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At least 397 records · Page 22Linked to original sources

PubMed2002

Correlations among angular wave component amplitudes in elastic multiple-scattering random media.

The propagation of scalar waves through random media that provide multiple elastic scattering is considered by derivation of an expression for the angular correlation of the scattered wave amplitudes. Coherent wave transmission is shown to occur through a mechanism similar to that responsible for coherent backscattering. While the properties of the scattered wave are generally consistent with radiative-transfer theory for sufficiently small incident and scattering angles, coherent transmission provides corrections to radiative-transfer results at larger angles. The theoretical angular correlation curves are fit, by specifying the probability densities of two random variables that correspond to material parameters, to measured data of laser light scattering from various polymer microsphere suspensions.

Light↗

PubMed2004

Scaling properties of self-expanding surfaces.

Scaling properties of self-expanding surfaces are studied with a comparison to those of self-flattening surfaces [Phys. Rev. E 66, 040602(R) (2002)]. The evolution of self-expanding surfaces is described by a restricted solid-on-solid type monomer deposition-evaporation model in which both deposition at the globally lowest site and evaporation at the globally highest site are suppressed. We find numerically that equilibrium surface fluctuation has a scaling behavior with a roughness exponent alpha approximately 1 in one dimension (1D). In contrast, 2D equilibrium surfaces show the same dynamical scaling behavior with alpha=0 (log) and dynamic exponent z approximately 5/2 as 2D self-flattening surfaces. Stationary roughness can be understood analytically by relating the self-expanding growth model to self-repelling random walks. In the case of nonequilibrium growing/eroding surfaces, self-expanding dynamics cause the fluctuation of surfaces to be characterized by alpha approximately 1 in both 1D and 2D.

Models, Statistical↗

PubMed2005

Electrostatics and the assembly of an RNA virus.

Electrostatic interactions play a central role in the assembly of single-stranded RNA viruses. Under physiological conditions of salinity and acidity, virus capsid assembly requires the presence of genomic material that is oppositely charged to the core proteins. In this paper we apply basic polymer physics and statistical mechanics methods to the self-assembly of a synthetic virus encapsidating generic polyelectrolyte molecules. We find that (i) the mean concentration of the encapsidated polyelectrolyte material depends on the surface charge density, the radius of the capsid, and the linear charge density of the polymer but neither on the salt concentration nor the Kuhn length, and (ii) the total charge of the capsid interior is equal but opposite to that of the empty capsid, a form of charge reversal. Unlike natural viruses, synthetic viruses are predicted not to be under an osmotic swelling pressure. The design condition that self-assembly only produces filled capsids is shown to coincide with the condition that the capsid surface charge exceeds the desorption threshold of polymer surface adsorption. We compare our results with studies on the self-assembly of both synthetic and natural viruses.

Capsid↗

PubMed2006

Fluctuation theorems for quantum master equations.

A quantum fluctuation theorem for a driven quantum subsystem interacting with its environment is derived based solely on the assumption that its reduced density matrix obeys a closed evolution equation--i.e., a quantum master equation (QME). Quantum trajectories and their associated entropy, heat, and work appear naturally by transforming the QME to a time-dependent Liouville space basis that diagonalizes the instantaneous reduced density matrix of the subsystem. A quantum integral fluctuation theorem, a steady-state fluctuation theorem, and the Jarzynski relation are derived in a similar way as for classical stochastic dynamics.

Entropy↗

PubMed2004

Roughness of stylolites: implications of 3D high resolution topography measurements.

Stylolites are natural pressure-dissolution surfaces in sedimentary rocks. We present 3D high resolution measurements at laboratory scales of their complex roughness. The topography is shown to be described by a self-affine scaling invariance. At large scales, the Hurst exponent is zeta(1) approximately 0.5 and very different from that at small scales where zeta(2) approximately 1.2. A crossover length scale at around L(c)=1 mm is well characterized. Measurements are consistent with a Langevin equation that describes the growth of a stylolitic interface as a competition between stabilizing long range elastic interactions at large scales or local surface tension effects at small scales and a destabilizing quenched material disorder.

Calcium Carbonate↗

PubMed2005

Stratification kinetics of polyelectrolyte solutions confined in thin films.

We have studied free horizontal liquid films made with semidilute polyelectrolyte solutions. A stratification phenomenon is observed during film thinning, with a step size close to the mesh size of the polymer network: dark domains nucleate and expand, the outer polymer layer dewetting a thinner film. The kinetics of dark spot expansion is not simply related to bulk viscosity and becomes faster when the film thickness decreases, suggesting an increase of the chain mobility of the confined polymer chains. These findings are similar to recent ones for other confined liquids and are the first reported so far for freely standing films.

