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At least 109 records · Page 6Linked to original sources
PubMed1994

Analysis of the spatial-frequency-dependent DQE of optically coupled digital mammography detectors.

The effect of optical coupling efficiency on the spatial-frequency-dependent propagation of signal and noise is considered for x-ray image detectors for digital mammography in which a phosphor screen is optically coupled to a charge-coupled device (CCD) image array. For experimental purposes, optical coupling between a Gd2O2S:Tb phosphor screen and a CCD image array was provided by relay lenses. Neutral density filters were inserted between the lenses to vary the optical coupling efficiency without altering the inherent spatial resolution. The total coupling efficiency, defined as the number of electrons (e-) recorded in the CCD per x-ray interaction in the phosphor, was calculated in each case. The modulation transfer function, and the contributions to the total noise power spectrum (NPS) of x-ray quantum noise, secondary quantum noise, and inherent detector noise were measured as a function of coupling efficiency. These data were used to calculate the spatial-frequency-dependent detective quantum efficiency [DQE(f)]. The NPS due to x-ray quantum noise had a significant spatial-frequency dependence for coupling efficiencies of more than 9 e- per x-ray interaction, but little spatial-frequency dependence for coupling efficiencies of less than 2 e- per x-ray interaction. These results indicate that to preserve high spatial-frequency values of DQE(f), and to ensure that images are x-ray quantum-noise limited at high spatial frequencies, a coupling efficiency on the order of 10 e- per x-ray interaction is required.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrons↗
PubMed2003

A spatial analysis of county-level variation in hospitalization rates for low back problems in North Carolina.

Hospitalization rates for low back problems vary widely. In previous non-spatial analyses, population-level socioeconomic and health resource characteristics have explained little of the variation in rates. This study examines geographic variation in hospitalization rates for low back problems while controlling for spatial dependence in the data. County-level surgical and medical hospitalization rates were calculated using North Carolina hospital (USA) discharge data from 1990-92. Non-spatial and spatial regression models were estimated using socioeconomic and health resource predictors. Both surgical and medical rates varied significantly among the 100 counties. Non-spatial models explained 62% of variation in log-transformed surgical rates and 66% of variation in log-transformed medical rates; however, residuals showed significant spatial dependence. Spatial lag models were therefore applied. Using simple contiguity spatial weights, surgery rates increased with higher percent urban population, primary care physician density, and discharge rate for other causes, and decreased with higher percent college graduates, percent disabled, occupied hospital bed density, and unoccupied hospital bed density. There was a nonlinear relationship between surgery rates and percent employed in heavy lifting/transportation industries. Medical rates increased with higher other-cause discharge rate and with MRI/CT scanner availability, and decreased with higher percent urban population, percent nonwhite population, percent in heavy lifting/transportation industries, and unoccupied hospital bed density. The results show that population-level socioeconomic and health resource characteristics are important determinants of variation in low back hospitalization rates. Independent of these variables, a separate spatial process produces geographic clustering of high-rate counties. Spatial effects are important and should be considered in small area analyses.

Catchment Area, Health↗
PubMed2006

Modeling the effect of liming on calcium concentration in Swedish lakes.

To abate the severe acidification of Swedish lakes are about 7% directly limed (AL-lakes). These are important to include when assessing acidification. Consequently, we need to estimate their chemical status as if they were not lime-affected. We hypothesize that there is a spatial dependence for calcium (Ca) and magnesium (Mg) concentrations. Spatial variation is determined using variograms. We study lakes within two 150 * 150 km quadrates (EMEP150-grid) in the middle and southern parts of Sweden. We model Ca in AL-lakes using the Ca and Mg concentrations in nearby, unaffected lakes (UL-lakes). The mean Ca/Mg for the three or seven closest UL-lakes is used. We assume that Ca/Mg is constant for UL-lakes and that Mg unaltered by liming. For UL-lakes, Ca and Mg are spatially dependent. For AL-lakes, there is a spatial dependence for Mg, but, as anticipated, not for Ca. The modeled Ca for UL-lakes show a good correspondence with measured Ca; r2 > 0.6; slopes close to 1 and intercepts almost 0. Using three or seven nearby lakes when modeling Ca give similar results. Modeled Ca was higher than measured Ca for 36% (three closest lakes) and 38% (seven closest) of the modeled AL-lakes in the middle part of Sweden. The corresponding results for the southern part were 10% and 9%, respectively. This model is an acceptable estimator of Ca in limed lakes and for estimating critical loads and acidification status on an EMEP-scale.

