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PubMed2004

Periodically varying externally imposed environmental effects on population dynamics.

Effects of externally imposed periodic changes in the environment on population dynamics are studied with the help of a simple model. The environmental changes are represented by the temporal and spatial dependence of the competition terms in a standard equation of evolution. Possible applications of the analysis are on the one hand to bacteria in Petri dishes and on the other to rodents in the context of the spread of the Hantavirus epidemic. The analysis shows that spatiotemporal structures emerge, with interesting features which depend on the interplay of separately controllable aspects of the externally imposed environmental changes.

Animals↗

PubMed2001

Determination of the detective quantum efficiency of a prototype, megavoltage indirect detection, active matrix flat-panel imager.

After years of aggressive development, active matrix flat-panel imagers (AMFPIs) have recently become commercially available for radiotherapy imaging. In this paper we report on a comprehensive evaluation of the signal and noise performance of a large-area prototype AMFPI specifically developed for this application. The imager is based on an array of 512 x 512 pixels incorporating amorphous silicon photodiodes and thin-film transistors offering a 26 x 26 cm2 active area at a pixel pitch of 508 microm. This indirect detection array was coupled to various x-ray converters consisting of a commercial phosphor screen (Lanex Fast B, Lanex Regular, or Lanex Fine) and a 1 mm thick copper plate. Performance of the imager in terms of measured sensitivity, modulation transfer function (MTF), noise power spectra (NPS), and detective quantum efficiency (DQE) is reported at beam energies of 6 and 15 MV and at doses of 1 and 2 monitor units (MU). In addition, calculations of system performance (NPS, DQE) based on cascaded-system formalism were reported and compared to empirical results. In these calculations, the Swank factor and spatial energy distributions of secondary electrons within the converter were modeled by means of EGS4 Monte Carlo simulations. Measured MTFs of the system show a weak dependence on screen type (i.e., thickness), which is partially due to the spreading of secondary radiation. Measured DQE was found to be independent of dose for the Fast B screen, implying that the imager is input-quantum-limited at 1 MU, even at an extended source-to-detector distance of 200 cm. The maximum DQE obtained is around 1%--a limit imposed by the low detection efficiency of the converter. For thinner phosphor screens, the DQE is lower due to their lower detection efficiencies. Finally, for the Fast B screen, good agreement between calculated and measured DQE was observed.

Biophysical Phenomena↗

PubMedDate not supplied

A laser-based multiformat camera for medical imaging.

The spatial resolution and contrast resolution required of a multiformat camera (MFC) for medical imaging are discussed. A typical cathode-ray tube (CRT) MFC and a prototype laser MFC are compared based on the following measured quantities: line spread function and associated contrast transfer function, noise characteristics, intensity transfer function (dynamic range), large-area contrast, and film irradiance. The laser MFC is found to provide significantly better performance than the CRT MFC in all of these areas.

Fourier Analysis↗

PubMed1997

Enhanced cross-talk correction technique for simultaneous dual-isotope imaging: a TL-201/Tc-99m myocardial perfusion SPECT dog study.

A newly developed cross-talk correction method for simultaneous dual-isotope SPECT imaging was tested in a canine model. The method is based on the assumption that the transformations, which modify the primary energy window images into the scatter images as viewed in the other energy windows, are known. These transformations were found by measuring the point spread functions (PSFs) in two different energy windows for both isotopes in water. The dual-isotope correction method is described by two convolution equations which were applied in frequency space. The equations take into account the different spatial distributions of the primary and scatter cross-talk photons. The new enhancement of the method was in applying restoration filters to the resulting corrected images. Three separate studies were acquired in our dog study: two single-isotope and one dual-isotope study. The single isotope images were used as references. The contrast between the left ventricle cavity (LVC) and the myocardium was used in transaxial and short-axis slices as a parameter to evaluate results of dual-isotope correction method with restoration. The change in contrast in the dual-isotope corrected images in both energy windows, i.e., Tc-99m primary window (140 keV) and Tl-201 primary window (70 keV), was significant. The only exception was for the short-axis Tc-99m window images. The corrected 140 keV dual-isotope short-axis slice had the contrast of 0.60 vs 0.58, which was the value in the noncorrected dual-isotope short-axis slice. For dual-isotope 140 keV transaxial slice, the contrast changed from 0.72 to 0.82 after correction. In comparison, for single-isotope Tc-99m 140 keV transaxial slice, contrast changed from 0.62 to 0.84 after restoration correction. There was less change in contrast in the short-axis Tc-99m 140 keV slice, i.e., from 0.56 to 0.61. In the Tl-201 primary window for the transaxial slices the improvement of contrast was from 0.38 to 0.64, and for short-axis slices from 0.22 to 0.32 after correction. In the same 70 keV energy window for single-isotope Tl-201 images, contrast improved from 0.61 to 0.69 and from 0.35 to 0.38 for transaxial and short-axis slice, respectively, after applying restoration correction. In conclusion, the presented dual-isotope correction method with restoration improves the quality of the simultaneous rest Tl-201/stress Tc-99m sestamibi SPECT imaging.

