848 research outputs found
Kinetics of helium bubble formation in nuclear materials
The formation and growth of helium bubbles due to self-irradiation in
plutonium has been modelled by a discrete kinetic equations for the number
densities of bubbles having atoms. Analysis of these equations shows that
the bubble size distribution function can be approximated by a composite of:
(i) the solution of partial differential equations describing the continuum
limit of the theory but corrected to take into account the effects of
discreteness, and (ii) a local expansion about the advancing leading edge of
the distribution function in size space. Both approximations contribute to the
memory term in a close integrodifferential equation for the monomer
concentration of single helium atoms.
The present boundary layer theory for discrete equations is compared to the
numerical solution of the full kinetic model and to previous approximation of
Schaldach and Wolfer involving a truncated system of moment equations.Comment: 24 pages, 6 figures, to appear in Physica
Retesting visual fields: Utilizing prior information to decrease test-retest variability in glaucoma
PURPOSE. To determine whether sensitivity estimates from an individual's previous visual field tests can be incorporated into perimetric procedures to improve accuracy and reduce test-retest variability at subsequent visits. METHODS. Computer simulation was used to determine the error, distribution of errors and presentation count for a series of perimetric algorithms. Baseline procedures were Full Threshold and Zippy Estimation by Sequential Testing (ZEST). Retest strategies were (1) allowing ZEST to continue from the previous test without reinitializing the probability density function [pdf], (2) running ZEST with a Gaussian pdf centered about the previous result; (3) retest minimizing uncertainty (REMU), a new procedure combining suprathreshold and ZEST procedures incorporating prior test information. Empiric visual field data of 265 control and 163 patients with glaucoma were input into the simulation. Four error conditions were modeled: patients who make no errors, 15% false-positive (FP) with 3% false-negative (FN) errors, 15% FN with 3% FP errors, and 20% FP with 20% FN errors. RESULTS. If sensitivity was stable from test to retest, an the retest algorithms were faster than the baseline algorithms by, on average, one presentation per location and are significantly more accurate (P < 0.05). When visual fields changed from test to retest, REMU was faster and more accurate than the other retest approaches and the baseline procedures. Relative to the baseline procedures, REMU showed decreased test-retest variability in impaired regions of Visual field. CONCLUSIONS. The obvious approaches to retest, such as continuing the previous procedure or seeding with previous values, have limitations when sensitivity changes between tests. REMU, however, significantly improves both accuracy and precision of testing and displays minimal bias, even when fields change and patients make errors
Individualized StructureâFunction Mapping for Glaucoma: Practical Constraints on Map Resolution for Clinical and Research Applications
yesPurpose: We have developed customized maps that relate visual field and optic nerve head (ONH) regions according to individual anatomy. In this study, we aimed to determine feasible map resolution for research use, and to make a principled recommendation of sector size for clinical applications.
Methods: Measurement variability in foveaâONH distance and angle was estimated from 10 repeat OCT scans of 10 healthy people. Errors in estimating axial length from refractive error were determined from published data. Structureâfunction maps were generated, and customized to varied clinically-plausible anatomical parameters. For each parameter set (n = 210), 200 maps were generated by sampling from measurement/estimation error distributions. Mapped 1° sectors at each visual field location from each parameter set were normalized to difference from their mean. Variation (90% ranges) in normalized mapped sectors represents the precision of individualized maps.
Results: Standard deviations of repeated measures of foveaâONH distance and angle were 61 ÎŒm and 0.97° (coefficients of variation 1.3% and 12.0%, respectively). Neither measure varied systematically with mean (Spearmans's Ï = 0.26, P = 0.47 for distance, Ï = â0.31, P = 0.39 for angle). Variation (90% ranges) in normalized mapped sectors varied across the visual field and ranged from 3° to 18° when axial length was measured accurately, and from 6° to 32° when axial length was estimated from refractive error.
Conclusions: The 90% ranges represent the minimum feasible ONH sector size at each visual field location. For clinical use an easily interpretable scheme of 30° sectors is suggested
Genomic variations associated with attenuation in Mycobacterium avium subsp paratuberculosis vaccine strains
BACKGROUND: Mycobacterium avium subspecies paratuberculosis (MAP) whole cell vaccines have been widely used tools in the control of Johne's disease in animals despite being unable to provide complete protection. Current vaccine strains derive from stocks created many decades ago; however their genotypes, underlying mechanisms and relative degree of their attenuation are largely unknown.
RESULTS: Using mouse virulence studies we confirm that MAP vaccine strains 316 F, II and 2e have diverse but clearly attenuated survival and persistence characteristics compared with wild type strains. Using a pan genomic microarray we characterise the genomic variations in a panel of vaccine strains sourced from stocks spanning over 40 years of maintenance. We describe multiple genomic variations specific for individual vaccine stocks in both deletion (26-32 Kbp) and tandem duplicated (11-40 Kbp) large variable genomic islands and insertion sequence copy numbers. We show individual differences suitable for diagnostic differentiation between vaccine and wild type genotypes and provide evidence for functionality of some of the deleted MAP-specific genes and their possible relation to attenuation.
