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At least 415 records · Page 23Linked to original sources
PubMed1993

NMR relaxation enhancement in gels polymerized and cross-linked by ionizing radiation: a new approach to 3D dosimetry by MRI.

A new type of tissue-equivalent medium for magnetic resonance imaging of the dose distributions produced by ionizing radiation has been developed. Agarose gel is infused with acrylamide and N,N'-methylene-bis-acrylamide (Bis) comonomers, which are readily polymerized by free radical initiators in de-aerated aqueous solutions. Polymerization and cross-linking induced locally by free radical products of water radiolysis increase the rate of water proton spin relaxation gradually up to doses of about 15 Gy. The slopes of the dose-response curves at 64 MHz are 0.015 and 0.28 s-1 Gy-1 for R1 and R2, respectively. The agarose matrix as well as the high (50% by weight) relative concentration of the cross-linker (Bis) per total comonomer limit the spread of polymerization so that the spatial distribution of the radiation dose is faithfully represented in the resultant spatial distribution of relaxation rates. The gel can be imaged with conventional magnetic resonance imaging devices with high spatial resolution and accuracy. In addition, due to the well established effect of the precipitation of insoluble agglomerates of highly cross-linked acrylamide, the optical turbidity of the gel increases gradually with the absorbed dose. This may provide an additional means of visualizing the dose distribution in three dimensions. The major advantage of the acrylamide-Bis-agarose gels over those that depend on ionic chemical dosimeters, for example, Fricke-infused gels, lies in the lack of diffusion of radiation-induced chemical changes subsequent to or concurrent with irradiation.

Acrylamide↗
PubMed2005

Dynamics of a novel pathogen in an avian host: Mycoplasmal conjunctivitis in house finches.

In early 1994, a novel strain of Mycoplasma gallisepticum (MG)--a poultry pathogen with a world-wide distribution--emerged in wild house finches and within 3 years had reached epidemic proportions across their eastern North American range. The ensuing epizootic resulted in a rapid decline of the host population coupled with considerable seasonal fluctuations in prevalence. To understand the dynamics of this disease system, a multi-disciplinary team composed of biologists, veterinarians, microbiologists and mathematical modelers set forth to determine factors driving and influenced by this host-pathogen system. On a broad geographic scale, volunteer observers ("citizen scientists") collected and reported data used for calculating both host abundance and disease prevalence. The scale at which this monitoring initiative was conducted is unprecedented and it has been an invaluable source of data for researchers at the Cornell Laboratory of Ornithology to track the spread and magnitude of disease both spatially and temporally. At a finer scale, localized and intensive field studies provided data used to quantify the effects of disease on host demographic parameters via capture-mark-recapture modeling, effects of host behavior on disease and vice-versa, and the biological and genetic profiles of birds with known phenotypic characteristics. To balance the field-based component of the study, experiments were conducted with finches held in captivity to describe and quantify the effects of experimental infections on hosts in both individual and social settings. The confluence of these various elements of the investigation provided the foundation for construction of a general compartmentalized epidemiological model of the dynamics of the house finch-MG system. This paper serves several purposes including (i) a basic review of the pathogen, host, and epidemic cycle; (ii) an explanation of our research strategy; (iii) a basic review of results from the diverse multi-disciplinary approaches employed; and (iv) pertinent questions relevant to this and other wildlife disease studies that require further investigation.

Animals↗
PubMed2006

[How do the temporal lobes communicate in medial temporal lobe seizures?].

