排序方式: 共有94条查询结果,搜索用时 15 毫秒
41.
Dynamic contrast-enhanced magnetic resonance imaging (MRI) is a technique used to study and track contrast kinetics in an area of interest in the body over time. Reconstruction of images with high contrast and sharp edges from undersampled data is a challenge. While good results have been reported using a radial acquisition and a spatiotemporal constrained reconstruction (STCR) method, we propose improvements from using spatially adaptive weighting and an additional edge-based constraint. The new method uses intensity gradients from a sliding window reference image to improve the sharpness of edges in the reconstructed image. The method was tested on eight radial cardiac perfusion data sets with 24 rays and compared to the STCR method. The reconstructions showed that the new method, termed edge-enhanced spatiotemporal constrained reconstruction, was able to reconstruct images with sharper edges, and there were a 36%±13.7% increase in contrast-to-noise ratio and a 24%±11% increase in contrast near the edges when compared to STCR. The novelty of this paper is the combination of spatially adaptive weighting for spatial total variation (TV) constraint along with a gradient matching term to improve the sharpness of edges. The edge map from a reference image allows the reconstruction to trade-off between TV and edge enhancement, depending on the spatially varying weighting provided by the edge map. 相似文献
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43.
This paper presents and analyzes a comprehensive model for the design of cellular manufacturing systems (CMS). A recurring theme in research is a piecemeal approach when formulating CMS models. In this paper, the proposed model, to the best of the authors’ knowledge, is the most comprehensive one to date with a more integrated approach to CMS design, where production planning and system reconfiguration decisions are incorporated. Such a CMS model has not been proposed before and it features the presence of alternate process routings, operation sequence, duplicate machines, machine capacity and lot splitting. The developed model is a mixed integer non-linear program. Linearization procedures are proposed to convert it into a linearized mixed integer programming formulation. Computational results are presented by solving some numerical examples, extracted from the existing literature, with the linearized formulation. 相似文献
44.
Video on demand (VoD) is a technology used to provide a number of programs to a number of users on request. In developing a VoD system, a fundamental problem is load balancing, which is further characterized by optimally placing videos to a number of predefined servers and routing the user program requests to available resources. In this paper, an exact solution algorithm is described to solve the video placement and routing problem. The algorithm is based on Lagrangean relaxation and decomposition. The novelty of the approach can be described as the use of integer programs to obtain feasible solutions in the algorithm. Computational experimentation reveals that for randomly generated problems with up to 100 nodes and 250 videos, the use of such integer programs help greatly in obtaining good quality solutions (typically within 5% of the optimal solution), even in the very early iterations of the algorithm. 相似文献
45.
Tolga ?mamo?lu 《European Polymer Journal》2004,40(8):1875-1880
Electrochemical polymerisation of 2-aminofluorene, 2AF, was investigated in ethylalcohol/water mixture (3:2, v:v) in the presence of HClO4 as the supporting electrolyte via constant potential electrolysis, CPE. Prior to CPE, electrochemical behaviour of the monomer was investigated in the same solvent-electrolyte couple utilising cyclic voltammetry, CV. Electrochemical polymerisation of the monomer yielded insoluble, dark bluish-green, conducting polymer deposit on the electrode surface. Characterisation of the polymer film has been carried out using FT-IR spectroscopic technique and thermal behaviour was studied using differential scanning calorimetry, DSC. Spectroelectrochemical, SPEL, behaviour of the polymer on ITO working electrode was studied by recording the electronic absorption spectra, in situ, in monomer-free solution at different potentials and it is found that the film can be reversibly cycled between −0.1 and 1.1 V vs SCE. Paramagnetic behaviour of the polymer was monitored using in situ ESR spectroscopy. The temperature dependence of conductivity supported the Mott's variable range hopping, VRH, mechanism for poly(aminofluorene), PAF. 相似文献
46.
The use of particle formulations with antifouling surface properties attracts increasing interest in several biotechnological applications. Majority of these studies utilize a poly(ethylene glycol) coating to render the corresponding surface nonrecognizable to biological macromolecules. Herein, we report a simple way to prepare novel antifouling colloids composed of oligo(ethylene glycol) backbones via surfactant-free emulsion polymerization. Monodisperse cross-linked poly(ethylene glycol) ethyl ether methacrylate particles were characterized by dynamic light scattering and transmission electron microscopy. The effects of monomer, cross-linker and initiator on particle characteristics were investigated. More importantly, a prominent blockage of bovine serum albumin adsorption was obtained for the poly(ethylene glycol)-based sub-micron (~200 nm) particles when compared with similar-sized poly(methyl methacrylate) counterparts. 相似文献
47.
