A Realistic Computer-Simulated Brain Phantom for Evaluation of PET Charactenstics

To evaluate accurately the imaging characteristics of positron emission tomography (PET), a realistic computer-simulated brain phantom was developed. A cross-sectional slice from a human cadaver brain was chosen for its combination of gray matter, white matter, and cerebrospinal fluid (CSF) regions. The slice was photographed and digitized into a gray-level image with a video digitizer, boundary edges were located around cerebral structures in the digitized image, and each structural region was assigned a uniform pixel value dependent on both the cerebral parameter (e.g., blood flow, oxygen uptake, metabolic rate) under investigation and the type of structure (gray matter, white matter, CSF). Line integrals through the regions were generated at various angular and transverse positions according to specific physical characteristics (such as detector line-spread function) of a tomographic scanner configuration to create a set of simulated but realistic projection measurements. The set of projection measurements can be processed with any standard reconstruction program to create a tomographic image to reveal the effects of various PET characteristics. Investigations with this computer-simulated brain phantom have demonstrated its usefulness for examining the interrelations among neuroanatomical structure volume, tomographic spatial resolution, partial volume effect, and nonlinear parameter estimation. Transportability of the simulated phantom and the procedure to other medical imaging environments is described, and limitations of this simulation procedure are discussed.     

AN EXPERIMENTAL-NUMERICAL METHOD FOR MEASURING CRACK PROPAGATING VELOCITIES UNDER STRESS WAVE LOADING

An experimental-numerical method for measuring dynamic crack propagating velocities under stress wave loading is established in this paper. The experiments of the three-point bend specimen are done on the improved Hopkinson bar. Deflection of loading point, dynamic load and instantaneous crack length are measured, then crack propagating velocities are calculated. Experiments on 40Cr steel show that the results given by this method have a good agreement with that obtained by the resistance fracture gage method. Therefore this method is feasible for measuring crack propagating velocities under high loading rate and will have wide application.     

储砂斗中铸造用砂的流动

叙述了从储砂斗中放砂的困难。借助于垂直流动的现代砂斗激活装置(Modeern bin activator)有助于解决这类难题。     

FORMATION OF MANGANESE SILICIDE THIN FILMS BY SOLID PHASE REACTION

1. IntroductionThe transition metal silicides have attraeted much attention because of their potentialapplicatiOIl in very large scale integration (VLSI) de.ice[1]. Near-noble metal silicides canbe used as Schottky al1d ohn1ic col1tacts in silicon-based d…     

钛合金铸造技术的发展(述评)

在许多情况下,由于大量地使用钛合金,使航空、航海和耐蚀系统的寿命延长、性能改善、但是,近几年来,钛合金的应用量却只有适度增加,其主要原因在于基本材料费用昂贵、加工件造价高。解决问题的途径是采用“成品形状”(net—shape)成形技术,减少了材料的浪费,降低了成本,钛合金铸造技术作为一种很有吸引力的“成品形状”成形途径。近来更加引人注目。本文从各个角度探讨了两种主要铸造方法——捣实石墨模铸造和熔模铸造,其中包括规格大小、制模、产品性能及其改进途径,铸件与铸锭加工材(1M)的机械性能比较表明,不仅两者的成本相当,而且很多性能诸如断裂韧性和疲劳裂纹扩展速度也很相近。这是由于铸态组织和使用热等静压具有使其内部缩孔完全愈合的能力所决定的。从而暗示铸造系数普遍地用于钛铸件是不适宜的,应认真考虑消除之。当前钛合金铸造技术的开发研究工作又有新的进展,作者对未来的发展动向提出了自己的见解。     

焊机动特性对熔滴短路过渡过程的影响

本文对MIG—MAG气体保护焊机的动特性作了分析,并提出了用电弧模拟装置作为负载的新测试方法。作者还利用汉诺威电弧分析仪对直流电感、焊机外特性以及电弧电压对短路过渡过程,特别是短路频率、短路时间的影响,进行了观察和讨论。     

第六章 未硫化胶料的流变行为(二)

基本概念 D.几种特殊类型的形变 1.剪切流动也许最重要的形变类型是剪切层流,所以我们必须研究高聚物流体这类流动。这些运动来自简单剪切流动(见图2):     

Random-effects regression analysis of correlated grouped-time survival data

Random-effects regression modelling is proposed for analysis of correlated grouped-time survival data. Two analysis approaches are considered. The first treats survival time as an ordinal outcome, which is either right-censored or not. The second approach treats survival time as a set of dichotomous indicators of whether the event occurred for time periods up to the period of the event or censor. For either approach both proportional hazards and proportional odds versions of the random-effects model are developed, while partial proportional hazards and odds generalizations are described for the latter approach. For estimation, a full-information maximum marginal likelihood solution is implemented using numerical quadrature to integrate over the distribution of multiple random effects. The quadrature solution allows some flexibility in the choice of distributions for the random effects; both normal and rectangular distributions are considered in this article. An analysis of a dataset where students are clustered within schools is used to illustrate features of random-effects analysis of clustered grouped-time survival data.     

低变质程度煤液化效果的同位素监测

通过在1升的带搅拌的高压釜中使煤的重量浓度从0依次增加到2~(7％)的一系列实验,研究了Vesta次烟煤对Athabasca抽提后沥青(切除了低于350℃馏份)共处理的效果。将Vesta煤与沥青(3.94‰)之间存在的~(13)C/~(12)C比例的差别用作天然同位素示踪,以监测煤和沥青进入每一种合成液体馏份中的比率。通过比较同位素测得的产品中来自煤的比率表明:煤进入到馏出物、甲苯可溶物、甲苯不溶物的比例随掺入的煤量的增加而逐渐增加。煤的重量百分比从15％增加到2~(6％),结果甲苯不溶物增加了三倍。相应地,沥青进入到合成液体中的碳的比例也随煤的浓度而变化。比较沥青加氢处理与共处理中同位素测得的产品量表明:煤的浓度从4％增加到8％,馏出物的量增加14—17％,而甲苯可溶物相应地减少32—49％。煤的浓度为15—26％时,共处理中单独由沥青提供的馏出物无明显增加,而沥青提供的甲苯不溶物的碳的比例从0.9％增加到5.0％。这些结果表明:由5—10％的煤组成的煤-沥青混合物共处理,能够促进沥青馏出物的生成。