Electrolytes↗

PubMed2005

Jamming as a critical phenomenon: a field theory of zero-temperature grain packings.

A field theory of frictionless grain packings in two dimensions is shown to exhibit a zero-temperature critical point at a nonzero value of the packing fraction. The zero-temperature constraint of force balance plays a crucial role in determining the nature of the transition. Two order parameters, , the deviation of the average number of contacts from the isostatic value, and , the average magnitude of the force per contact, characterize the transition from the jammed (high packing fraction) to the unjammed (low packing fraction state). The critical point has a mixed character with the order parameters showing a jump discontinuity but with fluctuations of the contact force diverging. At the critical point, the distribution of phi shows the characteristic plateau observed in static granular piles. The theory makes falsifiable predictions about the spatial fluctuations of the contact forces.

Algorithms↗

PubMed2004

A parametric model for studying organism fitness using step-stress experiments.

We propose a method based on parametric survival analysis to analyze step-stress data. Step-stress studies are failure time studies in which the experimental stressor is increased at specified time intervals. While this protocol has been frequently employed in industrial reliability studies, it is less common in the life sciences. Possible biological applications include experiments on swimming performance of fish using a step function defining increasing water velocity over time, and treadmill tests on humans. A likelihood-ratio test is developed for comparing the failure times in two groups based on a piecewise constant hazard assumption. The test can be extended to other piecewise distributions and to include covariates. An example data set is used to illustrate the method and highlight experimental design issues. A small simulation study compares this analysis procedure to currently used methods with regard to type I error rate and power.

Analysis of Variance↗

PubMedDate not supplied

Ovarian neoplasms and pregnancy.

The aim of our study was to investigate the incidence of ovarian malignant neoplasms in pregnancy. In the prospective study, during a 2-year period of time, we had the ethical dilemma concerning the conciliar treatment vs the patient's wish to deliver and save the reproductive capabilities. We examined 37 pregnant women; the incidence of ovarian cancer was 13.5% vs 6.5% in other works. Diagnoses were made by ultrasound criteria and physical examinations. Statistically significant results were obtained by Student t test. Mean gestational age was 20.1 weeks and mean age 31.1 years. Treatment depends on the neoplasms type, grade, and presence of the metastatic pathways. In benign neoplasms, we used laparoscopic treatment with minor invasion. Distribution of benign neoplasms was in the same range as that in other works.

Adult↗

PubMed1996

A randomized trial of dementia care in nursing homes.

OBJECTIVE: To evaluate the efficacy of a dementia care program to reduce behavior disorders in nursing home patients with dementia. DESIGN: Randomized controlled clinical trial with 6-month follow-up. SETTING: A 250-bed community nursing home. PATIENTS: The nursing home was screened to identify patients with dementia and behavior disorders. A total of 118 patients were eligible for randomization. Of these, 89 (75.4%) were randomized, and 81 of these (91.0%) completed the trial. INTERVENTION: The A.G.E. dementia care program consisted of Activities, Guidelines for psychotropic medications, and Educational rounds. The control treatment was usual nursing home care. MEASUREMENTS: Behavior disorders, antipsychotic drug and physical restraint use, patient activity levels, and cognitive and functional status. RESULTS: After 6 months, 12 of 42 (28.6%) intervention patients exhibited behavior disorders compared with 20 of 39 (51.3%) controls (OR = 0.38; 95% CI [0.15, 0.95]; P = .037). Controls were more than twice as likely to receive antipsychotics (OR = 2.55, 95% CI [0.96, 6.76]; P < .056), to be restrained during activity times (OR = 2.98, 95% CI [1.10, 8.04]; P < .028), and to be restrained on nursing units (OR = 2.14, 95% CI [0.9, 5.3]; P < .10). Intervention patients were much more likely to participate in activities (OR = 13.71; 95% CI [4.51, 41.73]; P = .001). CONCLUSIONS: The A.G.E. program reduces the prevalence of behavior disorders and the use of antipsychotic drugs and restraints. It is practical, feasible, and appears to improve the lives of patients with dementia in nursing homes.

Aged↗

PubMed1992

Comparative effectiveness of visual/tactile and simplified screening examinations in caries risk assessment.

