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At least 235 records · Page 13Linked to original sources
PubMed2004

Potential of a Compton camera for high performance scintimammography.

In this paper, we present a novel approach to scintimammography that is based on the Compton camera principle. We analyse the performance of our scheme using Monte Carlo simulations. In particular, we evaluate the detection efficiency, spatial resolution and lesion visibility of the system at several gamma photon energies. The simulation results show that the proposed technique achieves an absolute detection efficiency of 0.03 and a full width at half maximum resolution of 3.8 mm at 141 keV photon energy for point sources 2.5 cm deep in a 5 cm thick breast phantom using 5 mm thick silicon detectors. Furthermore, our approach shows good performance in lesion detection, especially at high gamma photon energies, where mechanically collimated systems perform poorly due to severe septal penetration. With total collected counts of 1.35 million, equivalent to a 30 s acquisition time for an activity concentration level of 3.7 kBq ml(-1) (100 nCi cm(-3)) in normal breast tissue, and a tumour-to-background ratio of 8:1, our system can clearly reveal an 8 mm diameter tumour that is located 2.5 cm deep in a 500 ml breast phantom. We also present a simulation-based quantitative performance comparison between the proposed scintimammographic system and the compact collimated scintimammographic system in the task of lesion detection under a clinical imaging situation using a non-prewhitening matched filter observer model. Our comparison demonstrates that for the same imaging time, the two systems have a comparable performance in detecting an 8 mm tumour at 141 keV, with the proposed system performing marginally better. However, the proposed scintimammographic system clearly outperforms the compact collimated counterpart in the detection of a 5 mm tumour. We also investigate the contribution of scatter and direct radiation from adjacent organs. We find that the background contribution of liver to the right breast is 30% at 141 keV, which can be reduced to 4.8% with shielding.

Breast Neoplasms↗
PubMed2000

Evolution of cooperative problem solving in an artificial economy

We address the problem of how to reinforce learning in ultracomplex environments, with huge state-spaces, where one must learn to exploit a compact structure of the problem domain. The approach we propose is to simulate the evolution of an artificial economy of computer programs. The economy is constructed based on two simple principles so as to assign credit to the individual programs for collaborating on problem solutions. We find empirically that starting from programs that are random computer code, we can develop systems that solve hard problems. In particular, our economy learned to solve almost all random Blocks World problems with goal stacks that are 200 blocks high. Competing methods solve such problems only up to goal stacks of at most 8 blocks. Our economy has also learned to unscramble about half a randomly scrambled Rubik's cube and to solve several commercially sold puzzles.

Journal Article↗
PubMed2006

Global optimization and modeling techniques for planar multilayered dielectric structures.

We present a multilevel global optimization strategy for synthesizing planar multilayered dielectric structures. A low discrepancy sequence of sample points with uniform variable space coverage allows a global-level search while systematic refinement using gradient-based techniques identifies optima at the local level. Since efficient local optimization is important for this method, a fast calculation approach based on mode matching is presented; this also facilitates the compact derivation of analytical gradients. The approach is compared with genetic and simulated annealing algorithms through an antireflection coating design. The method proves to be competitive in terms of its performance, nonadaptive algorithm, and ability to track local solutions.

Journal Article↗
PubMed2000

Hydrodynamic aspects of slurry packing processes in microcolumn liquid chromatography.

A Stokesian dynamics computer simulation based method is presented for the estimation of the bed porosity of slurry-packed capillary liquid chromatography (LC) columns. A colloidally well-described reversed-phase stationary phase-slurry liquid suspension was used as a model system. The applied simulation method takes into account the velocity of the slurry and colloidal interaction forces, as well as inter-particle hydrodynamic interactions. The predicted bed porosities suggest that a lower slurry velocity leads to a denser packing structure due to the increased effect of colloidal repulsion effects. The results of the simulations were compared with the external porosity and chromatographic performance of capillary LC columns that were packed at different filtration and compaction pressures. However, the trends that were observed in the experimental results suggest that hydrodynamic packing parameters have no or little effect on the chromatographic performance of capillary LC columns. Within the experimental parameter window, the chromatographic performance and the column porosity were not influenced by the filtration and compaction pressure, nor by the duration of the compaction process.

Chromatography, Liquid↗
PubMed2003

Accelerated landfill waste decomposition by external leachate recirculation from an old landfill cell.