INTRODUCTION: Spatial and temporal patterns of the spread of partial epileptic seizures depend on the site from which they originate. Characterising seizure propagation patterns may help to better define the seizure focus. In medial temporal lobe epilepsies, seizure propagation to the contralateral temporal lobe is especially studied. STATE OF THE ART: Intracranial EEG records permit more precise definition of patterns of contralateral propagation in medial temporal lobe epilepsies. Several pathways have been implicated, sometimes differently in distinct studies, in propagation to the contralateral temporal lobe: the three commissures (the corpus callosum, the ventral hippocampal commissure and the anterior commissure, which link the temporal lobes) and an indirect circuit via the frontal lobes. Delays measured for contralateral propagation of a seizure of temporo-limbic origin vary significantly around a mean value of about 30 seconds. This slow spread may depend in part on the relatively small size of human commissural projections from the amygdalo-hippocampal formation, which largely originate in the presubiculum. However, a larger commissural projection pathway originates in the paralimbic medial temporal lobe and electrical stimulation of the anterior perirhinal cortex can induce contralateral epileptic discharges with delays as short as 100 ms (Adam et al., 2004). Thus, seizure activity emerging from anterior paralimbic regions can propagate rapidly to the contralateral medial temporal lobe via the anterior commissure. PERSPECTIVES: While the propagation of medial temporal lobe seizures is still debated, further studies are merited since the extent and speed of spread governs the electro-clinical semiology of seizures and our ability to identify their initiation site. CONCLUSIONS: We review anatomical (Demeter et al., 1990) and neurophysiological (Wilson et al., 1990) data for a dual inter-temporal propagation of medial temporal lobe seizures at different speeds and via different pathways.

Brain Mapping↗
PubMed2003

Feasibility, sensitivity, and reliability of laser-induced fluorescence imaging of green fluorescent protein-expressing tumors in vivo.

Whole-body imaging of green fluorescent protein (GFP) can be used to test the efficiency of gene carriers for in vivo transduction. The aim of the current study was to determine the sensitivity and the accuracy of a GFP imaging procedure by in vivo investigation of GFP-expressing tumor cells. An improved method of whole-body GFP imaging made use of a laser excitation source and band-pass filters matched specifically to GFP and constitutive tissue fluorescence emission bands. Processing of the primary GFP fluorescence images acquired by the CCD camera subtracted background tissue autofluorescence. Our approach achieved 100% sensitivity and specificity for in vivo detection of 10%-transfected BxPc3 pancreatic tumor after subcutaneous grafting or orthotopical implantation in the pancreas of nude mice. It also detected less transfected tumors (i.e., 1 to 5%) but with a loss in sensitivity (50% of cases). The system was employed over a 5-week period to monitor the persistence of GFP expression in 10%-transfected BxPc3 tumors orthotopically implanted in the pancreas of two nude mice, allowing the direct visualization of tumor progression and spread. In facilitating the temporal-spatial follow-up of GFP expression in vivo, the optimized laser-induced fluorescence imaging device can support preclinical investigations of vectors for therapeutic gene transduction through regular, harmless, real-time monitoring of theirin vivo transductional efficacy and persistence.

Animals↗
PubMedDate not supplied

Novel feature extraction technique for fuzzy relational clustering of a flexible dopamine reuptake inhibitor.

This paper describes a novel clustering methodology for classifying over 700 conformations of a flexible analogue of GBR 12909, a dopamine reuptake inhibitor that has completed phase I clinical trials as a treatment for cocaine abuse. The major aspect of the clustering methodology includes an efficient data-conditioning scheme where a systematic feature extraction procedure based on the structural properties of the molecule was used to reduce the associated feature space. This allowed region-specific clustering that focused on individual pharmacophore elements of the molecule. For clustering of the reduced feature set, the fuzzy clustering partitional method was utilized. Due to the relational nature of the feature data, fuzzy relational clustering was employed, and it successfully detected natural groups defined by rotational minima around N(sp(3))-C(sp(3)), O(sp(3))-C(sp(3)), and C(sp(3))-C(sp(2)) bonds. The proposed clustering methodology also employed several cluster validity measures, which corroborated the partitions produced by the clustering technique and agreed with the results of hierarchical clustering using the XCluster program. Representative structures which exhibited a reasonable spread of energies and showed good spatial coverage of the conformational space were identified for use as putative bioactive conformations in a future Comparative Molecular Field Analysis of GBR 12909 analogues. The clustering methodology developed here is capable of handling other computational chemistry problems, and the feature extraction technique can be easily generalized to other molecules.