Animals↗

PubMed2006

Infection control and pandemic influenza.

If an influenza pandemic occurs, the spread of the virus should be reduced for as long as possible while an effective vaccine is produced. Influenza spreads mainly by large respiratory droplets (> 5 microm) depositing onto the mucosal surfaces of the eye, mouth or respiratory tract. Hands are another major means for spread, and are frequently contaminated by droplets. The most effective way to reduce the spread of the virus is with good infection control practices and social distancing. Infection control practices include the use of personal protective equipment (PPE), hand hygiene, and respiratory hygiene and cough etiquette. Infected people should be isolated and spatial separation observed in common areas where infected people may be present. Any practices that create aerosols (eg, nebulisation) should be avoided, unless performed with appropriate precautions, especially with all people in the room wearing appropriate PPE. Now is the time to re-examine all our current practices so that we are better prepared, well practised and have good infection control practices in place for all transmissible respiratory infections.

Australia↗

PubMed2002

Evolutionary speed of species invasions.

Successful invasion may depend of the capacity of a species to adjust genetically to a spatially varying environment. This research modeled a species invasion by examining the interaction between a quantitative genetic trait and population density. It assumed: (I) a quantitative genetic trait describes the adaptation of an individual to its local ecological conditions; (2) populations far from the local optimum grow more slowly than those near the optimum; and (3) the evolution of a trait depends on local population density, because differences in local population densities cause asymmetrical gene flow. This genetics-density interaction determined the propagation speed of populations. Numerical simulations showed that populations spread by advancing as two synchronic traveling waves, one for population density and one for trait adaptation. The form of the density wave was a step front that advances homogenizing populations at their carrying capacity; the adaptation wave was a curve with finite slope that homogenizes populations at full adaptation. The largest speed of population expansion, for a dimensionless analysis, corresponded to an almost homogeneous spatial environment when this model approached an ecological description such as the Fisher-Skellam's model. A large genetic response also favored faster speeds. Evolutionary speeds, in a natural scale, showed a wide range of rates that were also slower compared to models that only consider demographics. This evolutionary speed increased with high heritability, strong stabilizing selection, and high intrinsic growth rate. It decreased for steeper environmental gradients. Also indicated was an optimal dispersal rate over which evolutionary speed declined. This is expected because dispersal moves individuals further, but homogenizes populations genetically, making them maladapted. The evolutionary speed was compared to observed data. Furthermore, a moderate increase in the speed of expansion was predicted for ecological changes related to global warming.

Adaptation, Biological↗

PubMed1998

Mechanisms for positional signalling by morphogen transport: a theoretical study.