Calcium↗
PubMed2004

Spatial-frequency-dependent changes in cortical activation before and after patching in amblyopic children.

PURPOSE: To examine the cortical response under transient stimulus conditions in amblyopic children before and after eye patching. To determine whether improvement in acuity is associated with spatial-frequency-dependent changes in specific peaks of the cortical response. METHODS: Visual evoked potentials (VEPs) to check reversal (163-18 arc min) and onset of sine wave gratings (0.5-4 cyc/deg) were measured in 24 amblyopic children (<7 years of age) before eye patching. VEPs were repeated in nine subjects with 20/40 or better acuity after patching. Age, severity of amblyopia, and VEP amplitudes of positive peak (P)100, P1, and negative peak (N)2 were analyzed by multivariate statistics. RESULTS: Before patching, the amblyopic eye showed decreasing amplitude with increasing spatial frequencies (P < 0.05) when compared with the nonamblyopic eye. Reduced amplitudes occurred at frequencies well below acuity. Latencies were mildly prolonged. After patching, amplitudes increased in the amblyopic eye across all spatial frequencies (ANCOVA; P < 0.0001 for each peak). However, a spatial-frequency-dependent increase in amplitude was significant only for a late negative peak (N2). The patched eye showed no significant changes. CONCLUSIONS: Recovery of acuity after eye patching is associated with an overall increase in cortical activation across a wide range of spatial frequencies below the acuity threshold. A spatial-frequency-dependent increase in a late negative peak suggests that the cortical generator of this peak demonstrates plasticity of acuity recovery.

Amblyopia↗
PubMed1996

Spatial frequency dependent observer bias in the measurement of contrast sensitivity.

A previously reported spatial frequency dependent change in contrast sensitivity, apparently produced by the psychometric method used, was investigated in two experiments. A relative reduction in the measured contrast sensitivity function at higher spatial frequencies was found when using a simple single staircase procedure, even when all external characteristics of the presentation procedure were the same as a more sophisticated and more reliable psychometric method (adaptive probit estimation). The estimated false-negative response rate was found to be greater for the single staircase procedure and was greater at a higher spatial frequency, while the false-positive response rate to interleaved blank presentations did not vary between spatial frequencies and between the two psychometric methods. This supports the suggestion that some subjects, given a knowledge of the psychometric method controlling grating contrast, may adopt different decision criteria at different spatial frequencies.

Adult↗
PubMed2006

On Bayesian inference for the K function.

The K function is a summary of spatial dependence in spatial point processes. In practice one observes a realization of the spatial point process, called a spatial point pattern. Although the K function of a spatial point process is typically unknown, several estimators of the process K function have been put forth. These estimators, however, are based upon empirical averages; the complicated distributional properties of the estimators unfortunately complicates interval estimation. In this paper, we propose a Bayesian inferential framework, allowing inference for the K function of the spatial point process (including interval estimation). Of particular interest is the unique use of the posterior predictive distribution to (efficiently) enable such inferences. To demonstrate our technique, the well known Swedish pine sapling data (Strand, 1972) is analyzed, including a discussion on evaluating model fit.

Algorithms↗
PubMedDate not supplied

Contrast adaptation in retinal and cortical evoked potentials: no adaptation to low spatial frequencies.