We consider the stability of parallel server systems under the longest queue first (LQF) rule. We show that when the underlying
graph of a parallel server system is a tree, the standard nominal traffic condition is sufficient for the stability of that
system under LQF when interarrival and service times have general distributions. Then we consider a special parallel server
system, which is known as the X-model, whose underlying graph is not a tree. We provide additional “drift” conditions for
the stability and transience of these queueing systems with exponential interarrival and service times. Drift conditions depend
in general on the stationary distribution of an induced Markov chain that is derived from the underlying queueing system.
We illustrate our results with examples and simulation experiments. We also demonstrate that the stability of the LQF depends
on the tie-breaking rule used and that it can be unstable even under arbitrary low loads. 相似文献
48.
This study compared the influence of various surface treatments on the shear bond strength between resin cement and lithium disilicate glass-ceramics. A series of 120 lithium disilicate ceramic samples were prepared to compare the effect of different surface treatments on the shear strength of a luting cement bonded to two all-ceramic systems. IPS Empress 2 and IPS e.max Press ceramic samples were fabricated according to the manufacturer's instructions. The ceramic samples were divided into the following 6 surface treatment groups for each ceramic system: 1—no treatment (C), 2—airborne-particle abrasion (A), 3—acid etching (E), 4—airborne-particle abrasion + acid etching (AE), 5—Nd:YAG laser (L), 6—Nd:YAG laser + acid etching (LE). Resin cement was then bonded to the treated ceramic surfaces and light polymerized. The shear bond strengths of the specimens were measured using a universal testing machine. Two-way ANOVA and Tukey HSD (α = 0.05) test were used to determine differences in shear bond strength between the groups. The ANOVA revealed significant differences between the treatment groups and ceramic types (p < 0.05). The shear bond strengths of IPS Empress 2 were significantly higher than those of IPS e.max Press. 相似文献
49.
The objective of this paper is to present and to validate a new hybrid coupling (HC) algorithm for modeling of fluid-structure interaction (FSI) in incompressible, viscous flows. The HC algorithm is able to avoid numerical instability issues associated with artificial added mass effects, which are often encountered by standard loosely coupled (LC) and tightly coupled (TC) algorithms, when modeling the FSI response of flexible structures in incompressible flow. The artificial added mass effect is caused by the lag in exchange of interfacial displacements and forces between the fluid and solid solvers in partitioned algorithms. The artificial added mass effect is much more prominent for light/flexible structures moving in water, because the fluid forces are in the same order of magnitude as the solid forces, and because the speed at which numerical errors propagate in an incompressible fluid. The new HC algorithm avoids numerical instability issues associated with artificial added mass effects by embedding Theodorsen’s analytical approximation of the hydroelastic forces in the solution process to obtain better initial estimates of the displacements. Details of the new HC algorithm are presented. Numerical validation studies are shown for the forced pitching response of a steel and a plastic hydrofoil. The results show that the HC algorithm is able to converge faster, and is able to avoid numerical instability issues, compared to standard LC and TC algorithms, when modeling the transient FSI response of a plastic hydrofoil. Although the HC algorithm is only demonstrated for a NACA0009 hydrofoil subject to pure pitching motion, the method can be easily extended to model general 3-D FSI response and stability of complex, flexible structures in turbulent, incompressible, multiphase flows. 相似文献
50.
Plane strain analytical solutions to estimate purely elastic, partially plastic and fully plastic deformation behavior of
rotating functionally graded (FGM) hollow shafts are presented. The modulus of elasticity of the shaft material is assumed
to vary nonlinearly in the radial direction. Tresca’s yield criterion and its associated flow rule are used to formulate three
different plastic regions for an ideal plastic material. By considering different material compositions as well as a wide
range of bore radii, it is demonstrated in this article that both the elastic and the elastoplastic responses of a rotating
FGM hollow shaft are affected significantly by the material nonhomogeneity. 相似文献