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At least 1,099 records · Page 61Linked to original sources
PubMed1994

A theoretical model for magneto-acoustic imaging of bioelectric currents.

A theoretical model of magneto-acoustic current imaging is derived, based on fundamental equations of continuum mechanics and electromagnetism. In electrically active tissue, the interaction between an applied magnetic field, B, and action currents, J, creates a pressure distribution. In the near field limit, this pressure obeys Poisson's equation, with a source term (delta x J).B. The displacement and pressure fields are calculated for a dipole (q), oriented either parallel or perpendicular to the applied magnetic field (B), at the center of an elastic, conducting sphere (radius a, shear modulus G). Surface displacements are on the order of qB/(4 pi Ga), which is about 1 nm for typical biological parameters. If the applied magnetic field is changing with time, eddy currents induced in the tissue may be larger than the action currents themselves. The frequency of the pressure and displacement arising from these eddy currents, however, is twice the frequency of the applied magnetic field, so it may be possible to eliminate this artifact by filtering or lock-in techniques. Magneto-acoustic and biomagnetic measurements both image delta x J in a similar way, although magneto-acoustic current imaging has the disadvantage that acoustic properties vary among tissues to a greater degree than do magnetic properties.

Acoustics↗
PubMed2004

A stochastic model of tumor response to fractionated radiation: limit theorems and rate of convergence.

The iterated birth and death Markov process is defined as an n-fold iteration of a birth and death Markov process describing kinetics of certain population combined with random killing of individuals in the population at moments tau 1,...,tau n with given survival probabilities s1,...,sn. A long-standing problem of computing the distribution of the number of clonogenic tumor cells surviving an arbitrary fractionated radiation schedule is solved within the framework of iterated birth and death Markov process. It is shown that, for any initial population size iota, the distribution of the size N of the population at moment t > or = tau n is generalized negative binomial, and an explicit computationally feasible formula for the latter is found. It is shown that if i --> infinity and sn --> 0 so that the product iota s1...sn tends to a finite positive limit, the distribution of random variable N converges to a probability distribution, which for t = tau n turns out to be Poisson. In the latter case, an estimate of the rate of convergence in the total variation metric similar to the classical Law of Rare Events is obtained.

Cell Survival↗
PubMed1993

A quantitative framework for evaluating the effect of community treatment on the morbidity due to ascariasis.

There is growing emphasis on the use of community treatment to reduce the level of morbidity caused by helminth infection. The design of chemotherapy programmes, in terms of frequency of treatment and proportion of the community treated, would be assisted by a quantitative framework which enabled the morbidity reduction achieved by different approaches to be compared. The present study describes a model developed for this purpose which embodies two innovative features. First, a quantitative score of morbidity (the proportion of individuals harbouring an intense infection) is used to rate the success of a programme and, second, the distribution of helminths in the host population is generated by a mechanism that allows the distribution to change dynamically as a function of both treatment and reinfection. The model behaviour, using values typical of Ascaris lumbricoides, indicates that the benefit derived from community chemotherapy increases non-linearly with the coverage and efficacy of treatment.

Animals↗
PubMed2001

Random graphs with arbitrary degree distributions and their applications.

Recent work on the structure of social networks and the internet has focused attention on graphs with distributions of vertex degree that are significantly different from the Poisson degree distributions that have been widely studied in the past. In this paper we develop in detail the theory of random graphs with arbitrary degree distributions. In addition to simple undirected, unipartite graphs, we examine the properties of directed and bipartite graphs. Among other results, we derive exact expressions for the position of the phase transition at which a giant component first forms, the mean component size, the size of the giant component if there is one, the mean number of vertices a certain distance away from a randomly chosen vertex, and the average vertex-vertex distance within a graph. We apply our theory to some real-world graphs, including the world-wide web and collaboration graphs of scientists and Fortune 1000 company directors. We demonstrate that in some cases random graphs with appropriate distributions of vertex degree predict with surprising accuracy the behavior of the real world, while in others there is a measurable discrepancy between theory and reality, perhaps indicating the presence of additional social structure in the network that is not captured by the random graph.

Journal Article↗
PubMed2000

LROC analysis of detector-response compensation in SPECT.