Contrast adaptation occurs in both the retina and the cortex. Defining its spatial dependence is crucial for understanding its potential roles. We thus asked to what degree contrast adaptation depends on spatial frequency, including cross-adaptation. Measuring the pattern electroretinogram (PERG) and the visual evoked potential (VEP) allowed separating retinal and cortical contributions. In ten subjects we recorded simultaneous PERGs and VEPs. Test stimuli were sinusoidal gratings of 98% contrast with spatial frequencies of 0.5 or 5.0 cpd, phase reversing at 17 reversals/s. Adaptation was controlled by prolonged presentation of these test stimuli or homogenous gray fields of the same luminance. When adaptation and test frequency were identical, we observed significant contrast adaptation only at 5 cpd: an amplitude reduction in the PERG (-22%) and VEP (-58%), and an effective reduction of latency in the PERG (-0.95 ms). When adapting at 5 cpd and testing at 0.5 cpd, the opposite effect was observed: enhancement of VEP amplitude by +26% and increase in effective PERG latency by + 1.35 ms. When adapting at 0.5 cpd and testing at 5 cpd, there was no significant amplitude change in PERG and VEP, but a small effective PERG latency increase of +0.65 ms. The 0.5-cpd channel was not adapted by spatial frequencies of 0.5 cpd. The adaptability of the 5-cpd channel may mediate improved detail recognition after prolonged blur. The existence of both adaptable and nonadaptable mechanisms in the retina allows for the possibility that by comparing the adaptational state of spatial-frequency channels the retina can discern between overall low contrast and defocus in emmetropization control.

Adaptation, Physiological↗
PubMed2002

Photodynamic therapy creates fluence rate-dependent gradients in the intratumoral spatial distribution of oxygen.

In photodynamic therapy (PDT), treatment efficacy may be reduced by the presence of pre-existing tumor hypoxia or by oxygen depletion during the therapy. Tumor oxygenation during PDT has been measured with needle electrodes, but the intratumoral distribution of this oxygen is not known. In the present study, the spatial distribution of hypoxia during PDT was quantified using the hypoxia-labeling marker EF3. Mice bearing radiation-induced fibrosarcoma tumors were treated with Photofrin-mediated PDT to a total dose of 135 J/cm(2), delivered at a fluence rate of either 75 mW/cm(2) or 38 mW/cm(2). PDT-created hypoxia at each fluence rate was labeled by exposing tumors to EF3 (52 mg/kg) during the period of illumination. Cryosectioning, immunohistochemistry, and fluorescence microscopy were carried out to quantify EF3 binding as a function of distance to the nearest perfused blood vessels in sections cut from within the superficial (light-adjacent) 600 micro m or the deep (light-distant) 600 micro m of tumors (5-6 mm in diameter, approximately 3 mm in depth). In both superficial and deep sections, PDT at 75 mW/cm(2) resulted in the development of significant gradients in tumor hypoxia as a function of distance to a perfused blood vessel. Furthermore, significant hypoxia was detected even in vascular-adjacent tissue. These effects were associated with a significant decrease in the percentage of perfused vessels and a significant increase in the median distance of a cell to the nearest perfused blood vessel. In contrast, during PDT at 38 mW/cm(2), sections from deep tumor levels demonstrated only insignificant increases in the rise in hypoxia as a function of distance to a perfused vessel and in the level of hypoxia in vascular-adjacent tissue. No effects on tumor perfusion were detected during PDT at 38 mW/cm(2). Overall, these results demonstrate that spatially dependent depletion of oxygen can occur during PDT as a function of the fluence rate and that PDT can create significant hypoxia in even tissue adjacent to perfused blood vessels.

Animals↗
PubMed2005

[Spatial variability of nutrients in cultivated soils of Xinhui District, Jiangmen City].

