscape factors. An early-warning system for WNV transmission in equine populations could be developed.

Animals↗
PubMed1998

Spatial structure, environmental heterogeneity, and population dynamics: analysis of the coupled logistic map.

Spatial extent can have two important consequences for population dynamics: It can generate spatial structure, in which individuals interact more intensely with neighbors than with more distant conspecifics, and it allows for environmental heterogeneity, in which habitat quality varies spatially. Studies of these features are difficult to interpret because the models are complex and sometimes idiosyncratic. Here we analyze one of the simplest possible spatial population models, to understand the mathematical basis for the observed patterns: two patches coupled by dispersal, with dynamics in each patch governed by the logistic map. With suitable choices of parameters, this model can represent spatial structure, environmental heterogeneity, or both in combination. We synthesize previous work and new analyses on this model, with two goals: to provide a comprehensive baseline to aid our understanding of more complex spatial models, and to generate predictions about the effects of spatial structure and environmental heterogeneity on population dynamics. Spatial structure alone can generate positive, negative, or zero spatial correlations between patches when dispersal rates are high, medium, or low relative to the complexity of the local dynamics. It can also lead to quasiperiodicity and hyperchaos, which are not present in the nonspatial model. With density-independent dispersal, spatial structure cannot destabilize equilibria or periodic orbits that would be stable in the absence of space. When densities in the two patches are uncorrelated, the probability that the population in a patch reaches extreme low densities is reduced relative to the same patch in isolation; this "rescue effect" would reduce the probability of metapopulation extinction beyond the simple effect of spreading of risk. Pure environmental heterogeneity always produces positive spatial correlations. The dynamics of the entire population is approximated by a nonspatial model with mean patch characteristics. This approximation worsens as the difference between the patches increases and the dispersal rate decreases: Under extreme conditions, destabilization of equilibria and periodic orbits occurs at mean parameter values lower than those predicted by the mean parameters. Apparent within-patch dynamics are distorted: The local population appears to have the wrong growth parameter and a constant number of immigrants (or emigrants) per generation. Adding environmental heterogeneity to spatial structure increases the occurrence of spatially correlated population dynamics, but the resulting temporal dynamics are more complex than would be predicted by the mean parameter values. The three classes of spatial pattern (positive, negative, and zero correlation), while still mathematically distinct, become increasingly similar phenomenologically.

Animals↗
PubMed1994

Simulation of spatial and contrast distortions in keyhole imaging.

Keyhole imaging is a scheme introduced to improve temporal resolution in dynamic contrast-enhanced MRI by a factor of four or more. A "full" acquisition before contrast administration is followed by truncated acquisitions sensitive primarily to changes in image contrast. Simulations of the point-spread functions that obtain, and their effect on contrast and spatial resolution, reveal significant degradation only for the smallest objects. Our simulations also address the feasibility of three-dimensional keyhold imaging, and demonstrate a potential 16-fold increase in temporal resolution. This suggests roles for keyhole imaging in conventional (nondynamic) precontrast and postcontrast studies and other applications.

Contrast Media↗
PubMed1999

Theoretical limits of spatial resolution in elliptical-centric contrast-enhanced 3D-MRA.

The point spread function (PSF) for contrast-enhanced three-dimensional (3D) MR angiography using the elliptical centric view order is derived. This view order has been shown previously to provide high venous suppression thereby enabling long acquisition times capable of high spatial resolution. The dependence of the PSF on TR, field of view (FOV), scan time, and trigger time are shown explicitly. Theoretical predictions are corroborated with experimental results in phantoms and in vivo. The PSF width decreases as the square root of the product of TR and the two phase encoding FOV's for fixed nominal voxel size. The PSF peak amplitude increases as the reciprocal of this product. Theory and experiment demonstrate that acquisition times over 40 sec provide superior resolution compared to shorter acquisitions, despite falling levels of contrast agent concentration. The analysis predicts that an isotropic spatial resolution of 1 mm before zero filling is possible in a FOV large enough to encompass the carotid and vertebral arteries bilaterally. Magn Reson Med 42:1106-1116, 1999.

Carotid Arteries↗
PubMedDate not supplied

[The spatial synchronization of the potentials in the background and evoked ECoG of a neuronally isolated strip of rabbit cortex].

Conditions of formation of potentials' spatial synchronization in the neuronally isolated cortical slab were studied in anaesthetized rabbits in acute experiments. Cortical potentials in all recording points of the slab were similar to each other. Seizure potentials induced by electrical stimulation spread over the slab and became synchronous in all points of recording. The neuronal division of the slab in two parts did not lead to a disturbance of initial synchronization of the background potentials in the isolated parts of the slab, while seizure potentials after electrical stimulation spread only over one part of the slab. These results suggest that the spatial synchronization of the EEG activity is a result of activity physically spreading from subcortical structures.