Gradients of cellular activities are ubiquitous in embryonic development. It is widely believed that the inhomogeneous spatial distribution of a morphogen would be able to set up such gradients. But how then does the morphogen propagate in the first place? Straightforward molecular diffusion is often proposed as a possible mechanism. We first show that, surprisingly, the mere binding of the diffusing morphogen to its membrane receptors suffices to prevent the establishment of a concentration-based positional signalling system. Instead, a flat, saturated distribution of receptor-bound morphogen builds up. Because the distribution spreads gradually from the morphogen source, however, cells may still know their position if they are able to integrate the morphogen signal in time. The irregularities of diffusion in the complex extracellular medium would in fact be partially compensated for by such time summation. Another, non-exclusive possibility is that morphogen transport does not occur by simple diffusion only. We put forth a novel model of receptor-aided, directed diffusion that achieves a spatial distribution of morphogen. Our model is based, as an illustration, on the properties of members of the TGFbeta family of molecules. We show that two simple hypotheses regarding the kinetics of TGBbeta binding to its receptors suffice to establish a remarkable transfer mechanism whereby a morphogen such as activin could be both propagated along cell membranes, and transferred between cells that are in contact. The model predicts that morphogen propagation properties depend strongly on the closeness of cell-cell appositions, does not necessitate protein synthesis, accumulation or slow degradation (in contrast to the diffusion/time integration model), and that the morphogen is localised mostly on or close to cell membranes.

Animals↗

PubMed1975

Alleopathy and spatial competition among coral reef invertebrates.

Species of ectoprocts and solitary encrusting animals were subjected in aquaria to homogenates of 11 sympatric species of sponges and colonial ascidians. Five of the nine sponge species and one of the two ascidian species exhibited species-specific allelochemical effects. Evidence suggests that alleochemical provide a wide-spread, specific, and complex mechanism for interference competition for space among natural populations of coral reef organisms. The existence of such species-specific mechanisms may provide a basis for maintenance of diversity in space-limited systems in the absence of high levels of predation and physical disturbance.

Journal Article↗

PubMed2005

Lateral spacing of integrin ligands influences cell spreading and focal adhesion assembly.

Cell-extracellular matrix (cell-ECM) interactions mediated by integrin receptors are essential for providing positional and environmental information necessary for many cell functions, such as proliferation, differentiation and survival. In vitro studies on cell adhesion to randomly adsorbed molecules on substrates have been limited to sub-micrometer patches, thus preventing the detailed study of structural arrangement of integrins and their ligands. In this article, we illustrate the role of the distance between integrin ligands, namely the RGD (arginine-glycine-aspartate) sequence present in ECM proteins, in the control of cell adhesion. By using substrates, which carry cyclic RGD peptides arranged in highly defined nanopatterns, we investigated the dynamics of cell spreading and the molecular composition of adhesion sites in relation to a fixed spacing between the peptides on the surface. Our novel approach for in vitro studies on cell adhesion indicates that not only the composition, but also the spatial organization of the extracellular environment is important in regulating cell-ECM interactions.

Animals↗

PubMed2006

Diversity and evolution of West Nile virus in Illinois and the United States, 2002-2005.

Evolutionary analyses of West Nile virus (WNV) have been limited by uneven sampling across geographic regions and over time. In this study, an expanded data set of 68 WNV envelope gene sequences from the Midwest (Illinois) was created and combined with published sequences to investigate spatial and temporal structuring in the United States viral population. Results indicate an overall lack of geographic structure to WNV in the United States, supporting the notion of WNV as a rapidly expanding pathogen not significantly restricted in its spread by geographic distance. However, analyses of viral genetic diversity show a steady increase in WNV nucleotide-level diversity over time. Additionally, evolutionary rate calculations indicate that WNV has evolved at approximately 0.85 x 10(-3) substitutions/site/year, largely through neutral substitution and purifying selection. Overall, these results show WNV across the United States to be a panmictic viral population that is diversifying and evolving.

Analysis of Variance↗

PubMed1976

Multiple excitations in photosynthetic systems.

The yield of fluorescence in Chlorella from a 7 ns pulse of light is found to decrease gradually as a function of the number of hits in the photosynthetic units. The fivefold decrease in yield is spread over some three orders of magnitude of pulse energy and strongly suggests another random process in addition to that of photon absorption. Evidence supports the view that this random process is not in the time but in the spatial domain. The model used to fit the data is that of a unit with multiple traps for the singlet excitation. An excitation is captured by an open trap or destroyed by a filled trap with equal probability. These studies give evidence for the connectivity of the photosynthetic energy transfer apparatus on the short time scale. The short fluorescence lifetimes following picosecond pulse excitation of photosynthetic systems reported by several laboratories may be explained by the effect of multiple excitations.

Chlorella↗

PubMed1996

Outbreak of multi-drug resistant Staphylococcus aureus: a cluster analysis.