Central to the development of a model for identifying children at high risk to caries is a clinical evaluation to assess dental status and other conditions potentially useful in caries prediction. Traditionally, this evaluation has been based on a relatively lengthy visual/tactile examination conducted by a dentist. Replacing the dentist examination with a dental auxiliary conducted screening evaluation could lead to reduced time and costs. The 4-yr University of North Carolina Caries Risk Assessment Study involved approximately 5000 schoolchildren initially in Grades 1 and 5 living near Aiken, South Carolina, and Portland, Maine. The effectiveness of caries prediction models using visual/tactile examination data were compared with the same models using simplified screening evaluation data. Results showed sensitivity ranged from 0.57 to 0.61 for the visual/tactile and screening models by site and grade cohort. Specificity for the models ranged from 0.80 to 0.83. None of these differences in sensitivity and specificity between visual/tactile (dentist) and screening (hygienist) models was statistically significant. Findings show that for the prediction of children at high risk to dental caries the clinical evaluation may be conducted with no reduction of precision by using dental hygienist performed screening evaluations rather than dentist conducted visual/tactile examinations. While no cost data were collected, these results imply that costs to future prediction programs could be reduced by using screening evaluations.

Child↗

PubMed2001

Decaying source method for scintillation camera resolving times.

An earlier paper by the author showed that scintillation camera systems can be described effectively using the resolving times T and tau0 of the dominant nonparalyzable and paralyzable components, that is, the detector system and the computer interface, respectively. When used with a full spectrum window, the camera has a lower nonparalyzable and an upper paralyzable operating range with normalized threshold input rate n(t)= N(t)tau(0) = ln(1 +k(T)n(t)), where kT= T/(tau)0. Correct determination of T and tau(0) requires that both r12 and r in the normalized two-source equations kT=(2/r12- 1/r) and k0=(r12/2r2)ln(2r/r12) come from the nonparalyzable (n< or =n(t)) and paralyzable (n> or =n(t)) ranges, respectively. A serious constraint of the two-source method, therefore, is that the large ratio a = n12 ln = 2 can lead to an input rate range (n,n12), which includes the threshold point n,, and in which neither T nor tau0 can be measured correctly. The decaying source method constitutes a refinement of the two-source method, which enables smaller ratios 1<a<2 to be used, and also includes the two-source method as a special case (a=2). This new method requires just two consecutive readings on a single decaying source, as opposed to three measurements on two sources of activity, for each determination of T or tau0, thus also minimizing staff exposure. The fact that only count rates and time intervals need now be recorded greatly simplifies computerization of the data acquisition and analysis activities, and the potential for real-time applications is obvious. The method enables T and tau0 to be measured accurately and with sufficient resolution to reveal possible variations with input rate. Long measurement times using a decaying source can be avoided, if required, by using a set of decaying sources simultaneously to cover different portions of the count rate range. The application of the measurement procedure in real-time and the use of the resolving times in the accurate correction of deadtime losses, also in real-time, are treated separately in another paper.

Gamma Cameras↗

PubMedDate not supplied

A simplified approach for modulation transfer function determinations in computed tomography.

In order to determine the modulation transfer functions (MTF's) for x-ray computed tomography (CT) scanners, a measurement must be performed to obtain either the point spread function (PSF) or the line spread function (LSF). Thereafter, the usual procedure is to interpolate between the measured points and to determine the Fourier transforms numerically in order to obtain the MTF. Since this must usually be done many times to evaluate various reconstruction kernels and scan modalities, the process is tedious. Fortunately, it can be greatly simplified by utilizing a mathematical function to describe the PSF or LSF. Measured data for five CT scanners indicates that the PSF can usually be described by a Gaussian function. Hence, the MTF can be written in a generalized form eliminating the necessity of performing Fourier transformations each time. The MTF is determined directly from a single performance characteristic related to the full width at half maximum. The accuracy of the approach is compared with detailed MTF calculations for five CT scanners and it is shown to agree favorably with this data.

Fourier Analysis↗

PubMed2006

Computation of the homogeneous and forced solutions of a finite length, line-driven, submerged plate.

A formulation is developed to predict the vibration response of a finite length, submerged plate due to a line drive. The formulation starts by describing the fluid in terms of elliptic cylinder coordinates, which allows the fluid loading term to be expressed in terms of Mathieu functions. By moving the fluid loading term to the right-hand side of the equation, it is considered to be a force. The operator that remains on the left-hand side is the same as that of the in vacuo plate: a fourth-order, constant coefficient, ordinary differential equation. Therefore, the problem appears to be an inhomogeneous ordinary differential equation. The solution that results has the same form as that of the in vacuo plate: the sum of a forced solution, and four homogeneous solutions, each of which is multiplied by an arbitrary constant. These constants are then chosen to satisfy the structural boundary conditions on the two ends of the plate. Results for the finite plate are compared to the infinite plate in both the wave number and spatial domains. The theoretical predictions of the plate velocity response are also compared to results from finite element analysis and show reasonable agreement over a large frequency range.