Animals↗
PubMed2005

Optimizing spatiotemporal sampling for k-t BLAST and k-t SENSE: application to high-resolution real-time cardiac steady-state free precession.

In k-t BLAST and k-t SENSE, data acquisition is accelerated by sparsely sampling k-space over time. This undersampling in k-t space causes the object signals to be convolved with a point spread function in x-f space (x = spatial position, f = temporal frequency). The resulting aliasing is resolved by exploiting spatiotemporal correlations within the data. In general, reconstruction accuracy can be improved by controlling the k-t sampling pattern to minimize signal overlap in x-f space. In this work, we describe an approach to obtain generally favorable patterns for typical image series without specific knowledge of the image series itself. These optimized sampling patterns were applied to free-breathing, untriggered (i.e., real-time) cardiac imaging with steady-state free precession (SSFP). Eddy-current artifacts, which are otherwise increased drastically in SSFP by the undersampling, were minimized using alternating k-space sweeps. With the synergistic combination of the k-t approach with optimized sampling and SSFP with alternating k-space sweeps, it was possible to achieve a high signal-to-noise ratio, high contrast, and high spatiotemporal resolutions, while achieving substantial immunity against eddy currents. Cardiac images are shown, demonstrating excellent image quality and an in-plane resolution of approximately 2.0 mm at >25 frames/s, using one or more receiver coils.

Artifacts↗
PubMed1984

Sister chromatid exchange (SCE) induced by laser-UV-microirradiation: correlation between the distribution of photolesions and the distribution of SCEs.

Small, medium, and large nuclear areas comprising approximately 5, 30, or 80% of the total area of the interphase nuclei of Chinese hamster cells (M3-1) cultivated in vitro were irradiated with a laser-UV-microbeam of wavelength 257 nm. The DNA of the cells was substituted with 5-bromodeoxyuridine (BrdUrd) for 1 cell cycle in one set of experiments. After microirradiation the cells were grown for a second cycle in medium without BrdUrd (protocol A). In a second set, cells with nonsubstituted DNA were microirradiated and grown for 2 additional cycles, the first in the presence, the second in the absence of BrdUrd (protocol B). In situ chromosome preparation and differential chromatid staining was subsequently performed. The induction of sister chromatid exchanges (SCEs) was found to be dependent on both the ultraviolet (UV) dose and the spatial distribution of the UV energy within the cell nucleus. Following both protocols the average number of chromosomes with SCEs was significantly higher after microirradiation of a large nuclear area as compared to microirradiation of a small nuclear area. In the latter case, multiple SCEs were noted on individual chromosome arms at the first postirradiation mitosis (protocol A). In other cells, especially at higher doses, protocol A resulted in shattering of a few closely neighbored chromosomes which were surrounded by intact ones with normal SCE levels. Microirradiation of medium-sized nuclear areas produced high levels of SCEs over a number of chromosomes which still appeared spatially related in a part of the metaphase spread. Finally, high SCE levels could be observed over most or all chromosomes when a large nuclear area (up to 100%) was exposed to the microbeam. Following protocol B the increase of SCEs was much less pronounced. Microirradiation of a small part of the cytoplasm in addition to the nuclei did not induce SCEs. Our results support the concept (i) that interphase chromosomes occupy distinct nuclear domains and indicate (ii) that the induction of SCEs by UV light is restricted to microirradiated chromatin.

Animals↗
PubMed2003

Genetic structure of a population of the ectomycorrhizal fungus Russula vinosa in subtropical woodlands in southwest China.

The genetic structure of a population of the ectomycorrhizal fungus Russula vinosa was analyzed using random amplified polymorphic DNA markers. Of 121 bands, 114 (94.2%) were polymorphic and there was a high genetic diversity (H=34.98) in this population. Each sporocarp represented a different genet and the genet size was no larger than 1 m. Pairwise sporocarps closer than 10 m had significantly higher genetic similarity. Second-order analysis indicated clumps with a radius of about 20 m in the whole population as well as in three genetic groups, i.e. simple matching similarity coefficients (Sm) 0.5-0.6, 0.6-0.7, and 0.7-0.8, respectively. The high-genetic-similarity group tended to have small clumps with high density, whereas the low-genetic-similarity group tended to have large clumps with low density. The spatial pattern analysis showed that the population mainly spread by short-distance spore dispersal rather than vegetative growth of dicaryophytic mycelia or long-distance spore dispersal.