Employing geostatistical methods and GIS technology, this paper studied the spatial distribution characteristics of pH, organic matter, CEC, total nitrogen, available phosphorus and slowly available potassium in cultivated soils of Xinhui District, Jiangmen City. All the test variables were normally or log normally distributed. Semivariogram analysis showed that soil nutrients were moderately spatially-dependent in a given spatial range, except that soil total nitrogen was strongly spatially-dependent. It was shown from Kriging analysis that soil pH and CEC was the highest in the northeast, and soil organic matter content was higher in the middle and northeast investigation region. The area with 1.5-2.0 g x kg(-1) soil total nitrogen content accounted for 75.7% of the investigated region, mainly distributed in the west and east, that with > 40 mg x kg(-1) soil available phosphorus content was accounted for 48.7%, mainly distributed in the west and northeast, and the area that slowly available potassium content was 160-350 mg x kg(-1) accounted for 48.1%, mainly distributed in the east, northeast and the middle.

China↗
PubMed2006

Comprehensive time-lag measurement as a diagnostic and analytical tool for non-Fickian transport studies: a salient porous barrier-gaseous permeant test case.

Concentration-independent permeation of He, Ne and N2 through a porous barrier, specially constructed by multistep compaction of fine "Carbolac" carbon powder, has been studied. Emphasis was placed on measuring a variety of time lags, supplemented with transient permeation and sorption kinetics. Previous time-lag data, restricted to values of a single time-lag parameter, were instrumental in revealing deviations from Fick's law in similar solid barrier-penetrant systems; however, they could only provide indirect evidence as to the nature of the underlying causes, namely, time- or spatial dependence of the relevant transport parameters. Thus, an interpretation in terms of time-dependence (induced by the presence of blind pores) was proposed, predicated on calculations indicating that, in theory, spatial dependence (induced by non-uniform compaction) could be reduced to insignificance by adoption of a suitable multistep powder compaction technique. The comprehensive time-lag analysis approach applied here is a general diagnostic and analytical tool, previously successfully tested on graphite barriers, that can discriminate rigorously between the aforementioned non-Fickian functional dependences, as well as provide additional substantial information in either case. This capability is well illustrated by the results obtained in the system under study here, which provide unequivocal evidence of substantial spatial dependence. Furthermore, it is shown that the predicted functional form of the detected spatial dependence (i) can, in conjunction with an adequate physical model and with the results of a previous theoretical parametric study, account fully for all aspects of the observed non-Fickian time-lag behaviour and (ii) correlates satisfactorily with variation of local porosity across the barrier, detected by a suitable X-ray imaging technique. On the other hand, there is no evidence, from time lags or transient kinetics, of any significant time-dependence attributable to slow filling or emptying of blind pores or to other causes.

Journal Article↗
PubMed1997

Cell type and spatial location dependence of cytoplasmic viscosity measured by time-resolved fluorescence microscopy.

Information on the cell type and spatial location dependence of cytoplasmic viscosity would be very useful in understanding some of the processes occurring in the cell. For this purpose, fluorescent dye kiton red (sulforhodamine B) was loaded into a variety of cells such as Swiss 3T3 fibroblasts, human mononuclear cells, Sarcoma-180 tumor cells, Chinese hamster ovary cells, plant cells from Digitalis lanata, stamen hair cells of Tradescantia, and guard mother cells of Allium cepa. Space-resolved measurements of cytoplasmic viscosity were carried out by using an experimental set-up wherein a picosecond laser system was coupled with an epifluorescence microscope. The spatial resolution of this set-up was approximately 1.0 micron, and reliable dynamic fluorescence measurements could be obtained from 10(2) to 10(3) fluorescent molecules. Fluorescence lifetime measurements showed that a large fraction (approximately 70%) of kiton red was in the free form. Fluorescence anisotropy decay of kiton red in cells was analyzed by a two population (free and bound) model. The microviscosity of cytoplasm was estimated from the anisotropy decay kinetics of the free probe. It was found that the cytoplasmic viscosity is dependent on both the cell type and spatial location within a cell. Furthermore, both the average value of viscosity and spatial variation within a cell were larger in the plant cells when compared to the animal cells. Model studies in various simpler systems have shown that the higher viscosity observed in some part of the cell could be due to either physical restriction and/or the presence of high concentrations of small solutes and macromolecules.

