Single Volume Element Research Articles

Micromechanics-based variational phase-field modeling of Brazilian splitting tests

The simulation of the Brazilian splitting test can be challenging due to the presence of stress concentrations near the load points, which can lead to numerical problems and convergence issues. This paper aims to realistically model the Brazilian splitting test on mortar samples under monotonic loading, employing a micromechanics-based variational phase-field formulation. A micromechanics-based phase-field model connects the field variables at the macroscale and the physical dissipative mechanisms at the microcrack level, associating plasticity with the sliding of closed microcracks and damage with the growth of open microcracks. The distinction between these two mechanisms aids crack initiation and helps to overcome convergence issues. To further support convergence, a viscoplasticity regularization is embedded in the micromechanics-based phase field model. First, the homogeneous response of a single volume element is presented to demonstrate the effect of the viscosity parameter, and to provide insight of different failure modes that the model predicts. The model is next calibrated against experimental results. Hereto, Brazilian splitting tests have been performed using Digital Image Correlation technique. The calibrated numerical model predicts shear fracture mechanisms involved in the Brazilian splitting test and this has been further explained by studying the effect of confining pressure on modulated failure modes in the model. The calibrated parameters are then used for other samples with the same material but a different size and boundary conditions to assess the validity of the model and its prediction capabilities. Results show that the numerical failure prediction is consistent with the experiment. Overall, the calibration and comparison of numerical and experimental results confirm the validity and reliability of the simulation model and its ability to predict material behavior and fracture modes under different boundary conditions.

Modeling of two-photon polymerization in the strong-pulse regime

This paper reports a study of two-photon polymerization induced by femtosecond laser pulses having microjoule pulse energy and kilohertz repetition rates. Light-matter interaction and polymerization kinetics are modeled in highly confined spatiotemporal scales. The model employs a non-diffractive Bessel beam, considers the effects of temperature-dependent species diffusions, and regards propagation and termination kinetic constants as functions of double-bond conversion. The model is validated by comparing the size of features predicted from simulations to those generated experimentally. The model is used to investigate how the time and energy required to create a single volume element ("voxel") change under various conditions of irradiation. The results show that polymerizing a single voxel requires a minimum exposure time that is constant across a range of irradiation conditions, and is largely determined by the chemical kinetics. In the regime where the pulse energy is low (< 10 μ J), it is more energy-efficient to use fewer pulses having higher energy within the same total exposure time. However, this trend reverses in the regime where the pulse energy is high (10 μ J - 30 μ J), because radical-radical recombination becomes significant, which wastes absorbed energy. This work advances the understanding of two-photon polymerization in the strong-pulse regime and is a step toward increasing throughput to a level suitable for industrial applications. • Two-photon polymerization is induced by few and strong femtosecond laser pulses • A Bessel beam is utilized to rapidly fabricates high aspect ratio structures • Polymerizing a single voxel requires a minimum exposure time • It is more energy-efficient to use fewer pulses when the pulse energy is low (<10 μ J) • Radical-radical recombination is severe when the pulse energy is high (>10 μ J)

Microscopy tools for product innovation

Microscopy tools for product innovation

Real-time Simulation of Dual-Layer Catalytic Converters Based on the Internal Mass Transfer Coefficient Approach

A new numerically efficient simulator for dual layer catalysts is implemented and its performance is demonstrated using the example of the dual layer ammonia oxidation catalyst. For the solution of the radial mass balances, each washcoat layer is represented by a single volume element and the diffusive mass fluxes into and within the washcoat are computed based on the concept of the internal mass transfer coefficients. The performance of the new simulator is compared against a reference simulator that fully resolves the concentration profiles in the washcoat. For a steady state test case, the error introduced by the new solution scheme is below 2 % for all relevant exhaust components. For a transient test case, the deviations become more significant. For the simulation of a WHTC, the maximum deviation in the NH3 outlet concentration is 23.3 %, whereas the cumulated NH3 emissions deviate by 10.6 %.With the new simulator, the 1800 s of the transient WHTC cycle can be simulated within 53 s on a standard laptop, 30 times faster than real time.

