Experimental and numerical investigation of the 3D SPECT photon detection kernel for non-uniform attenuating media

Experimental tests for non-uniform attenuating media are performed to validate theoretical expressions for the photon detection kernel, obtained from a recently proposed analytical theory of photon propagation and detection for SPECT. The theoretical multi-dimensional integral expressions for the photon detection kernel, which are computed numerically, describe the probability that a photon emitted from a given source voxel will trigger detection of a photon at a particular projection pixel. The experiments were performed using a cylindrical water-filled phantom with large cylindrical air-filled inserts to simulate inhomogeneity of the medium. A point-like, a short thin cylindrical and a large cylindrical radiation source of 99Tcm were placed at various positions within the phantom. The values numerically calculated from the theoretical kernel expression are in very good agreement with the experimentally measured data. The significance of Compton-scattered photons in planar image formation is discussed and highlighted by these results. Using both experimental measurements and the calculated values obtained from the theory, the kernel's size is investigated. This is done by determining the square N x N pixel neighbourhood of the gamma camera that must be connected to a particular radiation source voxel to account for a specific fraction of all counts recorded at all camera pixels. It is shown that the kernel's size is primarily dependent upon the source position and the properties of the attenuating medium through Compton scattering events, with 3D depth-dependent collimator resolution playing an important but secondary role, at least for imaging situations involving parallel hole collimation. By considering small point-like sources within a non-uniform elliptical phantom, approximating the human thorax, it is demonstrated that on average a 12 cm x 12 cm area of the camera plane is required to collect 85% of the total count recorded. This is a significantly larger connectivity than the 3 cm x 3 cm area required if scattering contributions are ignored and only the 3D depth-dependent collimator resolution is considered.     

Individual differences as a crucible in theory construction.

Argues that nomothetic theories should be formulated in a way that will allow an immediate individual-differences test. 3 guidelines are discussed as means of developing such theories: (a) The theory must assume at least 2 intervening processes, and these processes must interact in some way to relate the independent variables to the dependent variable. (b) Any assumed process must be tied to at least 1 independent variable. (c) In choosing theoretical processes, if at all possible at least 1 must be chosen which has some possibility of yielding an individual-differences interpretation. It is suggested that such an approach would identify the process variables as a fallout from nomothetic theory construction if the nomothetic theories are dealing with fundamental processes of behavior. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Modeling the Dynamics of Heavy Equipment Management Practices and Downtime in Large Highway Contractors

Machine downtime is invariably perceived as one of the most critical problems faced by highway contractors. Attempts to reduce downtime often result in failure due to the dynamic behaviors between equipment management practices and downtime. This paper is thus intended to highlight the dynamics of heavy equipment management practices and downtime in large highway contractors and utilizes them as a framework in constructing a simulation model using a system dynamics approach. Face-to-face interviews were conducted with equipment managers from five different large highway contractors in Thailand. The finding reveals that, to be successful in alleviating downtime, contractors must view their practices on equipment management as an integration of multiple feedback processes, which are interrelated and interdependent with downtime. Based on various validation tests, the simulation model is deemed appropriate in representing the equipment management system as related to downtime of large highway contractors. The research is of value in facilitating better understanding on the dynamics of equipment management practices and downtime as well as their interdependency.     

Toward a unified conceptual framework of psychotherapy.

The increase in theoretical redundancy and the fact that all theories of psychotherapy possess weaknesses as well as strengths suggest that the time may be ripe for the emergence of a unified theory of psychotherapy. Three questions are addressed that are prerequisite to the development of a unified conceptual framework in psychotherapy: (1) What is the current state of the field of psychotherapy integration? (2) How do we move toward a unified theory of psychotherapy? and (3) What must a unified psychotherapy theory include? Unification is merely the next step in the evolution of psychotherapy theory, which is expected to oscillate between stages of unification and differentiation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Contextualist Research: Iterating between Methods While Following an Empirically Grounded Approach

An alternative approach to research is described that has been developed through a succession of significant construction management research projects. The approach follows the principles of iterative grounded theory, whereby researchers iterate between alternative theoretical frameworks and emergent empirical data. Of particular importance is an orientation toward mixing methods, thereby overcoming the existing tendency to dichotomize quantitative and qualitative approaches. The approach is positioned against the existing contested literature on grounded theory, and the possibility of engaging with empirical data in a “theory free” manner is discounted. Emphasis instead is given to the way in which researchers must be theoretically sensitive as a result of being steeped in relevant literatures. Knowledge of existing literatures therefore shapes the initial research design; but emergent empirical findings cause fresh theoretical perspectives to be mobilized. The advocated approach is further aligned with notions of knowledge coproduction and the underlying principles of contextualist research. It is this unique combination of ideas which characterizes the paper's contribution to the research methodology literature within the field of construction management. Examples are provided and consideration is given to the extent to which the emergent findings are generalizable beyond the specific context from which they are derived.     