This research is focused on the management of moisture regime for a young landfill site in terms of leachate recirculation which entails the containment, collection and reinjection of leachate back into the landfill to promote in situ anaerobic biological treatment. Moreover, an innovative leachate management strategy was developed by using leachate recirculation from a mature landfill site into a young landfill site to provide accelerated waste stabilization. For this purpose, two reactors simulating young and old landfills were used in the laboratory. These reactors were loaded with shredded and compacted municipal solid waste with a typical composition determined for Istanbul Region. Both reactors were operated in a constant temperature room at 32 degrees C to enhance the growth of anaerobic microorganisms. Moreover, water was added to the reactors in order to simulate the annual rainfall. The reactor having the properties of old landfills was used as a control reactor. The reactor which represented the characteristics of young landfill was operated under four operational stages to enhance the activity of methanogenic population and accelerate waste stabilization. Results of this study indicated that the utilization of leachate recirculation enhanced waste stabilization in the young landfill by increasing the uniformity, and providing additional substrate and nutrients. Additions of buffer solutions of KOH and Na2CO3 together with leachate recirculation enhanced further waste stabilization and prevented possible acid inhibition. The utilization of external leachate recycled from the old landfill having desired acclimated anaerobic microorganisms, low organic content and higher buffer capacity into a young landfill could be a promising leachate management strategy for faster and controlled waste stabilization.

Bacteria, Anaerobic↗
PubMed2002

Development of an acoustic actuator for launch vehicle noise reduction.

In many active noise control applications, it is necessary that acoustic actuators be mounted in small enclosures due to volume constraints and in order to remain unobtrusive. However, the air spring of the enclosure is detrimental to the low-frequency performance of the actuator. For launch vehicle noise control applications, mass and volume constraints are very limiting, but the low-frequency performance of the actuator is critical. This work presents a novel approach that uses a nonlinear buckling suspension system and partial evacuation of the air within the enclosure to yield a compact, sealed acoustic driver that exhibits a very low natural frequency. Linear models of the device are presented and numerical simulations are given to illustrate the advantages of this design concept. An experimental prototype was built and measurements indicate that this design can significantly improve the low-frequency response of compact acoustic actuators.

Acoustics↗
PubMed2026

Study of the densification mechanisms of Al-Fe-Cr-Ti alloys during high-velocity compaction based on 3D MPFEM.

Aluminum alloy materials are widely used in aerospace and related fields, among which Al-Fe-Cr-Ti alloys have attracted increasing attention owing to their low density and excellent comprehensive properties. However, the densification mechanisms of alloy powders during high-velocity compaction (HVC) remain insufficiently understood. In this study, a three-dimensional multi-particle finite element method (3D MPFEM) model was developed to simulate the HVC process of Al-Fe-Cr-Ti alloy powders and to evaluate the effects of friction coefficient μ, impact energy per unit mass Em, hammer mass M, and compaction velocity v on powder densification. The results show that increasing μ from 0.25 to 0.65 reduced kinetic-energy transfer and stress transmission, decreasing the relative density ρ of the green from 0.7076 to 0.6797. In contrast, increasing Em from 55.58 to 144.67 J/g markedly improved densification, with the maximum relative density reaching 0.8881. Displacement-field analysis further revealed that appropriate combinations of M and v promote particle rearrangement and plastic deformation. Experimental validation confirmed that the simulated density evolution agreed well with the measured trend, although the predicted values were slightly lower. These findings indicate that 3D MPFEM can reasonably describe the macroscopic densification trend and provide qualitative particle-scale insights into deformation and energy-transfer behavior during HVC.

3D MPFEM↗
PubMed2006

An ex vivo comparison of three different gutta-percha cones when compacted at different temperatures: rheological considerations in relation to the filling of lateral canals.