Agaricales↗
PubMed2004

New insights into the expression and function of neural connexins with transgenic mouse mutants.

Gap junctions represent direct intercellular conduits between contacting cells. The subunit proteins of these conduits are called connexins. To date, 20 and 21 connexin genes have been described in the mouse and human genome, respectively, many of them represent sequence-orthologous pairs. Targeted deletion of connexin genes in the mouse genome opened new insights into the biological function of these channel forming proteins, which, in some cases, could be correlated to phenotypic abnormalities in humans, suffering from inherited diseases caused by mutations in the corresponding orthologous connexin gene. Replacing the connexin coding DNA by an appropriate reporter gene has clarified in several cases its cell type specific expression in mouse brain. Various studies demonstrated that connexin36 is mainly expressed in interneurons of retina and brain. Targeted deletion of connexin36 evoked a loss of electrical signal transduction and interferes with synchrony which probably leads to defects in visual transmission and memory. Deletion of connexin43 in astrocytes of mouse brain resulted in increased spreading depression consistent with the notion of altered "spatial buffering" of K(+) ions and glutamate secreted by active neurons. General connexin30-deficiency led to hearing impairment and apoptosis of hair cells, similar to that observed in mice with cochlea specific deletion of connexin26. Reporter gene expression in connexin30-deficient mice indicated that astrocytes in certain brain regions and leptomeningeal as well as ependymal cells are labelled. Reporter gene expression in connexin45- and connexin47-deficient mice was used to reassign connexin45 expression to certain CNS neurons and connexin47 expression to oligodendrocytes.

Animals↗
PubMed2005

Monte Carlo simulation of classical swine fever epidemics and control. II. Validation of the model.

A stochastic and spatial simulation model was developed to simulate the spread of classical swine fever virus among herds in a certain area. A model is a simplification of a real system. The mechanisms and parameters are often not exactly known. Validation is necessary to gain insight into model behaviour and to identify risk factors with great impact on the response variables. Several risk factors such as incubation period, number of daily farm contacts, probability of detection, probability of infection after contact, probability of local spread and time from infection to infectivity were considered in the model as probability distributions in order to take the stochastic component of disease dynamics into account. In order to estimate the effects of the risk factors on the response variables mean size and duration of epidemics, a sensitivity analysis was performed. A fractional factorial design with two-level factors (2(7-2) design) was developed to gain the maximum strength with minimum demand on the calculating capacity. The main factors were unconfounded with any other main factor and also unconfounded with two-factor interactions. Apart from the time from infection to infectivity, all risk factors had a significant effect on the mean size and duration of epidemics (p<0.05). Eight two-factor interactions had a significant influence as well (p<0.05). Mainly, two-factor interactions with probability of detection were significant thus emphasising the impact of a rapid detection of outbreaks. The reaction of the simulation responses to changing of the parameter values was consistent with the expected reaction.

Animals↗
PubMed2004

Chromatic assimilation: spread light or neural mechanism?

Chromatic assimilation is the shift in color appearance of a test field toward the appearance of nearby light. Possible explanations of chromatic assimilation include wavelength independent spread light, wavelength-dependent chromatic aberration and neural summation. This study evaluated these explanations by measuring chromatic assimilation from a concentric-ring pattern into an equal-energy-white background, as a function of the inducing rings' width, separation, chromaticity and luminance. The measurements showed, in the s direction, that assimilation was observed with different inducing-ring widths and separations when the inducing luminance was lower or higher than the test luminance. In general, the thinner the inducing rings and the smaller their separation, the stronger the assimilation in s. In the l direction, either assimilation or contrast was observed, depending on the ring width, separation and luminance. Overall, the measured assimilation could not be accounted for by the joint contributions from wavelength-independent spread light and wavelength-dependent chromatic aberration. Spatial averaging of neural signals explained the assimilation in s reasonably well, but there were clear deviations from neural spatial averaging for the l direction.

Adult↗
PubMedDate not supplied

Percolation on heterogeneous networks as a model for epidemics.

We consider a spatial model related to bond percolation for the spread of a disease that includes variation in the susceptibility to infection. We work on a lattice with random bond strengths and show that with strong heterogeneity, i.e. a wide range of variation of susceptibility, patchiness in the spread of the epidemic is very likely, and the criterion for epidemic outbreak depends strongly on the heterogeneity. These results are qualitatively different from those of standard models in epidemiology, but correspond to real effects. We suggest that heterogeneity in the epidemic will affect the phylogenetic distance distribution of the disease-causing organisms. We also investigate small world lattices, and show that the effects mentioned above are even stronger.

Disease Outbreaks↗