Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “spatial spread”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 379 records · Page 21Linked to original sources
PubMed2001

Analytical model of a three-dimensional integral image recording system that uses circular- and hexagonal-based spherical surface microlenses.

A mathematical model for a three-dimensional omnidirectional integral recording camera system that uses either circular- or hexagonal-based spherical surface microlens arrays is derived. The geometry of the image formation and recording process is fully described. Matlab is then used to establish the number of recorded micro-intensity distributions representing a single object point and their dependence on spatial position. The point-spread function for the entire optical process for both close and remote imaging is obtained, and the influence of depth on the point-spread dimensions for each type of microlens and imaging condition is discussed. Comparisons of the two arrangements are made, based on the illustrative numerical results presented.

Journal Article↗
PubMed2005

Sex-linked differentiation between incipient species of Anopheles gambiae.

Emerging species within the primary malaria vector Anopheles gambiae show different ecological preferences and significant prezygotic reproductive isolation. They are defined by fixed sequence differences in X-linked rDNA, but most previous studies have failed to detect large and significant differentiation between these taxa elsewhere in the genome, except at two other loci on the X chromosome near the rDNA locus. Hypothesizing that this pericentromeric region of the X chromosome may be accumulating differences faster than other regions of the genome, we explored the pattern and extent of differentiation between A. gambiae incipient species and a sibling species, A. arabiensis, from Burkina Faso, West Africa, at 17 microsatellite loci spanning the X chromosome. Interspecific differentiation was large and significant across the entire X chromosome. Among A. gambiae incipient species, we found some of the highest levels of differentiation recorded in a large region including eight independent loci near the centromere of the X chromosome. Outside of this region, no significant differentiation was detected. This pattern suggests that selection is playing a role in the emergence of A. gambiae incipient species. This process, associated with efficient exploitation of anthropogenic modifications to the environment, has public health implications as it fosters the spread of malaria transmission both spatially and temporally.

Animals↗
PubMed2005

[Effects of fungicide on temporal and spatial niches of Rhizoctonia solani].

This paper studied the variation of the temporal and spatial niches of rice sheath blight-causing Rhizoctonia solani under spraying fungicide. The results showed that between fungicide treatments and control, the spatial niche breadth of Rhizoctonia solani was different, while its temporal niche breadth was similar. The spatial niche breadth of Rhizoctonia solani was 0.5240 when spraying fungicide at booting and full heading stages, 0.5742 at booting and milkfilling stages, and 0.8577 at tillering stage, while the control was 0.8563. Spraying fungicide had little effect on temporal niche breath. The spatial niche breath, percentage of diseased leaves and sheathes, index of disease, and control effect all suggested that spraying fungicide two times in rice growth period was better than spraying it one time. Spraying fungicide at booting and full heading stages and at booting stage and milkfilling stages had the best effects. Fungicide could not only narrow the spatial niche breadth, limit the spread of Rhizoctonia solani on top leaves which were important for the yield, but also narrow the temporal niche breadth, limit the spread of Rhizoctonia solani during the yield formation period of rice. But, spraying fungicide only changed the distribution of the fungus niches and narrowed the niches during rice yield formation period, with no changes in the whole niches of Rhizoctonia solani.

Fungicides, Industrial↗
PubMed1985

[Changes in the properties of smooth muscle cells of the fundus portion of the stomach after removal of the body and antral portion].

Sucrose-gap and microelectrode techniques revealed that the spatial constant (value of electrotonic spread of the slow potential) was greater in fundal portion of stomach with removed body and antral portion than in intact stomach. This finding suggests an increased conductivity of the fundal portion muscle tissue after removal of evacuatory and hormone--producing portions of the stomach. The sodium--calcium nature of the slow potential was revealed in the membrane of fundal smooth muscle cells. After removal of the stomach body and antral portion the permeability of smooth muscle cells is altered in the stomach fundus and the slow potential becomes the same as in the stomach wall of intact dogs. This mechanism of electrophysiological alteration maintains the evacuatory function after removal of the stomach body and antral portion.

Animals↗
PubMed1980

Feasibility of time-of-flight reconstruction in positron emission tomography.

A feasibility study was carried out to determine whether image quality can be improved by the use of time-of-flight (TOF) information in positron emission tomography (PET). The experiment used two fast cesium fluoride detectors followed by constant-fraction discriminators for coincidence-timing resolutions of 600 to 800 psec full width at half maximum, depending on the energy discrimination level. A point source was scanned to study the spatial response of the point spread function with and without the TOF information for nonfiltered back-projected data. Back-projected images of a simplified chest phantom, 42 cm in diameter and filled with relative activity concentrations of 1.0, and 5, are presented for the unfiltered data to demonstrate the improvement in image quality obtained with the use of TOF. Filtered and reconstructed images of this phantom are also presented to show the relative differences in the images obtained with PET and TOF-PET techniques for similar filter functions.