Reframing HIV prevention for gay men in the United States.

The HIV epidemic in the United States has affected at least two generations of gay men. Despite numerous efforts to intervene on this public health crisis, HIV infections continue to escalate, especially among young men. This condition is compounded by an ever-growing number of gay men who are aging and living with HIV. We must enact an innovative and proactive vision and framework for HIV prevention that moves us beyond the undertakings rooted in social–cognitive paradigms that have informed this work for the past 25 years. A new framework for HIV prevention must give voice to gay men; must consider the totality of their lives; must delineate the underlying logic, which directs their relation to sex and HIV; and must concurrently respect their diverse life experiences. This approach should be rooted in a biopsychosocial paradigm, should be informed by both theory and practice, and should be directed by three theoretical lenses—a theory of syndemics, developmental theories, and contextual understandings of HIV disease. Taken together, these elements are a call to action for research and practice psychologists who are working to improve the lives of gay men. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Nonspecific factors and theoretical choice.

Discusses the importance of nonspecific factors as major forces in therapeutic outcome. Their relation to the adoption of a theoretical framework for psychotherapy is examined. The position is taken that a theoretical choice must be made to bring some nonspecific factors into play and to most responsibly remain aware of the value biases and messages inherent in one's therapeutic techniques. Suggestions are made about the teaching of psychological theory, and possibilities for research on values and theory in psychotherapy are proposed. (39 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Computational modeling and elementary process analysis in visual word recognition.

An attempt is made to isolate the assumptions that make a connectionist approach to visual word recognition distinctive. These include the commitment to distributed representations, the claim that there is no distinction between lexical and nonlexical systems in the naming task, and the claim that it is possible to map from orthography to meaning without using localized representations. It is argued that merely demonstrating that a network model can perform these tasks is not sufficient and that a detailed theory of how the network performs its tasks must accompany the simulation, because a simulation is not equivalent to an explanation. It is argued that further progress requires detailed modeling and experimental study of the elementary processes assumed to be involved in networks and that it is premature to dismiss alternative models of lexical access such as serial search models. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

多孔介质细观流动理论研究进展

首先,从理论分析、实验研究和数值模型三个方面概述了当前多孔介质细观流动的研究现状,重点围绕纳微孔隙中流体流动界面作用与细观力学特性关系及表征、细观?宏观网络仿真模拟、细观尺度流体（油/水、气/水）流动细观动力学机制及数学模型等关键问题展开论述。在此基础上介绍了当前细观流动界面作用与细观力学特性研究情况,明确了细观尺度流体非线性流动机理,构建了反映微观力作用下细观尺度流动的数学模型,形成了网络仿真模拟方法。将为非常规油气开发过程中揭示影响流动细观成因,进一步阐明不同条件下的动用机理,确定高效开发方法提供指导,同时促进渗流力学学科的发展,具有重要的理论和现实意义。      

Ostwald ripening during liquid phase sintering—Effect of volume fraction on coarsening kinetics

Phase coarsening, also termed Ostwald ripening, is generally thought to be a slow, diffusion-controlled process which occurs subsequent to phase separation under extremely small under- or over-saturation levels. The theory due to Lifshitz, Slyozov, and Wagner (LSW), which predicts the coarsening kinetics and the particle distribution function, are applicable todilute systems only, in which particle-particle interactions are unimportant. Most liquid phase sintered systems, however, have large enough volume fractions of the dispersed phase to violate the essential assumptions of LSW theory. Recent progress will be described on simulating Ostwald ripening in randomly dispersed, high volume fraction systems. A fast algorithm for solving the multiparticle diffusion problem (MDP) will be described, permitting simulation of coarsening dynamics by cyclic time-stepping and updating the diffusion solution for large random particle arrays. The rate constants, controlling the growth of the average particle, and the particle distribution functions were obtained by numerical simulations up to a volume fraction of 0.55. A new statistical mean field theory has now been developed which reproduces the MDP simulation data accurately, and finally makes clear how the linear mean-field approximations employed by LSW theory must be modified to describe real systems. The predictions of the mean field are found to compare favorably with experimental measurements made over a wide range of volume fraction solid of the kinetics of Ostwald ripening in liquid phase sintered Fe-Cu alloys. The new theory provides a comprehensive approach to understanding microstructural coarsening in liquid phase sintered systems. This paper is based on a presentation delivered at the symposium “Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds” held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME.     