Cluster Analysis↗
PubMed2003

Determination of beam intensity in a single step for IMRT inverse planning.

In intensity modulated radiotherapy (IMRT), targets are treated by multiple beams at different orientations each with spatially-modulated beam intensities. This approach spreads the normal tissue dose to a greater volume and produces a higher dose conformation to the target. In general, inverse planning is used for IMRT treatment planning. The inverse planning requires iterative calculation of dose distribution in order to optimize the intensity profile for each beam and is very computation intensive. In this paper, we propose a single-step method utilizing a figure of merit (FoM) to estimate the beam intensities for IMRT treatment planning. The FoM of a ray is defined as the ratio between the delivered tumour dose and normal tissue dose and is a good index for the dose efficacy of the ray. To maximize the beam utility, it is natural to irradiate the tumour with intensity of each ray proportional to the value of the FoM. The nonuniform beam intensity profiles are then fixed and the weights of the beam are determined iteratively in order to yield a uniform tumour dose. In this study, beams are employed at equispaced angles around the patient. Each beam with its field size that just covers the tumour is divided into a fixed number of beamlets. The FoM is calculated for each beamlet and this value is assigned to be the beam intensity. Various weighting factors are incorporated in the FoM computation to accommodate different clinical considerations. Two clinical datasets are used to test the feasibility of the algorithm. The resultant dose-volume histograms of this method are presented and compared to that of conformal therapy. Preliminary results indicate that this method reduces the critical organ doses at a small expense of uniformity in tumour dose distribution. This method estimates the beam intensity in one single step and the computation time is extremely fast and can be finished in less than one minute using a regular PC.

Algorithms↗
PubMed2002

Three-dimensional crystalline ion beams.

We report on the experimental realization of two- and three-dimensional crystalline ion beams in the rf quadrupole storage ring PALLAS (LMU, Munich). At a beam energy around 1 eV the phase transition to the Coulomb ordered state is identified by a sudden decrease of the spatial width and of the velocity spread of the beam. The focusing conditions required to attain crystalline beams up to structures of helices surrounding a string are systematically investigated and related to the situation in typical heavy ion storage rings.

Journal Article↗
PubMed2006

Distance-based tests for homogeneity of multivariate dispersions.

The traditional likelihood-based test for differences in multivariate dispersions is known to be sensitive to nonnormality. It is also impossible to use when the number of variables exceeds the number of observations. Many biological and ecological data sets have many variables, are highly skewed, and are zero-inflated. The traditional test and even some more robust alternatives are also unreasonable in many contexts where measures of dispersion based on a non-Euclidean dissimilarity would be more appropriate. Distance-based tests of homogeneity of multivariate dispersions, which can be based on any dissimilarity measure of choice, are proposed here. They rely on the rotational invariance of either the multivariate centroid or the spatial median to obtain measures of spread using principal coordinate axes. The tests are straightforward multivariate extensions of Levene's test, with P-values obtained either using the traditional F-distribution or using permutation of either least-squares or LAD residuals. Examples illustrate the utility of the approach, including the analysis of stabilizing selection in sparrows, biodiversity of New Zealand fish assemblages, and the response of Indonesian reef corals to an El Niño. Monte Carlo simulations from the real data sets show that the distance-based tests are robust and powerful for relevant alternative hypotheses of real differences in spread.

Animals↗
PubMed1999

Retinal waves and visual system development.