Physical Phenomena↗
PubMed1994

Comparison of four scatter correction methods using Monte Carlo simulated source distributions.

UNLABELLED: Scatter correction in SPECT is important for improving image quality, boundary detection and the quantification of activity in different regions. This paper presents a comparison of four scatter correction methods, three using more than one energy window and one convolution-subtraction correction method using spatial variant scatter line-spread functions. METHODS: The comparison is based on Monte Carlo simulated data for point sources on- and off-axis, hot and cold spheres of different diameters, and a clinically realistic source distribution simulating brain imaging. All studies were made for a uniform cylindrical water phantom. Since the nature of the detected photon is known with Monte Carlo simulation, separate images of primary and scattered photons can be recorded. These can then be compared with estimated scatter and primary images obtained from the different scatter correction methods. The criteria for comparison were the normalized mean square error, scatter fraction, % recovery and image contrast. RESULTS: All correction methods significantly improved image quality and quantification compared to those obtained with no correction. Quantitatively, no single method was observed to be the best by all criteria for all the source distributions. Three of the methods were observed to perform the best by at least one of the criteria for one of the source distributions. For brain imaging, the differences between all the methods were much less than the difference between them and no correction at all. CONCLUSION: It is concluded that performing scatter correction is essential for accurate quantification, and that all four methods yield a good, but not perfect, scatter correction. Since it is hard to distinguish the methods consistently in terms of their performance, it may be that the choice should be made on the basis of ease of implementation.

Brain↗
PubMed2006

Spread of equine West Nile virus encephalomyelitis during the 2002 Texas epidemic.

Using reports of clinical West Nile virus (WNV) encephalomyelitis in Texas equids during 2002, the distribution of disease was analyzed using cluster statistics and spatial modeling to develop hypotheses of disease spread during the first year of its detection. Significant (P < 0.05) clusters of cases reported early during the outbreak were identified in east, northcentral, and north Texas, and significant (P < 0.05) clusters late during the outbreak were detected in central, south, and west Texas. Two counties on the south Texas coast first reported disease significantly (P < 0.05) earlier than their 10 nearest neighboring counties. The estimated incidence of disease was greatest in the high plains of north Texas and in northcentral Texas. Higher rates were also estimated in eastern and southern areas of the Gulf Coast. The spatial and temporal distribution observed indicates that the equine WNV epidemic began in two parts of Texas and spread elsewhere throughout the state. The mechanism of introduction and spread remains speculative.

Animals↗
PubMed2004

Nest architecture, activity pattern, worker density and the dynamics of disease transmission in social insects.

The role of disease in the organization of insect colonies has become an important focus of research in evolutionary pathobiology, in which the relationship of sociality and disease transmission can be comparatively and experimentally analysed. In this paper we use an individual-based model of disease transmission to assess how an epidemic is influenced by worker density and activity level, the probability of disease transmission, and the structural organization of the nest. First, we observed in our model a nonlinear interaction between worker density and the probability of disease transmission: high levels of both factors interact to enhance the likelihood of an epidemic. Additionally, when we incorporated in our model the empirical observation that only a fraction of the worker population in social insect colonies is active at any given point in time, results suggested that relatively low levels of worker movement can have a significant impact on the spread of disease, slowing its transmission through the colony. Finally, we found that nests having even a simple spatial separation of chambers could delay the spread of infection and diminish the severity of an outbreak. The effect of nest structure in delaying infection spread became more pronounced as nest architecture became increasingly unidimensional, as in the case of simple gallery nests. Therefore, nest architecture and worker activity patterns might indeed exert considerable influence on the dynamics of epidemics in social insects and should be incorporated into models of disease transmission.

Animals↗
PubMed2003

Identification of geographic factors associated with early spread of foot-and-mouth disease.