Localization ROC (LROC) observer studies examined whether detector response compensation (DRC) in ordered-subset, expectation-maximization (OSEM) reconstructions helps in the detection and localization of hot tumors. Simulated gallium (Ga-67) images of the thoracic region were used in the study. The projection data modeled the acquisition of attenuated 93- and 185-keV photons with a medium-energy parallel-hole collimator, but scatter was not modeled. Images were reconstructed with five strategies: 1) OSEM with no DRC; 2) OSEM preceded by restoration filtering; 3) OSEM with iterative DRC; 4) OSEM with an ideal DRC; and 5) filtered backprojection (FBP) with no DRC. All strategies included attenuation correction. There were four LROC studies conducted. In a study using a single tumor activity, the ideal DRC offered the best performance, followed by iterative DRC, restoration filtering, OSEM with no DRC, and FBP. Statistical significance at the 5% level was found between all pairs of strategies except for restoration filtering and OSEM with no DRC. A similar ranking was found for a more realistic study using multiple tumor activities. Additional studies considered the effects of OSEM iteration number and tumor activity on the detection improvement that iterative DRC offered with respect to OSEM with no DRC.

Algorithms↗
PubMed2004

Living alone and outpatient care use by older veterans.

In nonveteran older adults, living alone influences outpatient care use, but its importance in the veteran population has not been well studied. The aims of this study are to describe the use of outpatient care by older veterans who live alone versus those who live with others and determine whether living alone influences outpatient use by older veterans. The data come from the 2001 Veteran Identity Program Survey designed to measure Department of Veterans Affairs (VA) and non-VA outpatient care use. Univariate and bivariate analyses were conducted to examine distributional properties, associations, and subgroup differences in outpatient care use. Poisson regression was used to assess the role of living alone on outpatient care use, controlling for predisposing, other enabling, and need factors. Results found that older veterans who use the VA, whether they live alone or not, have similar numbers of VA outpatient visits. Older veterans who use VA and non-VA facilities and who live alone have greater total outpatient visits than those who live with others. Regression results indicate that living alone is a predictor of VA routine medical visits, VA prescription refill visits, and total VA and non-VA outpatient visits but does not influence VA emergency room visits. These findings suggest that living alone is associated with differences in outpatient care use by older veterans. It is important for the VA to understand this relationship with the aim of developing interventions to improve access, effectiveness, and efficiency of health services for older veterans.

Activities of Daily Living↗
PubMed2007

Maximum-likelihood methods in wavefront sensing: stochastic models and likelihood functions.

Maximum-likelihood (ML) estimation in wavefront sensing requires careful attention to all noise sources and all factors that influence the sensor data. We present detailed probability density functions for the output of the image detector in a wavefront sensor, conditional not only on wavefront parameters but also on various nuisance parameters. Practical ways of dealing with nuisance parameters are described, and final expressions for likelihoods and Fisher information matrices are derived. The theory is illustrated by discussing Shack-Hartmann sensors, and computational requirements are discussed. Simulation results show that ML estimation can significantly increase the dynamic range of a Shack-Hartmann sensor with four detectors and that it can reduce the residual wavefront error when compared with traditional methods.

Electrons↗
PubMed1986

[Assessment of irradiation uniformity using the results of a cytogenetic study of human peripheral blood lymphocytes].

A study was made of chromosome aberration during the I mitosis in PHA-stimulated lymphocytes from normal donors' peripheral blood following gamma-radiation in vitro in doses of 1 to 8 Gy. The dose-effect curves were plotted for different cytogenetic characteristics (the percentage of cells with dicentrics, dicentrics per 100 cells and aberrant cell). It is shown that during even or relatively even radiation, the doses calculated on the basis of the different cytological characteristics do not substantially differ and distribution of the dicentrics in the cells is governed by Poisson's law. During uneven radiation simulated by numeric experiments with formation of cell mixtures radiated in different doses, one could observe superdispersion of the cells with a different number of the dicentrics relative to Poisson's distribution, whereas the dose calculated according to the dicentrics (per 100 cells) was respectively higher and less than the doses calculated according to the number of the cells and dicentrics (per aberrant cell). A method for computerization of dicentric distribution in the cells to lymphocyte distribution according to the dose is described.

Cells, Cultured↗
PubMedDate not supplied

Analysis of exacerbation rates in asthma and chronic obstructive pulmonary disease: example from the TRISTAN study.

Recurrent events in clinical trials have typically been analysed using either a multiple time-to-event method or a direct approach based on the distribution of the number of events. An area of application for these methods is exacerbation data from respiratory clinical trials. The different approaches to the analysis and the issues involved are illustrated for a large trial (n = 1465) in chronic obstructive pulmonary disease (COPD). For exacerbation rates, clinical interest centres on a direct comparison of rates for each treatment which favours the distribution-based analysis, rather than a time-to-event approach. Poisson regression has often been employed and has recently been recommended as the appropriate method of analysis for COPD exacerbations but the key assumptions often appear unreasonable for this analysis. By contrast use of a negative binomial model which corresponds to assuming a separate Poisson parameter for each subject offers a more appealing approach. Non-parametric methods avoid some of the assumptions required by these models, but do not provide appropriate estimates of treatment effects because of the discrete and bounded nature of the data.