An ecological theory of expertise effects in memory recall.

Previous research has shown a significant correlation between domain expertise and memory recall performance after a very brief exposure time. Despite the large number of such studies, several findings in the literature have no satisfactory theoretical explanation. A novel theory based on an ecological approach is proposed to explain these results. This constraint attunement hypothesis provides a framework for identifying and representing the various levels of goal-relevant constraint in a domain. The theory predicts that there will be a memory expertise advantage in cases in which experts are attuned to the goal-relevant constraints in the material to be recalled and that the more constraint available, the greater the expertise advantage can be. The theory explains a number of diverse empirical findings in the literature in a coherent, unique, and parsimonious fashion and suggests a number of promising issues for future research. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Energy transfer in photosynthesis

A theoretical model is presented to account for the physical mechanism of energy transfer from antenna molecules to the reaction centers in photosynthesis. The energy transfer is described by a generalized transport equation or "master equation". The solution of this equation for the proposed model gives a relationship between the antennae interaction energy and the transfer rate. The results are shown to be in agreement with inter-antenna transfer rates calculated from experimental fluorescence lifetimes. Previous theories were based either on the F?rster mechanism, which is valid for very small interaction energies, or an exciton model valid for very large interactions, but experimental results seemed to indicate that the actual situation was intermediate between these two. The F?rster theory and the exciton model are limiting cases of the master equation.     

冲击式预剥离液压特性分析

以大极板自动剥锌机预剥离液压系统为研究对象,分析了冲击式预剥离设备剥片过程的液压特性,建立了预剥离过程的仿真模型,并通过仿真分析,得到了蓄能器油压、冲击活塞速度及冲击功与活塞行程的关系,为液压系统的优化提供了理论支撑,为自动剥锌机预剥离设备的设计提供了理论参考和技术支持。     

"Individual heuristics and the dynamics of cooperation in large groups": Additional results using analytical methods.

The authors elaborate on the simulation study "Individual Heuristics and Dynamics of Cooperation in Large Groups" by D. M. Messick and W. B. G. Liebrand (see record 1995-17369-001). Messick and Liebrand demonstrated that, in problematic social situations in large groups, stable levels of cooperation can be established even if actors are assumed to be guided by simple behavioral strategies. Among other things, they provided a theory underlying their simulation system. The present article elaborates on Messick and Liebrand's theory and provides a model that predicts the results of simulation runs for a set of simulation systems with more general properties. This enabled deduction of additional, sometimes counterintuitive results. For example, it is shown that it need not be the case that the equilibrium level of cooperation increases with an increasing payoff for mutual cooperation and that it is likely that the equilibrium level of cooperation increases with an increasing payoff for mutual defection. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Phosphorus Removal of High Phosphorus Iron Ore byGasBased Reduction and Melt Separation

A large deposit of high phosphorus iron ore in China contains an average of 1.2% phosphorus and 50% iron and it has not been utilized. In current work, a novel process to remove phosphorus of the ore has been proposed. The novel process has been demonstrated theoretically and experimentally. The theoretical work (numerical simulation) was carried out with HSC chemistry package and a mathematical model developed using the coexistence theory of slag structure. Gas-based reduction and melt separation experiments were then designed and conducted. Simulation results shows that that all iron compounds in the ore could be reduced to metallic iron using CO/ H2 under temperature above 1000K and the yield of iron is more than 90% under either atmosphere; P can not be reduced and exists as Ca3(PO4)2; in the melt separation process, iron metallization ratio, melting temperature and CaO-adding ratio affect the phosphorus partition between slag and molten metal and CaO-adding ratio is the most distinguished parameter. Results of gas-based reduction agreed well with the simulation except for iron metallization ratio being less than predicted. This difference is mainly attributed to kinetic condition. Results of melt separation experiment show most P is left in the slag sample and some P in the metal sample exists as slag inclusion..     