CONCLUSIONS: This study shows how culture environments have influenced MAP genome diversity resulting in large tandem genomic duplications, deletions and transposable element activity. In combination with classical selective systematic subculture this has led to fixation of specific MAP genomic alterations in some vaccine strain lineages which link the resulting attenuated phenotypes with deficiencies in high reactive oxygen species handling
StructureâFunction Mapping: Variability and Conviction in Tracing Retinal Nerve Fiber Bundles and Comparison to a Computational Model
yesPurpose: We evaluated variability and conviction in tracing paths of retinal nerve fiber bundles (RNFBs) in retinal images, and compared traced paths to a computational model that produces anatomically-customized structureâfunction maps.
Methods: Ten retinal images were overlaid with 24-2 visual field locations. Eight clinicians and 6 naïve observers traced RNFBs from each location to the optic nerve head (ONH), recording their best estimate and certain range of insertion. Three clinicians and 2 naïve observers traced RNFBs in 3 images, 3 times, 7 to 19 days apart. The model predicted 10° ONH sectors relating to each location. Variability and repeatability in best estimates, certain range width, and differences between best estimates and model-predictions were evaluated.
Results: Median between-observer variability in best estimates was 27° (interquartile range [IQR] 20°â38°) for clinicians and 33° (IQR 22°â50°) for naĂŻve observers. Median certain range width was 30° (IQR 14°â45°) for clinicians and 75° (IQR 45°â180°) for naĂŻve observers. Median repeatability was 10° (IQR 5°â20°) for clinicians and 15° (IQR 10°â29°) for naĂŻve observers. All measures were worse further from the ONH. Systematic differences between model predictions and best estimates were negligible; median absolute differences were 17° (IQR 9°â30°) for clinicians and 20° (IQR 10°â36°) for naĂŻve observers. Larger departures from the model coincided with greater variability in tracing.
Conclusions: Concordance between the model and RNFB tracing was good, and greatest where tracing variability was lowest. When RNFB tracing is used for structureâfunction mapping, variability should be considered
PCN322 Opportunities with Accelerated Approval Times, but Stumbles at Hta? the Impact of a Limited Evidence Package
Enhanced structure-function relationship in glaucoma with an anatomically and geometrically accurate neuroretinal rim measurement
yesPurpose: To evaluate the structureâfunction relationship between disc marginâbased rim area (DM-RA) obtained with confocal scanning laser tomography (CSLT), Bruch's membrane openingâbased horizontal rim width (BMO-HRW), minimum rim width (BMO-MRW), peripapillary retinal nerve fiber layer thickness (RNFLT) obtained with spectral-domain optical coherence tomography (SD-OCT), and visual field sensitivity.
Methods: We examined 151 glaucoma patients with CSLT, SD-OCT, and standard automated perimetry on the same day. Optic nerve head (ONH) and RNFL with SD-OCT were acquired relative to a fixed coordinate system (acquired image frame [AIF]) and to the eye-specific fovea-BMO center (FoBMO) axis. Visual field locations were mapped to ONH and RNFL sectors with fixed Garway-Heath (VFGH) and patient-specific (VFPS) maps customized for various biometric parameters.
Results: Globally and sectorally, the structureâfunction relationships between DM-RA and VFGH, BMO-HRWAIF and VFGH, and BMO-HRWFoBMO and VFPS were equally weak. The R2 for the relationship between DM-RA and VFGH ranged from 0.1% (inferonasal) to 11% (superotemporal) whereas that between BMO-HRWAIF and VFGH ranged from 0.1% (nasal) to 10% (superotemporal). Relatively stronger global and sectoral structureâfunction relationships with BMO-MRWAIF and with BMO-MRWFoBMO were obtained. The R2 between BMO-MRWAIF and VFGH ranged from 5% (nasal) to 30% (superotemporal), whereas that between BMO-MRWFoBMO and VFPS ranged from 5% (nasal) to 25% (inferotemporal). The structureâfunction relationship with RNFLT was not significantly different from that with BMO-MRW, regardless of image acquisition method.
Conclusions: The structureâfunction relationship was enhanced with BMO-MRW compared with the other neuroretinal rim measurements, due mainly to its geometrically accurate properties
Pathogenic Potential to Humans of Bovine Escherichia coli O26, Scotland
Escherichia coli O26 and O157 have similar overall prevalences in cattle in Scotland, but in humans, Shiga toxinâproducing E. coli O26 infections are fewer and clinically less severe than E. coli O157 infections. To investigate this discrepancy, we genotyped E. coli O26 isolates from cattle and humans in Scotland and continental Europe. The genetic background of some strains from Scotland was closely related to that of strains causing severe infections in Europe. Nonmetric multidimensional scaling found an association between hemolytic uremic syndrome (HUS) and multilocus sequence type 21 strains and confirmed the role of stx<sub>2</sub> in severe human disease. Although the prevalences of E. coli O26 and O157 on cattle farms in Scotland are equivalent, prevalence of more virulent strains is low, reducing human infection risk. However, new data on E. coli O26âassociated HUS in humans highlight the need for surveillance of non-O157 enterohemorrhagic E. coli and for understanding stx<sub>2</sub> phage acquisition
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