3T3 Cells↗
PubMed2003

Sensitivity analyses for ecological regression.

In many ecological regression studies investigating associations between environmental exposures and health outcomes, the observed relative risks are in the range 1.0-2.0. The interpretation of such small relative risks is difficult due to a variety of biases--some of which are unique to ecological data, since they arise from within-area variability in exposures/confounders. The potential for residual spatial dependence, due to unmeasured confounders and/or data anomalies with spatial structure, must also be considered, though it often will be of secondary importance when compared to the likely effects of unmeasured confounding and within-area variability in exposures/confounders. Methods for addressing sensitivity to these issues are described, along with an approach for assessing the implications of spatial dependence. An ecological study of the association between myocardial infarction and magnesium is critically reevaluated to determine potential sources of bias. It is argued that the sophistication of the statistical analysis should not outweigh the quality of the data, and that finessing models for spatial dependence will often not be merited in the context of ecological regression.

Bias↗
PubMed2006

Spatial distribution pattern of oviposition in the mosquito Aedes aegypti in relation to urbanization in Buenos Aires: southern fringe bionomics of an introduced vector.

The distribution of Aedes aegypti (L) (Diptera: Culicidae) oviposition in Buenos Aires City is spatially heterogeneous. Oviposition activity was monitored for a year with a grid of 279 traps at 850-m intervals that were serviced weekly. Geostatistics were used for the spatial analysis and generalized linear regression to model oviposition as a function of demographic and environmental variables. The proportion of weeks infested and the total number of eggs showed spatial continuity and were higher in areas that had higher densities of houses and were closer to industrial sites; they were lower in areas with higher human populations or higher densities of flats. When all sites were considered, the spatial structure showed a strong trend, but after regression, the residuals presented lower spatial dependence. When only infested sites were considered, the oviposition variables were spatially autocorrelated and the regression residuals showed little or no spatial dependence. The spatial pattern of Ae. aegypti oviposition in a highly urbanized city such as Buenos Aires seems to be related to the urbanization gradient. These urban environments might present different resource availability or continuity between patches of resources.

Aedes↗
PubMed2004

Spatial reference memory in GluR-A-deficient mice using a novel hippocampal-dependent paddling pool escape task.

Genetically modified mice lacking the L-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit, GluR-A (GluR1), and deficient in hippocampal CA3-CA1 long-term potentiation (LTP), were assessed on a novel, hippocampal-dependent spatial reference memory, paddling pool escape task. The mice were required to use the extramaze cues around the laboratory to find a hidden escape tube that was in a constant location at one of 12 possible positions around the perimeter of the paddling pool, in order to escape from shallow water. The knockout mice performed well on this task. They displayed a small initial impairment (in terms of both escape latencies and choice errors), but they were soon as efficient as the wild-type mice in escaping from the water. This was further demonstrated by performance during a 20-s probe trial in which the exit tube was blocked. Both groups of mice spent most of the time searching in the quadrant of the pool in which the exit tube had previously been located. In a subsequent experiment, entirely normal spatial acquisition was observed in the knockout mice when the paddling pool was moved to a novel spatial environment. The GluR-A -/- mice were also unimpaired in a further reversal phase in which the correct exit location was moved by 180 degrees around the perimeter wall. These results are consistent with previous watermaze studies, providing further demonstration of intact hippocampus-dependent spatial reference memory in GluR-A knockout mice. They contrast strikingly with the profound deficits in hippocampus-dependent, short-term, flexible spatial working memory observed in these knockout mice. This study also demonstrates a novel behavioral task for assessing spatial memory in genetically modified mice. This task shares the behavioral profile of the well-established watermaze paradigm, but may have advantages for the study of genetically modified mice.

Animals↗
PubMed1983

Spatial frequency dependence of accommodative responses in amblyopic eyes.