Many pathways in the developing visual system are restructured and become highly organized even before vision occurs. Yet the developmental processes underlying the remodeling of visual connectivity are crucially dependent on retinal activity. Surprisingly, the immature and light-insensitive retina spontaneously generates a pattern of rhythmic bursting activity during the period when the connectivity patterns of retinal ganglion cells are shaped. Spatially, the activity is seen to spread across the retina in the form of waves that bring into synchrony the bursts of neighboring cells. Waves are present in the developing retina of higher and lower vertebrates, which suggests that this form of activity may be a common and fundamental mechanism employed in the activity-dependent refinement of early patterns of visual connections. Unraveling the cues encoded by the waves promises to provide important insights into how interactions driven by specific patterns of activity could lead to the modification of connectivity during development.

Animals↗
PubMedDate not supplied

Effects of nerve and fibroblast growth factors on the production of nitric oxide in experimental model of Huntington's disease.

The role of nitric oxide (NO) in neurological diseases represents one of the most studied, yet controversial subjects in physiology. The aim was to examine the effects of intrastriatal injection neurotrophins (nerve growth factors-NGF, fibroblast growth factors-FGF) in order to investigate the possible involvement of NO in quinolinic acid (QA) induced striatal toxicity in the rat model of Huntington's disease (HD). QA was administered unilaterally into the striatum of adult Wistar rats in a single dose of 150 nM. The other two groups of animals were pretreated immediately before QA application with NGF and FGF, respectively. Control group was treated with 0.9% saline solution in the same manner. Animals were decapitated 7 days after the treatment. Nitrite levels were significantly decreased both in the ipsi- and contralateral striatum and forebrain cortex of NGF- and FGF-treated animals compared with QA treatment. These results indicated a temporal and spatial propagation of oxidative stress and spread protective effects of NGF and FGF on the forebrain cortex, the distant structure, but tightly connected with striatum, the place of direct neurotoxic damage. Neurotrophins could be the potential neuroprotective agents in HD.

Animals↗
PubMedDate not supplied

Effects of nerve growth factor on antioxidative system in the thalamus of MPTP treated Wistar rats.

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism is one of the most useful models for the study of that disease. It has been suggested that MPTP-induced neurotoxicity may involve the production of reactive oxygen species. MPTP was applied intracerebrally, unilaterally, in the striatum in single dose of 0.09 g/kg b.w. The second group was treated both with MPTP and nerve growth factor (NGF) in dose of 7 ng/ml. NGF was applied immediately after the neurotoxin. Control group was treated with 0.9% saline solution in the same manner. Animals were decapitated 7 days after the treatment. In the group treated with MPTP, the activity of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) was decreased in ipsilateral thalamus, compared to control values as well as to the contralateral thalamus. In the same structures superoxide anion production was increased, compared to controls. Following the application of both MPTP and NGF, the activity of SOD and GSH-Px remained on control values, while the superoxide anion content was decreased, compared to controls. These results indicate a temporal and spatial propagation of oxidative stress and spread protective effects of NGF on the thalamus, the structure that is distant, but very tightly connected with striatum, the place of direct neurotoxic damage.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗
PubMed2005

Effectiveness evaluation of different suction systems.

Microorganisms of the patient's oral cavity and his/her blood and saliva may cause different air-borne and blood-borne infectious diseases among odontologists and their assistants who work with patients. Quantitative analysis and spatial distribution analysis of the environmental spread of oral liquid and cooling liquid mixture were performed during this study. Effectiveness of suction systems of four types was evaluated: without suction, using a small-size suction pump alone, using a small-size and large-size suction pumps, using a small-size suction pump together with an experimental extra-oral aspirator. Quantitative changes of the water aerosol, which enters the environment during the preparation of teeth, were determined in respect of the used suction systems. The small-size pump system together with an experimental extra-oral suction system eliminated best the aerosol formed during the preparation.

Aerosols↗
PubMed1991

[Transmembrane signal. Respective role of free cytosol calcium and of protein kinase C].