Rapid assessments of hydrate blockage risk in oil-continuous flowlines

As industry moves toward the production of oil and gas resources in deep offshore environments, the prospective formation of natural gas hydrates under low temperature, high-pressure conditions poses an increasing risk of pipeline blockage. Successful management of this risk requires a precise forecast of the hydrate growth rate. Such predictions should incorporate quantitative descriptions of both deterministic and probabilistic hydrate phenomena. In this work, we present a first step towards such a quantitative risk assessment for systems that form hydrate slurries, based on a consideration of the stochastic nature of hydrate formation. Our framework introduces a new approach to risk assessment, by coupling a laboratory-derived probabilistic nucleation model with existing deterministic calculations for hydrate slurry viscosification. This new approach is used to extend a previously-described hydrate risk algorithm, the Hydrate Flow Assurance Simulation Tool (HyFAST), which enables the rapid assessment of hydrate slurry viscosification using the most advanced models available. Importantly, while the previous version of HyFAST was constrained to calculations in flowloop and autoclave geometries, for which it is sufficient to consider a single volume element, the new version of the HyFAST algorithm allows calculations for flowlines wherein a series of multiple volume elements are considered. The advanced hydrate formation models within this algorithm allow identification of critically important hydrate formation scenarios, and may serve as a screening tool for cases that then require study within rigorous hydrodynamic packages such as OLGA® or LedaFlow® to examine behaviour during key transient events. By coupling the outputs of our predictive algorithm and quantitative risk framework, we demonstrate a new capability to assess what constitutes an acceptable level of hydrate formation. We use this algorithm in a series of case studies that compare the effectiveness of common hydrate mitigation strategies over the life of a reservoir, including the optimization of a wellhead choke opening for both production rate and hydrate blockage risk.

Quantitative one-dimensional imaging using picosecond dual-broadband pure-rotational coherent anti-Stokes Raman spectroscopy

We employ picosecond dual-broadband pure-rotational coherent anti-Stokes Raman spectroscopy (CARS) in a one-dimensional (1D) imaging configuration. Temperature and O(2):N(2) concentration ratios are measured along a 1D line of up to 12 mm in length. The images consist of up to 330 individual rotational CARS (RCARS) spectra, corresponding to 330 spatially resolved volume elements in the probe volume. Signal levels are sufficient for the collection of single-laser-pulse images at temperatures of up to approximately 1200 K and shot-averaged images at flame temperatures, demonstrated at 2100 K. The precision of picosecond pure-rotational 1D imaging CARS is assessed by acquiring a series of 100 single-laser-pulse images in a heated flow of N(2) from 410 K-1200 K and evaluating a single volume element for temperature in each image. Accuracy is demonstrated by comparing temperatures from the evaluated averaged spectra to thermocouple readings in the heated flow. Deviations from the thermocouple of <30 K in the evaluated temperature were found at up to 1205 K. Accuracy and single-shot precision are compared to those reported for single-point nanosecond dual-broadband pure-RCARS and nanosecond 1D vibrational CARS.

Proton MR spectroscopic imaging in multiple sclerosis

We studied 24 patients with multiple sclerosis (MS) by proton magnetic resonance spectroscopic imaging (1H-MRSI) to assess the neurochemical pathology of the white-matter lesions (WML) and normal-appearing white matter (NAWM). Our 1H-MRSI technique allowed simultaneous measurement of N-acetylaspartate (NAA), choline-containing compounds (Cho), and creatine plus phosphocreatine (Cr) signal intensities from four 15-mm slices divided into 0.84 ml single-volume elements. In WML we found significantly lower NAA/Cr and NAA/Cho ratios and a significantly higher Cho/Cr ratio than in NAWM or control white matter. In NAWM, NAA/Cr and Cho/Cr were significantly lower than in control white matter. 1H-MRSI was compatible with damage to myelin in WML, and with axonal damage and/or dysfunction in WML and NAWM. These findings extend data on involvement of NAWM in MS beyond the abnormalities visible on MRI.

On the formulation of geometrically nonlinear 3D-Rebar-elements using the enhanced assumed strain method

This paper presents the combination of the Rebar-concept with the enhanced assumed strain method for geometrically nonlinear 3D-bricks. Finite element calculations with Rebar elements are used to model composite materials with single fibers or fiber layers within one single volume element. The application of global geometric functions to describe single fiber courses, allows an easy computing of single fiber composite structures. In combination with work equivalent nodal forces for the load case prestress, prestressed concrete beams can be calculated. Rebar fiber layer elements are employed successfully for modelling composite laminate structures.

Common pattern of cortical pathology in childhood-onset and adult-onset schizophrenia as identified by proton magnetic resonance spectroscopic imaging.