Albuterol↗
PubMed2004

Chromosome fragmentation after irradiation with C ions.

The premature chromosome condensation (PCC) technique has been used to compare chromatin breakage and repair in non-cycling CHO-K1 cells following high LET (C ions) and low LET (X-rays) irradiation. For both radiation qualities the average initial number of excess PCC fragments increases linearly with dose. However, the frequency of chromatin breaks follows the pattern of energy deposition and at higher LET values reveals clustering due to the large number of ionizing events being concentrated in a small volume of the cell nucleus. In consequence, the distribution of PCC chromosomes plus excess fragments among cells has followed Poisson statistics after X-ray irradiation while the overdispersion of the frequencies has been observed after C-irradiation indicating that a single particle traversal through a cell nucleus can produce multiple chromatin lesions.

Animals↗
PubMed2000

Fixation probabilities when the population size undergoes cyclic fluctuations.

Let us assume that there is a monoecious random mating population that changes cyclically in size. Then, the probability that a nonrecessive favorable mutant is ultimately fixed, if it is originally present in a single heterozygote, is approximately proportional to the harmonic mean of the effective population sizes in the cycle and inversely proportional to the population size when the mutant appears. This approximation works well if the selective advantage s of the mutant is small and the length k of a cycle is small in comparison with the population sizes in a cycle. If k is large the harmonic mean is, in general, replaced by a weighted harmonic mean that puts the largest weights on reciprocals of effective population sizes in the first few generations after the mutant appears.

Alleles↗
PubMed1995

The effect of cervical loop electrosurgical excision on subsequent pregnancy outcome: North American experience.

OBJECTIVE: Our purpose was to determine pregnancy outcome and parturition in women who were treated by loop electrosurgical excision for squamous intraepithelial lesions. STUDY DESIGN: A series of 574 consecutive women of reproductive age (15 to 44 years old) were treated by loop electroexcision for low- and high-grade squamous intraepithelial lesions. The incidence of pregnancies was determined in these women within a 3-year period after therapy by comparing the "observed" number of pregnancies with the "expected" number of pregnancies in the study population, assuming that fertility rates in these women were the same as in the untreated general female population. Pregnancy outcome was correlated in the 53 women who were delivered of an infant with the posttreatment appearance of the cervix and birth weight with maternal smoking. RESULTS: The incidence of pregnancy in the study population was 8.5 per 100 woman years compared with 7.4 per 100 woman years in the untreated central population. Fifty-three women had 54 pregnancies, of which 46 (84%) were live births either at term (40) or between 37 and 39 weeks of gestation (6), and three patients are at present at 24, 34, and 36 weeks of gestation. There were two stillbirths and three first-trimester spontaneous abortions. Premature delivery was not observed. External os stenosis (one case), shortening of the cervix (one case), and repeat electroexcision (four cases) had no adverse effect on pregnancy and parturition. Smoking > or = 10 cigarettes per day before and during pregnancy, rather than loop electrosurgical excision per se, was associated with lower-birth-weight babies than those of nonsmokers (p < 0.01). CONCLUSION: Loop electrosurgical excision to a maximum depth of 1.5 cm and a mean frontal diameter of 1.8 cm does not appear to have adverse effects on subsequent pregnancy outcome and parturition.

Adolescent↗
PubMed1991

Air pollution and acute respiratory illness in five German communities.

To assess the impact of short-term exposure to air pollution on respiratory illness in children we recruited pediatricians and hospitals in five German cities to report daily counts of children's visits for croup symptoms and obstructive bronchitis. Data were collected for at least 2 years in each location. These symptoms are predominantly found in very young children, with the croup reporting peaking at 2 years of age and obstructive bronchitis at 1 year. Attacks of croup and obstructive bronchitis were relatively rare events: the mean number of cases of croup per day in each city ranged from 0.5 to 3, and obstructive bronchitis was even less frequent. A total of 6330 cases of croup and 4755 cases of obstructive bronchitis were observed during the study. The distributions of these events were quite skewed and were modeled as a Poisson process. To focus the analysis on short-term correlations and avoid seasonal confounding, biannual, annual (seasonal), and six shorter term cycles were controlled for in the regression models. After controlling for short-term weather factors, total suspended particulate matter (TSP) and nitrogen dioxide (NO2) were associated with croup cases. An increase in TSP levels from 10 micrograms/m3 to 70 micrograms/m3 was associated with a 27% increase in cases of croup; the same increase in NO2 levels resulted in a 28% increase in cases. No pollutant was associated with daily cases of obstructive bronchitis.