AIM: To compare ex vivo the penetration of three brands of gutta-percha cones, compacted under a constant force and heated to different temperatures, into artificial lateral canals. METHODOLOGY: Resin blocks with simulated main canals, each having two lateral canals (C at 6.5 and A at 13 mm from the surface of the resin block), were selected. A gutta-percha cone, either Mynol MF, Hygenic MF or GT Tulsa 0.04 was compacted into each main canal for 5 s using a wire rod with a diameter of 0.7 mm soldered to the bottom of a metal cylinder, with a force of 2.7 kg at controlled temperatures of 37, 42, 47, 52, 60 degrees C. The penetration of each brand of gutta-percha into 60 lateral canals (10 at each temperature) was measured using a stereomicroscope. Statistical analysis was performed using the ANOVA, the Scheffè test and the t-test. RESULTS: None of the three brands of cones entered up to 0.1 mm within either lateral canal until a temperature of 47 degrees C was reached; at that temperature only Mynol cones (P < or = 0.05) penetrated in four of 10 A-level canals (mean 0.13 +/- 0.19 mm) and in all 10 C-level canals (mean 0.43 +/- 0.12 mm). The A-level lateral canals were penetrated at 52 degrees C by Mynol cones (mean 0.76 +/- 0.34 mm) to a significantly greater distance (P < or = 0.05) than Tulsa cones (mean 0.31 +/- 0.12 mm) and Hygenic cones (mean 0.11 +/- 0.08 mm). At 60 degrees C the Mynol cones (mean 1.93 +/- 0.34 mm) penetrated significantly more (P < or = 0.05) than the Tulsa cones (mean 0.86 +/- 0.22 mm) and Hygenic cones (mean 0.67 +/- 0.19 mm). The C-level lateral canals were penetrated at 52 degrees C by Mynol cones (mean 0.91 +/- 0.29 mm) to a significantly greater distance (P < or = 0.05) than Tulsa cones (mean 0.47 +/- 0.16 mm) and Hygenic cones (mean 0.46 +/- 0.15 mm), whilst no significant difference was found at 60 degrees C. CONCLUSIONS: When heated and compacted, the three gutta-percha cones penetrated the lateral canals to different degrees. They penetrated more than 0.43 mm into the lateral canals only at temperatures higher than 47 degrees C.

Dental Pulp Cavity↗
PubMed1999

Two-body in vitro wear study of some current dental composites and amalgams.

STATEMENT OF PROBLEM: Wear resistance of restorative materials is a major concern in clinical practice for restorations involving occlusion. The relatively poor wear resistance of dental composites in stress-bearing posterior situations has restricted even wider clinical application of this type of restorative material. PURPOSE: This in vitro study compared the relative wear resistance of a selection of current dental composites and amalgams under cyclic loading to explore the wear mechanisms operating on these materials and to assess their relative potential clinical wear resistance under variable masticatory loads. MATERIAL AND METHODS: A 2-body in vitro wear test was undertaken on a selection of 2 Ultrafine Compact-Filled composites, 1 microfilled composite, and 3 dental amalgams. An alternating sine curve load (0.5 to 15 N) was applied to the wear surface through a rotating countersample with a purpose-built system to simulate the variable loading pattern that occurs during mastication. RESULTS: Under conditions simulating the repetitive cyclic loading pattern that may occur intraorally in high stress occlusal contact situations, the Ultrafine Compact-Filled composites exhibited a delamination wear pattern and had a significantly lower wear resistance than the amalgams. By comparison, the microfilled composite (Silux Plus) displayed an improved wear resistance that was superior to some amalgams. Dispersalloy had the best wear resistance among the materials selected. CONCLUSION: The wear of Ultrafine Compact-Filled composite and microfilled composite differed and reflect different operative wear mechanisms. For amalgams, the size, shape, and composition of the particles had an effect on the wear resistance of the materials.

Analysis of Variance↗
PubMed2001

Structural transitions in neutral and charged proteins in vacuo.

In vacuo proteins provide a simple laboratory to explore the roles of sequence, temperature, charge state, and initial configuration in protein folding. Moreover, by the very absence of solvent, the study of anhydrous proteins in vacuo will also help us to understand specific environmental effects. From the experimental viewpoint, these systems are now beginning to be characterized at low resolution. Molecular dynamics (MD) simulations, in combination with tools for protein shape analysis, can complement experiments and provide further insights on the folding-unfolding transitions of these proteins. We review some aspects of this issue by using the results from a detailed MD study of hen egg-white lysozyme. For lysozyme ions, unfolding can be triggered by Coulombic repulsion. In neutral lysozyme, unfolding can be induced by centrifugal forces and also by weakening the monomer-monomer interaction. In both cases, the resulting unfolded transients can be used as initial configurations for relaxation dynamics. All trajectories are analyzed in terms of global molecular shape features of the backbone, including its anisometry and chain entanglement complexity. This strategy allows us to quantify separately the degree of polymer collapse and the evolution of large-scale folding features. Using these last two notions, we discuss some basic questions regarding the nature of the accessible paths associated with unfolding from, and refolding into, compact conformers.

Animals↗
PubMed1997

Mechanical stability of compact modules of barnase.