BACKGROUND: Outbreaks of multi-drug resistant Staphylococcus aureus over eight years were investigated to prevent future endemic. METHODS: Isolates of multiresistant S. aureus underwent a cluster analysis combined with canonical discriminant analysis using bacteriologic biotyping and sensitivity to 21 drugs. RESULTS: Of a total of 786 strains recovered from 155 in-patients, the specialty surgical ward (SW) exhibited 470 isolates (59.8%) and the general SW 214 (27.2%). Among six clusters formed, four clusters were predominant in the general SW. An ordination diagram from the canonical discriminant analysis revealed a distribution in which clusters were localized temporally (year) and spatially (ward). A yearly shift of clusters indicated emergence of a new phenotype of multiresistant S. aureus. CONCLUSIONS: The cluster analysis of isolates of multiresistant S. aureus using biotyping and sensitivity may supplement the classical method of tracing the spreading patterns of this microbe.

Anti-Bacterial Agents↗

PubMed2006

Drift of particles in self-similar systems and its Liouvillian interpretation.

We study the dynamics of classical particles in different classes of spatially extended self-similar systems, consisting of (i) a self-similar Lorentz billiard channel, (ii) a self-similar graph, and (iii) a master equation. In all three systems, the particles typically drift at constant velocity and spread ballistically. These transport properties are analyzed in terms of the spectral properties of the operator evolving the probability densities. For systems (i) and (ii), we explain the drift from the properties of the Pollicott-Ruelle resonance spectrum and corresponding eigenvectors.

Journal Article↗

PubMedDate not supplied

Signal and noise in modulation transfer function determinations using the slit, wire, and edge techniques.

The modulation transfer function (MTF) of an idealized imaging system can be determined from the Fourier transform of the system's line-spread function (LSF). Three techniques of experimentally determining the LSF require imaging either a slit, wire, or edge. In this paper, these three techniques are modeled theoretically to determine the noise in the calculated MTFs as a function of spatial frequency resulting from both quantum fluctuations and stochastic detector noise. The techniques are compared using the signal-to-noise ratio (SNR) in the MTF, defined as the ratio of the MTF value to the standard deviation in an ensemble of MTF determinations from independent measurements. It is shown that for a specified photon fluence, the edge method MTF has the highest SNR at low spatial frequencies, while that of the slit method is superior at high frequencies. The wire method SNR is always inferior to that of the slit technique. This suggests that the edge method is preferable for measuring parameters such as the low-frequency drop, and the slit method is preferable for determining high-frequency response. The cross-over frequency at which the slit and edge methods are equal (f(e)) for quantum-noise limited systems is a function of the slit width and the length over which the LSF is measured. For detector-noise limited systems, f(e) is dependent on the slit width only. The SNR in all but the quantum-noise limited slit method can therefore be increased by decreasing the length over which the LSF is measured, smoothing the tails of the LSF, or by fitting the tails to an analytic expression.

Fourier Analysis↗

PubMed1989

Phase aberration correction in medical ultrasound using speckle brightness as a quality factor.

Medical ultrasonic images are degraded by tissues with inhomogeneous acoustic velocities. The resulting phase aberration raises the off-peak response of the imaging system's point spread function (PSF), decreasing dynamic range. In extreme cases, multiple images of a single target are displayed. Phase aberration may become a limiting factor to image quality as ultrasonic frequency and aperture size are increased in order to improve spatial resolution. A method is proposed to correct for unknown phase aberration, which uses speckle brightness as a quality factor. The phase delays of a phased array transducer are modified, element by element, to maximize mean speckle brightness in a region of interest. The technique proposed is analogous to the correction technique used by Muller and Buffington [J. Opt. Soc. Am. 64 (9), 1200-1209 (1974)] to adaptively focus incoherent optical telescopes. The method is demonstrated using a computer model with several different simulated aberration profiles. With this model, mean speckle brightness is calculated using the two-dimensional PSF. Experiments have also been conducted in which speckle brightness is shown to increase as the phase delays of an ultrasonic scanner are modified in order to compensate for a rippled aberrating layer made of silicone rubber. The characteristics of the proposed method, and the possibility of employing it clinically to correct for unknown inhomogeneities in acoustic velocity, are discussed.