Air Pollution↗
PubMed1998

A stochastic model of schistosomiasis immuno-epidemiology.

Schistosomiasis is a helminth (worm) infection with approximately 200 million people infected worldwide. There is still controversy on whether differing worm burdens between individuals is a result of differing contact rates or of acquired immunity. In this paper, we present a stochastic modelling framework to address these issues. By using appropriate approximations for the higher moments of the joint distributions, differential equations for the means, variances and co-variances of infection and immunity can be obtained. Numerical solutions of these equations to obtain age profiles of the above properties were compared with Monte Carlo simulations of the stochastic process. Simulations showed that the results depended on whether between host heterogeneity was generated by differing contact rates or differing immune responses. Heterogeneity in contact rates produced a highly aggregated distribution of parasites with a large variance/mean ratio. Heterogeneity in the immune response had very little effect on the overall dynamics. This agrees with the predominant field evidence which would suggest that infection is mainly determined by ecology with a smaller contribution of immunity.

Age Factors↗
PubMed1999

Heterogeneity in the distribution of Strongyloides ratti infective stages among the faecal pellets of rats.

The distribution of helminth parasites within their host population is usually overdispersed and can be described by the negative binomial distribution. The causes of this overdispersion are poorly understood, but heterogeneity in the distribution of infective stages within the environment has been implicated as a possible factor. Here we describe the distribution of infective stages of the rat intestinal nematode parasite Strongyloides ratti among the faecal pellets of its host. The distribution of infective stages between faecal pellets is overdispersed and well described by the negative binomial distribution. This overdispersion increases during the course of infection and occurs over a range of infection intensities. Overdispersion of nematode infective stages among faecal pellets may result in increased spatial heterogeneity of the infective stages in the environment and thus may contribute to the generation of overdispersion of adult parasitic stages. In addition, these findings raise important issues regarding the accurate quantification of helminth egg counts from faecal samples.

Animals↗
PubMed2001

Fatal falls in the US construction industry, 1990 to 1999.

The Occupational Safety and Health Administration's (OSHA's) Integrated Management Information System (IMIS) database allows for the detailed analysis of risk factors surrounding fatal occupational events. This study used IMIS data to (1) perform a risk factor analysis of fatal construction falls, and (2) assess the impact of the February 1995 29 CFR Part 1926 Subpart M OSHA fall protection regulations for construction by calculating trends in fatal fall rates. In addition, IMIS data on fatal construction falls were compared with data from other occupational fatality surveillance systems. For falls in construction, the study identified several demographic factors that may indicate increased risk. A statistically significant downward trend in fatal falls was evident in all construction and within several construction categories during the decade. Although the study failed to show a statistically significant intervention effect from the new OSHA regulations, it may have lacked the power to do so.

Accidental Falls↗
PubMed1994

The Charcot-Marie-Tooth syndrome: clinical aspects from a population study in South Wales, UK.

The Charcot-Marie-Tooth (CMT) syndrome comprises a heterogeneous group of disorders affecting the peripheral nerves and anterior horn cells of the spinal cord. They constitute a significant proportion of the burden of disability caused by single gene neurological disorders, with a disease prevalence of 16.7 per 10(5) of the South Wales adult population. The clinical and genetic characteristics of these diseases in the population are described. In those cases identified as type I hereditary motor and sensory neuropathy (HMSN I), we have shown that it is possible to correlate disease severity with age, and that there is no clinical evidence for imprinting, manifest either in age of onset or disease severity.

Adolescent↗
PubMed1997

A regional profile of suicide in Queensland.

A regional profile of suicide in Queensland follows on from earlier work which failed to find urban-rural differences in suicide rates. Suicide rates were higher in the north of the state and lower in the more developed south Rural Queensland health regions displayed both the lowest (Darling Downs and Mackay) and the highest (Peninsula) suicide rates. Socioeconomic factors correlated with these differences in some regions. Suicide rates for Aboriginal and Torres Strait Islanders, especially the young, were high, and may have contributed to elevated suicide rates in northern Queensland, but the numbers were insufficient to account for observed differences.

Adolescent↗