Globular proteins are composed of structural elements such as secondary structures and modules. Modules are compact segments consisting of 10-40 contiguous amino acid residues and are often encoded by exons. Therefore, the view that the modular organization of proteins is a result of exon-shuffling or -fusion is given support. Secondary structures such as alpha-helix and beta-sheet are stabilized by hydrogen bonds and are thus considered to be stable, structural elements of a globular domain. Since module boundaries are often located on alpha-helices or beta-sheets, it is not obvious whether the modules are mechanically stable. We carried out molecular dynamics simulations on modules of barnase, a bacterial RNase from Bacillus amyloliquefaciens, for 1 ns in vacuo and 150 ps in water. Five of six modules (M1, 1-24; M2, 25-52; M3, 53-73; M4, 74-88; M5, 89-98) retained native-like conformations during these simulations. Only the C-terminal module (M6, 99-110) was deformed; it is less compact than the other modules. As the modules are mechanically stable they are suitable as parts combined into proteins. Together with RNase activity of the three isolated modules of barnase, M2, M3 and M6, our study supports the view that modules were indeed original building blocks of proteins.

Bacterial Proteins↗
PubMed1994

Laboratory interproximal access efficacy of two compact manual toothbrushes.

The Colgate Precision Compact, soft texture toothbrush and the Oral-B 35 toothbrush were compared using a laboratory device designed to simulate clinical toothbrushing motions and pressures. The toothbrushing time was sixty seconds for each vertical or horizontal toothbrushing sequence, for each of the three brushing weights tested (250, 500 or 750 g). Interproximal access efficacy (IAE) was determined by measuring the maximum width of the brushing stroke on pressure-sensitive paper placed around simulated anterior or posterior teeth. Twenty-four toothbrushes of each design were evaluated for each toothbrushing motion, tooth shape and toothbrushing weight. Using the vertical toothbrushing motion on anterior teeth, IAE means for the Colgate Precision Compact toothbrush were significantly higher (p < 0.001) than the Oral-B 35 toothbrush at 250 and 750 g of brushing weight. With vertical toothbrushing across posterior-shaped teeth, IAE values for the Colgate Precision Compact toothbrush were significantly higher (p < 0.001) than the Oral-B 35 toothbrush at each of the 250, 500 and 750 g brushing weights tested. With horizontal toothbrushing motions, the Colgate Precision Compact toothbrush had significantly higher (p < 0.001) IAE means, compared to the Oral-B 35 toothbrush, on both anterior and posterior tooth shapes and at each of the brushing weights tested. When all factors tested were combined, the total IAE for the Colgate Precision Compact toothbrush was significantly superior (p < 0.001) to the Oral-B 35 toothbrush.

Analysis of Variance↗
PubMed2006

Generalized RLS approach to the training of neural networks.

Recursive least square (RLS) is an efficient approach to neural network training. However, in the classical RLS algorithm, there is no explicit decay in the energy function. This will lead to an unsatisfactory generalization ability for the trained networks. In this paper, we propose a generalized RLS (GRLS) model which includes a general decay term in the energy function for the training of feedforward neural networks. In particular, four different weight decay functions, namely, the quadratic weight decay, the constant weight decay and the newly proposed multimodal and quartic weight decay are discussed. By using the GRLS approach, not only the generalization ability of the trained networks is significantly improved but more unnecessary weights are pruned to obtain a compact network. Furthermore, the computational complexity of the GRLS remains the same as that of the standard RLS algorithm. The advantages and tradeoffs of using different decay functions are analyzed and then demonstrated with examples. Simulation results show that our approach is able to meet the design goals: improving the generalization ability of the trained network while getting a compact network.

Algorithms↗
PubMed2002

Origins of protein denatured state compactness and hydrophobic clustering in aqueous urea: inferences from nonpolar potentials of mean force.

Free energies of pairwise hydrophobic association are simulated in aqueous solutions of urea at concentrations ranging from 0-8 M. Consistent with the expectation that hydrophobic interactions are weakened by urea, the association of relatively large nonpolar solutes is destabilized by urea. However, the association of two small methane-sized nonpolar solutes in water has the opposite tendency of being slightly strengthened by the addition of urea. Such size effects and the dependence of urea-induced stability changes on the configuration of nonpolar solutes are not predicted by solvent accessible surface area approaches based on energetic parameters derived from bulk-phase solubilities of model compounds. Thus, to understand hydrophobic interactions in proteins, it is not sufficient to rely solely on transfer experiment data that effectively characterize a single nonpolar solute in an aqueous environment but not the solvent-mediated interactions among two or more nonpolar solutes. We find that the m-values for the rate of change of two-methane association free energy with respect to urea concentration is a dramatically nonmonotonic function of the spatial separation between the two methanes, with a distance-dependent profile similar to the corresponding two-methane heat capacity of association in pure water. Our results rationalize the persistence of residual hydrophobic contacts in some proteins at high urea concentrations and explain why the heat capacity signature (DeltaC(P)) of a compact denatured state can be similar to DeltaC(P) values calculated by assuming an open random-coil-like unfolded state.