Computer Simulation↗

PubMed2006

GABAergic protection of hippocampal pyramidal neurons against glutamate insult: deficit in young animals compared to adults.

Hypoxia-ischemia (HI) injury in neonatal animals leads to selective regional loss of neurons including CA1 and CA3 pyramidal neurons of the hippocampus as well as nonlethal pathologies. Glutamate-receptor over-activation and Ca2+ influx are involved in these neonatal changes. We examined glutamate-evoked Ca2+ responses in neonatal (PN 7-13) and young adult (PN 21-27) CA1 pyramidal neurons in acute slices from rats. In neonates, transient exposure to glutamate produced large Ca2+ increases throughout neurons, including distal dendrites (primary Ca2+ responses). Repeated exposures resulted in sustained Ca2+ increases in apical dendrites (secondary Ca2+ responses) that were independent of continued glutamate exposure. These responses propagated and invaded the soma, resulting in irrecoverably high Ca2+ elevations. In neurons from adults, identical glutamate exposure evoked primary Ca2+ responses only in somata and proximal apical dendrites. Repeated glutamate exposures in the adult neurons also led to secondary Ca2+ responses, but they arose in the peri-somatic region and then spread outward to distal apical dendrites, again without recovery. More stimuli were required to initiate secondary responses in neurons from adult versus neonates. Block of GABAA receptors in adults caused the primary and secondary responses to revert to the spatial pattern seen in the neonates and greatly increased their vulnerability to glutamate. These findings suggest that neurodegenerative secondary Ca2+ events may be important determinants of susceptibility to HI injury in the developing CNS and that immature CA1 neurons may be more susceptible to excitotoxic injury due at least in part to insufficient development of GABAergic inputs to their dendrites.

Aging↗

PubMed2000

Origin sites of calcium release and calcium oscillations in frog sympathetic neurons.

In many neurons, Ca(2+) signaling depends on efflux of Ca(2+) from intracellular stores into the cytoplasm via caffeine-sensitive ryanodine receptors (RyRs) of the endoplasmic reticulum. We have used high-speed confocal microscopy to image depolarization- and caffeine-evoked increases in cytoplasmic Ca(2+) levels in individual cultured frog sympathetic neurons. Although caffeine-evoked Ca(2+) wave fronts propagated throughout the cell, in most cells the initial Ca(2+) release was from one or more discrete sites that were several micrometers wide and located at the cell edge, even in Ca(2+)-free external solution. During cell-wide cytoplasmic [Ca(2+)] oscillations triggered by continual caffeine application, the initial Ca(2+) release that began each Ca(2+) peak was from the same subcellular site or sites. The Ca(2+) wave fronts propagated with constant amplitude; the spread was mostly via calcium-induced calcium release. Propagation was faster around the cell periphery than radially inward. Local Ca(2+) levels within the cell body could increase or decrease independently of neighboring regions, suggesting independent action of spatially separate Ca(2+) stores. Confocal imaging of fluorescent analogs of ryanodine and thapsigargin, and of MitoTracker, showed potential structural correlates to the patterns of Ca(2+) release and propagation. High densities of RyRs were found in a ring around the cell periphery, mitochondria in a broader ring just inside the RyRs, and sarco-endoplasmic reticulum Ca(2+) ATPase pumps in hot spots at the cell edge. Discrete sites at the cell edge primed to release Ca(2+) from intracellular stores might preferentially convert Ca(2+) influx through a local area of plasma membrane into a cell-wide Ca(2+) increase.

Aniline Compounds↗

PubMedDate not supplied

[Spatial synchronization of rabbit brain biopotentials upon repeated electrical stimulation of the cortex].

A study was made on 23 rabbits of the degree of spatial synchronization in response to repeated electrical stimulations of one or simultaneously two cortical areas: the sensorimotor or visual, or simultaneously the sensorimotor and visual, or sensorimotor and precentral respectively. Enhancement of the synchronization induced by the stimulation, globally spread over the entire cortex and was followed by its decrease. However, in the two directly stimulated areas of the cortex, the similarity of bioelectrical oscillations persisted for a long time, which testifies to the establishment of selective relations between them.

Animals↗