Multislice proton magnetic resonance spectroscopic imaging (1H-MRSI) permits simultaneous acquisition and mapping of signal intensities of N-acetyl-containing compounds (mainly N-acetylaspartate, NAA), choline-containing compounds (CHO), and creatine plus phosphocreatine (CRE) from multiple whole-brain slices consisting of small single-volume elements. Previous 1H-MRSI studies of adult patients with schizophrenia showed small NAA relative signals in the hippocampal area and in the dorsolateral prefrontal cortex in comparison with healthy subjects. As part of a program to address the pathophysiological continuity between childhood-onset and adult-onset schizophrenia, the authors performed 1H-MRSI of patients with childhood-onset schizophrenia to specifically test whether the hippocampal area and dorsolateral prefrontal cortex show the same abnormalities as seen in adult-onset schizophrenia. A 1.5-T nuclear magnetic resonance machine was used to test 14 patients (mean age, 16.4 years) and 14 comparison subjects. Ratios of areas under the metabolite peaks of the proton spectra were determined (i.e., NAA/CRE, NAA/CHO, CHO/CRE) for multiple cortical and subcortical regions. The patients showed significantly lower NAA/CRE ratios bilaterally in the hippocampal area and the dorsolateral prefrontal cortex than the comparison subjects. There were no significant differences in CHO/CRE or in NAA ratios in any other area sampled. The present study shows that patients with childhood-onset schizophrenia have smaller than normal regional NAA relative signals, suggesting neuronal damage or malfunction in the hippocampal area and dorsolateral prefrontal cortex. These differences were similar in magnitude to those found in patients with adult-onset schizophrenia. The present data extend other evidence of a biological continuum between childhood- and adult-onset schizophrenia.

Proton magnetic resonance spectroscopic imaging in the clinical evaluation of patients with Niemann-Pick type C disease

OBJECTIVES10 patients with Niemann-Pick disease type C (NP-C) were studied by proton magnetic resonance spectroscopic imaging (1H-MRSI) to assess the biochemical pathology of the brain and to determine whether this...

Reconstruction and display of curvilinear objects from optical section data using 3-D curve fitting algorithms.

Biological objects resembling filaments are often highly elongated while presenting a small cross-sectional area. Examination of such objects requires acquisition of images from regions large enough to contain entire objects, but at sufficiently high resolution to resolve individual filaments. These requirements complicate the application of conventional optical sectioning and volume reconstruction techniques. For example, objective lenses used to acquire images of entire filaments or filament networks may lack sufficient depth (Z) resolution to localize filament cross-sections along the optical axis. Because volume reconstruction techniques consider only the information represented by a single volume element (voxel), views of filament networks reconstructed from images obtained at low Z-resolution will not accurately represent filament morphology. A possible solution to these problems is to simultaneously utilize all available information on the path of an object by fitting 3-D curves through data points localized in 2-D images. Here, we present an application of this approach to the reconstruction of microtubule networks from 2-D optical sections obtained using confocal microscopy, and to synthesized curves which have been distorted using a simple mathematical model of optical sectioning artefacts. Our results demonstrate that this strategy can produce high resolution 3-D views of filamentous objects from a small number of optical sections.

Proton magnetic resonance spectroscopic imaging in progressive supranuclear palsy, Parkinson's disease and corticobasal degeneration.

We used proton magnetic resonance spectroscopic imaging (1H-MRSI) to assess the in vivo cortical and subcortical neuronal involvement in progressive supranuclear palsy, Parkinson's disease and corticobasal degeneration. This technique permitted the simultaneous measurement of compounds containing N-acetylaspartate (NA), choline (Cho), creatine-phosphocreatine (Cre) and lactate, from four 15-mm slices divided into 0.84-ml single-volume elements. The study included 12 patients with progressive supranuclear palsy, 10 with Parkinson's disease, nine with corticobasal degeneration and 11 age-matched normal control subjects. Regions of interest were selected from the brainstem, caudate, thalamus, lentiform nucleus, centrum semiovale, and from frontal, parietal, precentral, temporal and occipital cortices. Progressive supranuclear palsy patients, compared with control subjects, had significantly reduced NA/Cre in the brainstem, centrum semiovale, frontal and precentral cortex, and significantly reduced NA/Cho in the lentiform nucleus. Corticobasal degeneration patients, compared with control subjects, had significantly reduced NA/Cre in the centrum semiovale, and significantly reduced NA/Cho in the lentiform nucleus and parietal cortex. There were no significant differences between Parkinson's disease patients and control subjects, or between patients groups in any individual region of interest. In the parietal cortex of corticobasal degeneration patients, NA/Cho was significantly reduced contralateral to the most affected side. There were statistically significant group differences in the regional pattern of NA/Cre and NA/Cho reduction, comparing normal control subjects with all patient groups, Parkinson's disease with corticobasal degeneration, and Parkinson's disease with progressive supranuclear palsy. Although the occurrence of significant groups differences does not imply that it is possible to differentiate between individual patients using 1H-MRSI in progressive supranuclear palsy and corticobasal degeneration, detection of specific cortical and subcortical patterns of neuronal involvement is possible with this technique. We suggest that this regional pattern of neuronal involvement found in progressive supranuclear palsy and corticobasal degeneration may help in the diagnostic evaluation of affected individuals.