Angiotensin II (Ang II) belongs to the family of the calcium-mobilizing hormones which includes other vasoactive hormones such as vasopressin, endothelin, serotonin. Angiotensin can be considered as an archetype for ligands activating the calcium messenger system. Observation of the changes occurring in the two branches of the calcium messenger system--the inositol 1, 4, 5-trisphosphate/calcium branch and the diacylglycerol/protein kinase branch--upon activation by Ang II in various target cells (adrenal zona glomerulosa cells, vascular smooth muscle cells and cardiomyocytes) emphasized common features but also revealed variation in the responses and in the interaction between the two branches (so-called cross-talk). For example, the use of single cell microfluorometry with fura-2 shows that, in adrenal glomerulosa cells, Ang II induces sinusoidal oscillations of cytosolic free calcium concentration which are typical of excitable cells; by contrast in vascular smooth muscle cells, one observes transient oscillations indicative of a mechanism of calcium-induced calcium release. Furthermore, the activation of protein kinase C by angiotensin II leads to negative feed-back mechanisms on the final biological response in adrenal cells and cardiomyocytes, whereas it has a potentiating effect in vascular smooth muscle cells. On-line video microscopy allows one to follow in real time the changes in cytosolic free calcium concentration in vascular smooth muscle cells and spontaneous beating cultured cardiomyocytes thereby revealing the spatial origin of the calcium "tide" spreading throughout the cytosol. The task is now to superimpose these calcium signals, these biochemical triggers and the framework of the cytoskeleton and intracellular organelles forming the stage of this play.

Angiotensin II↗
PubMed2001

Depolarisation-evoked Ca2+ waves in the non-excitable rat megakaryocyte.

1. A combination of patch clamp, confocal microscopy and immunohistochemistry was used to examine the spatial properties of Ca2+ signalling in the rat megakaryocyte, a non-excitable cell type in which membrane potential can markedly modulate agonist-evoked Ca2+ release. 2. Intracellular calcium ion concentration ([Ca2+]i) increases, stimulated by both ADP and depolarisation, frequently originated from a peripheral locus and spread as a wave throughout the cell. Spatially restricted [Ca2+]i increases, consistent with elementary Ca2+ release events, were occasionally observed prior to ADP-evoked waves. 3. ADP- and depolarisation-evoked Ca2+ waves travelled approximately twice as fast around the periphery of the cell compared to across its radius, leading to a curvilinear wavefront. There was no significant difference between wave velocities generated by the two stimuli. 4. Immunohistochemical staining of type III IP3 receptors, the endoplasmic reticulum-specific protein GRP78/BiP and calreticulin indicated a major peripheral location of the cellular Ca2+ stores which probably accounts for the accelerated wave velocity at the cell periphery. 5. These data demonstrate that [Ca2+]i increases, stimulated by depolarisation or the agonist ADP, have indistinguishable spatial properties, providing evidence that similar underlying mechanisms are responsible for their generation.

Adenosine Diphosphate↗
PubMed2005

Assessing the role of long-distance translocation and spatial heterogeneity in the raccoon rabies epidemic in Connecticut.

Spatial heterogeneity and long-distance translocation (LDT) play important roles in the spatio-temporal dynamics and management of emerging infectious diseases and invasive species. We assessed the influence of LDT events on the invasive spread of raccoon rabies through Connecticut. We identified several putative LDT events, and developed a network-model to evaluate whether they became new foci for epidemic spread. LDT was fairly common, but many of the LDTs were isolated events that did not spread. Two putative LDT events did appear to become nascent foci that affected the epidemic in surrounding townships. In evaluating the role of LDT, we simultaneously revisited the problem of spatial heterogeneity. The spread of raccoon rabies is associated with forest cover--rabies moves up to three-times slower through the most heavily forested townships compared with those with less forestation. Forestation also modified the effect of rivers. In the best overall model, rabies did not cross the river separating townships that were heavily forested, and the spread slowed substantially between townships that were lightly forested. Our results suggest that spatial heterogeneity can be used to enhance the effects of rabies control by focusing vaccine bait distribution along rivers in lightly forested areas. LDT events are a concern, but this analysis suggests that at a local scale they can be isolated and managed.