Monocular, steady-state accommodative responses were measured as a function of spatial frequency of simple sinusoidal gratings presented at high contrast and target vergence levels in amblyopes, as well as in strabismics without amblyopia and in visually-normal control subjects. In general, spatial frequency dependence of the accommodative response was the rule. However, the amblyopic eyes exhibited markedly reduced accommodative responses over most of the spatial frequency range tested, and this was attributed to reduced accommodative controller gain in the sensory pathways involved in the control of accommodation in the amblyopic eye. Due to the diversity of accommodative response spatial frequency profiles found across all groups, the results suggest that reflex, voluntary, and higher-level perceptual aspects of accommodation may interplay in a complex manner in the act of accommodation on a simple sinusoidal grating.

Accommodation, Ocular↗
PubMed2006

[Spatial variability of farmland heavy metals contents in Qianan City].

By the methods of geostatistics and GIS, this paper studied the spatial variability of Cu, Zn, Ni, Cr As, Hg, Cd and Pb contents in the farmland topsoil (0 -20 cm) of Qianan City, Hebei Province. The results showed that the average contents of test metals were under the secondary standard of environmental quality standard for soil (GB 15618-1995), and belonged to moderate variability. The theoretical models of semi-variance of Cu, Zn, Ni, Cr and As contents fitted exponential models, and their spatial correlations were strongly spatial dependence; while the theoretical models of Hg and Cd contents fitted spherical models, and their spatial correlations were moderately spatial dependence. The Pb content had pure nugget effect, and was weakly spatial dependence. In whole research scale, Pb content had lasting variability, while the other seven heavy metals contents had the spatial variability mainly caused by their spatial correlation parts. The ranges of heavy metals spatial correlations were about 11 km to 20 km. The Kriging' s interpolation of Cu, Zn, Ni, Cr, Hg, As and Cd contents showed that their contents were higher in northern mountainous area, but lower in mid basin.

China↗
PubMed1994

A spatial-frequency dependent quantum accounting diagram and detective quantum efficiency model of signal and noise propagation in cascaded imaging systems.

The detective quantum efficiency (DQE) is a system parameter that can be used to accurately describe image noise transfer characteristics through many imaging systems. A simpler approach used by some investigators, particularly when evaluating new ideas and system designs, is to describe the system as a series of cascaded stages. Each stage may correspond to either an increase in the number of quanta (e.g., conversion from x-ray to optical quanta in a radiographic screen), or a loss (a detection or coupling probability). The number of secondary quanta at each stage per incident primary quantum is given by the product of all preceding gains, and can be displayed graphically for convenient interpretation. The stage with the fewest quanta is called the "quantum sink," limiting the pixel signal-to-noise ratio to less than the square root of the number of quanta per pixel. This conventional zero-spatial-frequency "quantum accounting diagram" (QAD), however, neglects the spatial spreading of secondary quanta and can seriously underestimate image noise. It is shown that this problem is avoided with the introduction of a spatial-frequency dependent QAD, expressed as the product of the gains and squared modulation-transfer functions (MTF) of each stage. A generalized expression is developed for the DQE of a cascaded imaging system that is dependent only on the gain, gain Poisson excess (related to the variance), and MTF, of each stage. A direct relationship is then shown to exist between the DQE and values in the QAD. The QAD of a hypothetical system consisting of a charge-coupled device camera and a scintillating screen is evaluated as an illustrative example. The conventional zero-frequency analysis suggests two quantum sinks occur with approximately equal importance: one in the number of x rays, and one in the number of optical quanta. The spatial-frequency dependent analysis, however, shows the optical quantum sink becomes severe and dominates at nonzero frequencies. The necessary increase in gain or optical numerical aperture required to prevent the optical quantum sink for spatial frequencies of interest is determined from the QAD analysis. The visual impact of this nonzero spatial-frequency quantum sink is shown in images generated using a Monte Carlo simulation of the cascading process.

Algorithms↗