闭式高速压力机组合机身的载荷谱分析

基于力学的基本理论对带有平衡机构的曲柄滑块机构进行运动学和动力学分析.通过近似简化曲轴的力学模型,获得了曲轴与上横梁轴承座之间相互作用的载荷谱理论公式.利用Matlab对空载和打桩2种工况下组合机身的载荷谱数据进行仿真,分析这2种工况下载荷谱曲线的规律.仿真结果表明,在空载情形下,曲轴和上横梁轴承座之间相互作用的理论与仿真载荷谱曲线呈现周期性的正弦波形式;在打桩工况下,曲轴和上横梁轴承座之间相互作用的理论和仿真载荷谱曲线呈现周期性的脉冲波形式.该结果能够为有限元法分析闭式高速压力机组合机身动力学性能过程中载荷的确定提供理论基础.     

Theory of Seepage into an Auger Hole in a Confined Aquifer

A steady-state theory is presented for predicting flow into an auger hole partially penetrating a homogeneous and anisotropic confined aquifer that is underlain by an impermeable layer. The developed equations can be directly applied (i.e., without resorting to a coordinate transformation) to translate the rate of rise of the water in a pumped auger hole into directional conductivities of soil. The study shows that the conductivity values calculated by neglecting the confining pressure of an artesian aquifer (i.e., by applying the existing unconfined auger-hole seepage theories to experimental auger data obtained from a confined aquifer) may lead to serious error; hence, the confining head of an aquifer must be considered while the conductivity values are computed. Further, the distance of the outer layer also plays an important role in determining the flow to an auger hole penetrating a confined aquifer, and this parameter must therefore be included in the theoretical analysis of the problem. The validity of the proposed theory is checked by comparing a few results obtained from the theory with corresponding results obtained from numerical and analytical works. The developed theory is an addition to already existing auger-hole seepage theories for water-table aquifers; together with the available theories, the proposed solution is expected to cover the most commonly encountered auger hole experimental flow situations in the field.     

The phenomenological theory of martensite crystallography: Interrelationships

The phenomenological theory of martensite crystallography is based on the observation that the habit plane is an invariant plane in which all lines in it are neither distorted nor rotated by the displacement vector of the invariant plane strain. The structural change is effected by a Bain deformation which follows intuitively from the orientation relationship and through which atomic interchanges do not occur. A lattice invariant deformation is also incorporated to insure that the martensite-parent interface is semicoherent and glissile. These factors and the orientation relationship as well are uniquely interrelated within the theoretical framework and must be so for any transformation alleged to follow the theory. This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

氮在液态铁合金中溶解度的影响因素

由于高氮钢特殊的优异综合性能，使其在许多领域得到了应用。本文从压力、合金成分、温度三方面研究了影响氮在液态铁合金中溶解度的热力学因素。发现在设计了合适的合金钢成分后，增大系统的氮气分压可以明显地提高氮的溶解度，同时提出了利用铬当量法预测铁合金中氮溶解度的理论方法，从而为我国高氮钢的大规模生产提供理论依据。     

龙首矿充填体悬臂结构的稳定性分析与评价

安全是矿山生产中极其重要的一环,充填体悬臂结构是龙首矿进路回采中十分常见的安全隐患形式,由于自重和深部地应力的作用,容易发生崩塌破坏现象。根据龙首矿实际情况,其充填体悬臂结构失稳破坏主要有崩落式和拉裂—坠落式2种形式。基于悬臂梁理论,建立进路人工假顶悬臂和充填围岩悬臂力学计算模型,通过理论计算悬臂极限长度、数值模拟验证和稳定性系数K并进行综合评价,以保障矿山安全生产。结果表明：充填体悬臂的稳定性受顶部裂缝深度比和实际长度的影响,其极限长度与数值模拟结果基本一致。同时,稳定性系数Ｋ能够有效统计不同裂缝条件下充填体悬臂状态,判断充填体悬臂顶部裂缝深度比情况。因此,悬臂极限长度和稳定性系数Ｋ能够有效评价龙首矿充填体悬臂的稳定性,实际应用简单,计算结果与现场调查情况基本吻合,对于潜在的充填体悬臂结构安全隐患能起到及时预警的效果。