Acoustics↗

PubMed2006

Study of the comparison of the methods of equivalent sources and boundary element methods for near-field acoustic holography.

Boundary element methods (BEM) based near-field acoustic holography (NAH) has been used successfully in order to reconstruct the normal velocity on an arbitrarily shaped structure surface from measurements of the pressure field on a nearby conformal surface. An alternative approach for this reconstruction on a general structure utilizes the equivalent sources method (ESM). In ESM the acoustic field is represented by a set of point sources located over a surface that is close to the structure surface. This approach is attractive mainly for its simplicity of implementation and speed. In this work ESM as an approximation of BEM based NAH is studied and the necessary conditions for the successful application of this approach in NAH is discussed. A cylindrical fuselage surface excited by a point force as an example to validate the results is used.

Acoustics↗

PubMed2006

Modal decomposition method for acoustic impedance testing in square ducts.

Accurate duct acoustic propagation models are required to predict and reduce aircraft engine noise. These models ultimately rely on measurements of the acoustic impedance to characterize candidate engine nacelle liners. This research effort increases the frequency range of normal-incidence acoustic impedance testing in square ducts by extending the standard two-microphone method (TMM), which is limited to plane wave propagation, to include higher-order modes. The modal decomposition method (MDM) presented includes four normal modes in the model of the sound field, thus increasing the bandwidth from 6.7 to 13.5 kHz for a 25.4 mm square waveguide. The MDM characterizes the test specimen for normal- and oblique-incident acoustic impedance and mode scattering coefficients. The MDM is first formulated and then applied to the measurement of the reflection coefficient matrix for a ceramic tubular specimen. The experimental results are consistent with results from the TMM for the same specimen to within the 95% confidence intervals for the TMM. The MDM results show a series of resonances for the ceramic tubular material exhibiting a monotonic decrease in the resonant peaks of the acoustic resistance with increasing frequency, resembling a rigidly-terminated viscous tube, and also evidence of mode scattering is visible at the higher frequencies.

Acoustics↗

PubMed1991

Effect of marker distance and orientation on recombinant formation in poxvirus-infected cells.

Little is known about the mechanism of poxvirus recombination even though construction of recombinant viruses by recombination-dependent methods is a widely adopted technique. We have shown previously that transfected DNAs are efficiently recombined while replicating in cells infected with Shope fibroma virus. Because recombinant DNA can be recovered from infected cells as a high-molecular-weight head-to-tail concatemer, it was possible to transfect genetically marked lambda DNAs into infected cells and assay recombinants as bacteriophage particles following in vitro packaging. This approach was used in this study to examine how marker distance and marker orientation influence recombination in Shope fibroma virus-infected cells. Simple two-factor crosses were readily modelled by using a mapping function derived from classical phage studies and showed low negative interference (I = -2.8 +/- 0.5) in crosses involving markers greater than 100 bp apart. More complex four- and five-factor crosses showed that the recombination frequency per unit distance was not constant (rising as the marker separation was reduced from 100 to 1 bp) and that crosses performed in poxvirus-infected cells are subject to high negative interference. One consequence is that marker orientation does not dramatically influence the outcome of most Shope fibroma virus-catalyzed crosses in clear contrast to what is observed in adenovirus or simian virus 40-infected cells. These results can be interpreted to indicate that similar statistical and physical constraints influence both viral and phage recombination and suggest that heteroduplexes may be important intermediates in the poxvirus recombination process.

Animals↗

PubMed1993

The calibration of a sit-ups task using the Rasch Poisson Counts model.

The purpose of this study was to calibrate a nationally used sit-ups test using the Rasch Poisson Counts model and evaluate the model-data fit. The total number of subjects was 8,723, consisting of 4,486 girls and 4,237 boys, ages 10 to 18. The estimated difficulty of the sit-ups task was -2.80, which was appropriate for a majority of examinees whose ability levels ranged from .09 to 1.39. After the calibration, boys and girls as well as different age groups were compared under the same metric. Graphs of the model-data fit demonstrated that the model-data fit at a low ability level was not as good as the fit at a high ability, which could be caused by violation of assumptions of the model that examinees have the same performance speed throughout the test and that the speed at a given time is independent of the number of sit-ups completed so far.

Adolescent↗