Calorimetry↗
PubMed1994

A biphasic poroelastic analysis of the flow dependent subcutaneous tissue pressure and compaction due to epidermal loadings: issues in pressure sore.

A layer of skin and subcutaneous tissue on a bony substratum was modeled as a homogeneous layer of biphasic poroelastic material with uniform thickness. The epidermal surface and the bony interface were taken to be impervious. The soft tissue on the bony interface was assumed either fully adhered or completely free to slide on the bone. The cases for surface pressure loadings and displacement controlled indentations were simulated. The resultant biomechanical responses of the layer, including the transient tissue hydrostatic pressure and the tissue compaction, were presented. A new hypothesis is offered to interpret the threshold pressure-time curve for pressure sores in term of the time required for a particular area in the tissue layer to reach a critical compaction for a given level of applied pressure.

Biomechanical Phenomena↗
PubMed1997

Mechanism and computer simulation of a new robot hand for potential use as an artificial hand.

A prosthetic hand is essential to provide rehabilitation for individuals who lose a hand. A prosthetic hand serves two purposes: cosmetic and functional. In this paper, a prototype of the artificial hand with an emphasis on the functionality purpose is presented. A new mechanism, the NTU-Hand (NTU-Hand, patent number 107115, Taiwan, R.O.C.), which has 5 fingers with 17 degrees of freedom, has been designed and fabricated in our laboratory. Due to the special design of the mechanism, the hand has an uncoupled configuration in which each finger and joint are all individually driven. The size of the hand is almost the same as a human hand. All actuators, mechanical parts, and sensors are on the hand. The compact design makes it feasible to adapt the hand to the injured wrist. A computer simulation with three-dimensional graphics was also built to evaluate the manipulative range of the artificial hand. From the results of this simulation, the relationship between the hand and the grasped object in a specific viewpoint can be obtained.

Artificial Limbs↗
PubMed2004

Estimating transmitter release rates and quantal amplitudes in central synapses from postsynaptic current fluctuations.

Several approaches recently introduced to analyze release rates in central synapses advanced our understanding of synaptic neurotransmission, however, leaving many questions still unresolved. In this work we present evidence that a new method recently developed by Sakaba and Neher to study neurotransmission in calyx of Held, a giant glutamatergic synapse, could be also applied for estimating release rate functions and averaged quantal sizes in small central synapses. By means of different simulation approaches applied to reproduce GABAergic neurotransmission in the hippocampus we have shown that possible problems with a spatial voltage clamp which can occur in synaptic connections distributed over a large area of dendritic tree are not crucial for applicability of the method when synapses are compactly distributed or located proximally and when release rates are below 1 ms(-1). In another set of simulations we have also shown that at above mentioned release rates desensitization and/or saturation of postsynaptic GABAA receptors does not prevent accurate estimates of release rate and averaged quantal size. Thus, we conclude that the new approach based on analysis of fluctuations of postsynaptic currents under conditions of stationary release or moderately nonstationary conditions might be applicable to studies of small central synapses.

Animals↗
PubMed2005

Reduced-size polarized basis sets for calculations of molecular electric properties. II. Simulation of the Raman spectra.

The accuracies of the calculated vibrational frequencies and Raman intensities given by two new, highly compact Pol-type basis sets, Z2PolX and Z3PolX, have been determined and compared to the 6-31G(d), PolX, and aug-cc-pVTZ basis sets. Calculation of accurate Raman intensities has previously required large basis sets, but the ZmPolX basis sets are smaller even than PolX, which are the most compact basis sets able to calculate accurate Raman intensities. For the largest compound studied, C5H10O2, Z3PolX required more than an order of magnitude less CPU time than PolX, which has been shown to be 10 times faster than aug-cc-pVTZ. Two sets of test molecules were studied: one was a series of small molecules for which experimental values for absolute Raman activities were available; the second was a series of medium-sized molecules (mainly common organic solvents) where only relative Raman band intensities were available. The accuracies of the Raman intensities given by both of the ZmPolX basis sets were good compared to those of the PolX and aug-cc-pVTZ sets, and much better than the 6-31G(d) values. The errors in even unscaled frequency values <2000 cm(-1) were also acceptable and were slightly lower for Z3PolX than Z2PolX (30 cm(-1) vs. 48 cm(-1)). The combination of good intensity and frequency data meant that for the medium-sized organic molecules there was a close correspondence between the simulated Raman spectra and experimental data, and that the observed bands could easily be assigned on the basis of these calculations. Achieving this level of accuracy in the simulations at modest computational cost should now allow computational methods to be combined with experimental Raman studies much more widely than is currently the case.

Journal Article↗