Regionally specific pattern of neurochemical pathology in schizophrenia as assessed by multislice proton magnetic resonance spectroscopic imaging.

Several single-voxel proton magnetic resonance spectroscopy (1H-MRS) studies of patients with schizophrenia have found evidence of reductions of N-acetyl-aspartate (NAA) concentrations in the temporal lobes. Multislice proton magnetic resonance spectroscopy imaging (1H-MRSI) permits simultaneous acquisition and mapping of NAA, choline-containing compounds (CHO), and creatine/phosphocreatine (CRE) signal intensities from multiple whole brain slices consisting of 1.4-ml single-volume elements. We have used 1H-MRSI to assess the regional specificity of previously reported changes of metabolite signal intensities in schizophrenia. Hippocampal volume was also measured to test the relationship between 1H-MRSI findings and tissue volume in this region. Ratios of areas under the metabolite peaks of the proton spectra were determined (i.e., NAA/CRE, NAA/CHO, CHO/CRE) for multiple cortical and subcortical regions in 10 inpatients with schizophrenia. Patients showed significant reductions of NAA/CRE and NAA/CHO bilaterally in the hippocampal region and in the dorsolateral prefrontal cortex. There were no significant changes in CHO/CRE or in NAA ratios in any other area sampled. No significant correlation was found between metabolite ratios in the hippocampal region and its volume. NAA-relative signal intensity reductions in schizophrenia appear to be remarkably localized, involving primarily the hippocampal region and the dorsolateral prefrontal cortex, two regions implicated prominently in the pathophysiology of this disorder.

Cortical and subcortical chemical pathology in Alzheimer's disease as assessed by multislice proton magnetic resonance spectroscopic imaging.

Multislide proton magnetic resonance spectroscopic imaging (1H-MRSI) permits the simultaneous acquisition of N-acetylaspartate (NA), choline (Cho), creatine/phosphocreatine (Cre), and lactate (Lac) signal intensities from four 15-mm slices divided into 0.84-ml single-volume elements. NA is inferred to be a neuron-specific molecule, whereas Cho mainly reflects glycerophosphocholine and phosphocholine, compounds involved in phospholipid metabolism. To assess whether 1H-MRSI could detect a regional pattern of cortical and subcortical involvement in the brain of Alzheimer's disease (AD) patients. 1H-MRSI was performed in 15 patients with probable AD and 15 age-matched healthy controls. Regions of interest (ROIs) were selected from frontal (FC), temporal (TC), parietal (PC), occipital, and insular cortices, subcortical white matter (WM), and thalamus. In AD patients, we found a significant reduction of NA/Cre in the FC, TC, and PC and a significant reduction of Cho/Cre in the WM. This 1H-MRSI study of AD patients shows a regional pattern of neuronal damage in the associative cortices, as revealed by significant reduction of NA/Cre in the FC, TC, and PC, and regional derangement of phospholipid metabolism, as revealed by significant reduction of Cho/Cre in the WM.

A precise light attenuation correction for confocal scanning microscopy with O(n4/3) computing time and O(n) memory requirements for n voxels

Some dyes and tissues observed by confocal fluorescence microscopy show remarkable opacity, caused by absorption and scattering within the scanned volume. The efficiency of the excitation and fluorescence process may vary by a factor of 10 from top to bottom regions even in not very extended scan volumes. Based on known attenuation properties of a specimen, the amount of damping is computed by integrating the attenuation along all light paths within the numerical aperture of the objective, Thus, to correct a single volume element (voxel), one has to take into account the damping within the whole extended conical volume between the lens and the focus, resulting in intolerable execution times. Common approaches reduce resolution or simplify the integration paths, thereby resulting in either a loss of fine resolution or showing a low resolution versus computation time ratio. This paper presents a more efficient reformulation of this spatial integration process without simplifying the physical background. This algorithm requires computing times slightly longer than those of programs using simplified physical and statistical approaches. However, the algorithm may be tuned to achieve a precision and stability comparable to exhaustive integration.

Proton magnetic resonance spectroscopic imaging in patients with cerebellar degeneration.