OBJECTIVE: To explore whether early analysis of spatial data may result in identification of variables associated with epidemic spread of foot and mouth disease. SAMPLE POPULATION: 37 farms with infected cattle (ie, case farms) reported within the first 6 days of the 2001 Uruguayan foot-and-mouth disease epidemic. PROCEDURE: A georeferenced database was created and retrospective analysis was performed on case farm location in relation to farm density, cattle density, farm type (ie, beef vs dairy cattle production), road density, case farm distance to the nearest road, farm size, farm ownership, and day of infection. Mean or median results of 1 to 3 day versus 4 to 6 day spatial data were compared. Spatial-temporal associations were investigated by correlation analysis. RESULTS: Comparison of mean or median values between the first 3 days and days 4 to 6 of the epidemic and results of correlation analysis indicated a significant increase in road density, cattle density, and dairy cattle production and a significant decrease in farm size and case farm distance to the nearest road that developed over time. A route that linked most case farms by the shortest possible distance and also considered significantly associated variables was created. It included 86.1% of all case farms reported by 60 days into the epidemic. CONCLUSIONS AND CLINICAL RELEVANCE: Epidemic direction can be assessed on the basis of road density and other spatial variables as early as 6 days into an epidemic. Epidemic control areas may be more effectively identified if local and regional georeferenced data are considered.

Agriculture↗
PubMed1980

Contrast thresholds versus border enhancement: effect of scattered light.

Contrast thresholds with sine wave grids, spatial frequency thresholds and border enhancement spread were measured while the amount of scattering in the retinal image was varied. Narrow-angle scattering similar to that produced by the cornea and crystalline lens was generated by specially designed filters. The purpose was to assess the effectivity of these widely different responses as measures of small order scattering effects. As noted by other investigators, spatial frequency thresholds proved insensitive to small amounts of scattering. On the other hand, contrast thresholds for high frequency targets and border enhancement spread both showed significant changes with small amounts of scattering. Of these two criteria, border enhancement proved to be the more consistent scattering index.

Humans↗
PubMed2006

Plant-soil feedbacks and invasive spread.

Plant invaders have been suggested to change soil microbial communities and biogeochemical cycling in ways that can feedback to benefit themselves. In this paper, we ask when do these feedbacks influence the spread of exotic plants. Because answering this question is empirically challenging, we show how ecological theory on 'pushed' and 'pulled' invasions can be used to examine the problem. We incorporate soil feedbacks into annual plant invasion models, derive the conditions under which such feedbacks affect spread, and support our approach with simulations. We show that in homogeneous landscapes, strong positive feedbacks can influence spreading velocity for annual invaders, but that empirically documented feedbacks are not strong enough to do so. Moreover, to influence spread, invaders must modify the soil environment over a spatial scale larger than is biologically realistic. Though unimportant for annual invader spread in our models, feedbacks do affect invader density and potential impact. We discuss how future research might consider the way landscape structure, dispersal patterns, and the time scales over which plant-soil feedbacks develop regulate the effects of such feedbacks on invader spread.

Models, Theoretical↗
PubMed2005

Wave-like spread of Ebola Zaire.

In the past decade the Zaire strain of Ebola virus (ZEBOV) has emerged repeatedly into human populations in central Africa and caused massive die-offs of gorillas and chimpanzees. We tested the view that emergence events are independent and caused by ZEBOV variants that have been long resident at each locality. Phylogenetic analyses place the earliest known outbreak at Yambuku, Democratic Republic of Congo, very near to the root of the ZEBOV tree, suggesting that viruses causing all other known outbreaks evolved from a Yambuku-like virus after 1976. The tendency for earlier outbreaks to be directly ancestral to later outbreaks suggests that outbreaks are epidemiologically linked and may have occurred at the front of an advancing wave. While the ladder-like phylogenetic structure could also bear the signature of positive selection, our statistical power is too weak to reach a conclusion in this regard. Distances among outbreaks indicate a spread rate of about 50 km per year that remains consistent across spatial scales. Viral evolution is clocklike, and sequences show a high level of small-scale spatial structure. Genetic similarity decays with distance at roughly the same rate at all spatial scales. Our analyses suggest that ZEBOV has recently spread across the region rather than being long persistent at each outbreak locality. Controlling the impact of Ebola on wild apes and human populations may be more feasible than previously recognized.

Animals↗
PubMed1995

Spatial encoding and reconstruction in MRI with quadratic phase profiles.

Application of a slice-selective frequency modulated radiofrequency excitation pulse in a magnetic resonance (MR) imaging experiment creates a quadratic phase profile along the direction of slice selection. This quadratic phase profile inherently localizes the signal contribution to its vertex, which can be shifted with the application of a linear magnetic field gradient. Different methods for reconstruction of the spatial profile are discussed. The point spread functions of these reconstructions are similar to that of conventional Fourier transforms; one important difference is the elimination of aliasing at the expense of MR signal amplitude when performing 'local,' or limited, reconstruction. Limited reconstruction together with spatially limited excitation combines some of the advantages of both three-dimensional phase-encoded and two-dimensional multislice techniques in appropriate situations. Applications discussed are three-dimensional gradient echo experiments applied to time of flight MR angiography and T2(+)-weighted data collection.