Animals↗
PubMedDate not supplied

[Spatial distribution of tegumentary leishmaniasis in the city of Rio de Janeiro].

The authors analyze the historical and spatial determinants of the implantation, persistence, and spread of cutaneous leishmaniasis in the municipality of Rio de Janeiro and the links between the disease and the organization of urban space and occupation processes on the periphery of the city beginning in the early 20th century through the 1980s. A pattern of outbreaks that was restricted and spatially discontinuous but sequenced was observed and seen to be linked to a dynamic process of urban real estate appreciation, comprising a large endemic area for cutaneous leishmaniasis. An analysis of the occupation and organization of urban space in the so-called Western Zone of Rio de Janeiro was conducted, considering new functions of spatial elements expressed through changing work relations, land use, and land value. Urbanization of the area produced the necessary conditions to intensify an endemic pattern of well-defined outbreaks where human mobility and the work process increased contact between susceptible individuals and vectors. Analysis showed spatial units with differentiated risk levels.

Animals↗
PubMed2005

A neural network to improve dim-light vision? Dendritic fields of first-order interneurons in the nocturnal bee Megalopta genalis.

Using the combined Golgi-electron microscopy technique, we have determined the three-dimensional dendritic fields of the short visual fibres (svf 1-3) and first-order interneurons or L-fibres (L1-4) within the first optic ganglion (lamina) of the nocturnal bee Megalopta genalis. Serial cross sections have revealed that the svf type 2 branches into one adjacent neural unit (cartridge) in layer A, the most distal of the three lamina layers A, B and C. All L-fibres, except L1-a, exhibit wide lateral branching into several neighbouring cartridges. L1-b shows a dendritic field of seven cartridges in layers A and C, dendrites of L2 target 13 cartridges in layer A, L3 branches over a total of 12 cartridges in layer A and three in layer C and L4 has the largest dendritic field size of 18 cartridges in layer C. The number of cartridges reached by the respective L-fibres is distinctly greater in the nocturnal bee than in the worker honeybee and is larger than could be estimated from our previous Golgi-light microscopy study. The extreme dorso-ventrally oriented dendritic field of L4 in M. genalis may, in addition to its potential role in spatial summation, be involved in edge detection. Thus, we have shown that the amount of lateral spreading present in the lamina provides the anatomical basis for the required spatial summation. Theoretical and future physiological work should further elucidate the roles that this lateral spreading plays to improve dim-light vision in nocturnal insects.

Animals↗
PubMed1995

Spatial structure of two-locus genotypes under isolation by distance.

Extensive Monte Carlo simulations are conducted of spatial distributions of two-locus genotypes in large, continuous populations under isolation by distance models. The results show that substantial patches of double homozygotes are present in the spatial structures, even when loci are unlinked. The stochastic spread of identical two-locus genotypes largely outpowers the tendency for recombination to decouple patterns for separate loci. A spatial patch is a large area containing mostly one double homozygous genotype in a highly contiguous constellation. This patch structure is reflected in high positive spatial autocorrelations and large excesses of pairs, or joins, of identical double homozygotes at short-to-intermediate distances of spatial separation. Although spatial patches of double homozygotes are the dominant spatial feature, and the major contributors to overall high levels of autocorrelations among two-locus genotypes, other substantial features include areas of concentrations of identical genotypes heterozygous at only one locus. One implication of the patch structure is the presence of high levels of linkage disequilibrium, caused by isolation by distance even for unlinked loci, at some spatial scales; yet the disequilibrium in the large total populations is near 0. Thus linkage disequilibrium produced by isolation by distance is highly dependent on spatial scale. Another implication is that high degrees of spatial structuring and autocorrelations are produced for genetic variation controlling quantitative traits, at least when the number of loci is relatively small, under a wide range of situations, even if the trait is selectively neutral. The significance of the results to field studies is also examined.

Diploidy↗