Using proton magnetic resonance spectroscopic imaging, we studied the cerebellum of 9 patients with cerebellar degeneration and of 9 age-matched normal control subjects. This technique permits the simultaneous measurement of N-acetylaspartate, choline-containing compounds, creatine/phosphocreatine, and lactate signal intensities from four 15-mm slices divided into 0.84-ml single-volume elements. Because patients with cerebellar degeneration often show substantial atrophy on magnetic resonance imaging (MRI), we specifically chose to analyze the spectroscopic signals only from tissue that did not have an atrophic appearance on the MRI. The spectroscopic findings showed a significant reduction of N-acetylaspartate in all parts of the cerebellum, a significant correlation with MRI scores of cerebellar atrophy, and a significant correlation with clinical rating scores of cerebellar disturbance. Our method of analysis suggests the presence of a neurodegenerative process in cerebellar areas that do not appear to be atrophic on the MRI. Some limitations of proton magnetic resonance spectroscopic imaging in the present study were related to the partial field inhomogeneity characteristics of the posterior fossa, the anatomical location of the cerebellum, and the particularly severe cerebellar atrophy in some of the patients.

Multislice proton magnetic resonance spectroscopic imaging in X-linked adrenoleukodystrophy.

Multislice proton magnetic resonance spectroscopic imaging permits metabolic analysis of brain tissue in vivo by data acquisition in four oblique axial slices, each 15-mm thick and divided into 0.8-ml single-volume elements. We applied this technique to the systematic study of 25 patients with adrenoleukodystrophy: 3 with the severe childhood or adult cerebral form of the disease, 5 with adrenomyeloneuropathy, 12 with no demonstrable neurological involvement, and 5 women heterozygous for adrenoleukodystrophy who had some degree of neurological disability. Abnormalities on magnetic resonance spectroscopic imaging included a reduction in N-acetyl aspartate, an increase in choline-containing compounds, and at times, an increase in lactate. Five patients showed abnormalities in the presence of normal-appearing magnetic resonance images, and in 8 other patients the alterations on spectroscopic images were more severe than those demonstrable by magnetic resonance imaging. Correlation with clinical course suggests that an increase in the choline-containing compounds is associated with an active demyelinative process, whereas such compounds are not elevated in lesions that are stable. We conclude that magnetic resonance spectroscopic imaging is a more sensitive indicator of early neurological involvement than is magnetic resonance imaging, and that the character of abnormalities detected by the former technique may serve as a gauge of the degree of activity of the demyelinating process and as a guide to the selection and evaluation of therapeutic approaches.

Imaging cellular dynamics using scanning laser confocal microscopy

In recent years, the ability to image morphological dynamics and physiological changes in living cells and tissues has been greatly advanced by the advent of scanning laser confocal microscopy. Confocal microscopes employ optical systems in which both the condenser and objective lenses are focused onto a single volume element of the specimen. In practice, galvanometer-driven mirrors or acousto-optical deflectors are used to scan a laser beam over the specimen in a raster-like fashion through an epifluorescence microscope. The incident laser beam, as well as the collected fluorescent light, are passed through pinhole or slit apertures in image planes that are conjugate to the plane of the specimen. This method of illumination and detection prevents fluorescent light which is generated above and below the plane-of-focus from impinging on the imaging system's photodetector, thus rejecting much of the fluorescent light which normally blurs the image of a three-dimensional fluorescent specimen.

Partial volume tissue classification of multichannel magnetic resonance images-a mixel model

A single volume element (voxel) in a medical image may be composed of a mixture of multiple tissue types. The authors call voxels which contain multiple tissue classes mixels. A statistical mixel image model based on Markov random field (MRF) theory and an algorithm for the classification of mixels are presented. The authors concentrate on the classification of multichannel magnetic resonance (MR) images of the brain although the algorithm has other applications. The authors also present a method for compensating for the gray-level variation of MR images between different slices, which is primarily caused by the inhomogeneity of the RF field produced by the imaging coil.

Application of Digital Simulation to a Mass Transport Problem: Calculation of the Velocity of Flow at a Rotating Disk

AbstractFor the rotating disk a simulation model has been established, in which the forces acting on a single volume element are considered in order to calculate the velocity of flow. Thus the complicated solving of the Navier‐Stokes‐equation could be avoided. The simulation results have been compared with the analytical solution obtained by Cochran, leading to the conclusion that velocities of flow can successfully be calculated by digital simulation. This is especially important for systems with unknown convection. The velocity of flow obtained from digital simulation can be used to calculate the convective diffusion as a function of the Prandtl number, resulting in a dimensionless current parameter similar to the one derived from analytical calculations.