Computer Simulation↗
PubMed2000

Regularization for uniform spatial resolution properties in penalized-likelihood image reconstruction.

Traditional space-invariant regularization methods in tomographic image reconstruction using penalized-likelihood estimators produce images with nonuniform spatial resolution properties. The local point spread functions that quantify the smoothing properties of such estimators are space-variant, asymmetric, and object-dependent even for space-invariant imaging systems. We propose a new quadratic regularization scheme for tomographic imaging systems that yields increased spatial uniformity and is motivated by the least-squares fitting of a parameterized local impulse response to a desired global response. We have developed computationally efficient methods for PET systems with shift-invariant geometric responses. We demonstrate the increased spatial uniformity of this new method versus conventional quadratic regularization schemes in simulated PET thorax scans.

Computer Simulation↗
PubMed1999

Optical Spatial Solitons and Their Interactions: Universality and Diversity.

Spatial solitons, beams that do not spread owing to diffraction when they propagate, have been demonstrated to exist by virtue of a variety of nonlinear self-trapping mechanisms. Despite the diversity of these mechanisms, many of the features of soliton interactions and collisions are universal. Spatial solitons exhibit a richness of phenomena not found with temporal solitons in fibers, including effects such as fusion, fission, annihilation, and stable orbiting in three dimensions. Here the current state of knowledge on spatial soliton interactions is reviewed.

Journal Article↗
PubMed2003

Linear system models for ultrasonic imaging: application to signal statistics.

Linear equations for modeling echo signals from shift-variant systems forming ultrasonic B-mode, Doppler, and strain images are analyzed and extended. The approach is based on a solution to the homogeneous wave equation for random inhomogeneous media. When the system is shift-variant, the spatial sensitivity function--defined as a spatial weighting function that determines the scattering volume for a fixed point of time--has advantages over the point-spread function traditionally used to analyze ultrasound systems. Spatial sensitivity functions are necessary for determining statistical moments in the context of rigorous image quality assessment, and they are time-reversed copies of point-spread functions for shift variant systems. A criterion is proposed to assess the validity of a local shift-invariance assumption. The analysis reveals realistic situations in which in-phase signals are correlated to the corresponding quadrature signals, which has strong implications for assessing lesion detectability. Also revealed is an opportunity to enhance near- and far-field spatial resolution by matched filtering unfocused beams. The analysis connects several well-known approaches to modeling ultrasonic echo signals.

Computer Simulation↗
PubMed1996

[Social networks and diffusion of AIDS in Brazil].

The HIV/AIDS epidemic has been simplistically interpreted as a phenomenon restricted to risk groups that are socially and spatially circumscribed. However, epidemic trends in recent years have demonstrated the need to employ open diffusion models that emphasize social interaction as a means of spread of HIV. This study is a spatial analysis of the AIDS epidemic in Brazil, which sought to incorporate variables reflecting economic and demographic events into a system for processing geographically referenced health information. Findings indicate that metropolises and regional urban centers, mainly those in the Southeast, play an important role in the spread of the epidemic, not only because of their population density but also because they are centers of trade and social interaction. In smaller cities located in the state of São Paulo, a large number of AIDS cases among injecting drug users are concentrated, revealing the routes and centers of cocaine use.

Acquired Immunodeficiency Syndrome↗
PubMed1992

Steady-state heat transfer and thermal zone spreading in gel isoelectric focusing.

The zone spreading caused by a transverse pH profile due to a temperature gradient through the thickness of a gel slab is estimated. The temperature difference (delta T) between the upper and lower gel surfaces can be calculated as a function of the electric power dissipated in the gel and the gel dimensions. It is found that when delta T is only 1 degree C the zone spreading due to this thermal excursion is as high as 0.5 mm. Thus, an admissible delta T is found to be equal to 0.2 degrees C, since this corresponds to a thermal zone spreading of only 0.1 mm, i.e. the same order of magnitude as the spatial resolution of a laser scanner. A thermal zone spreading of 0.1 mm is compatible with a resolving power of 0.01 pH unit, the current limit of conventional isoelectric focusing in amphoteric buffers. A requirement for the thickness of a gel slab is formulated: e.g., at 40 W applied power (over a gel surface area of 25 x 11 cm), a thermal zone spreading limited to 0.1 mm can only be obtained with a gel thickness of approximately 170 microns.

Hot Temperature↗