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1.
INTRODUCTION Image quality in nuclear medicine tomography is critically dependent on the activity undergone by the patient. The optimal activity is the smallest amount of activity which preserves diagnostic accuracy. Nev- ertheless, the optimum depends on the gamma camera used for imaging, the size of the patient and the im- aging application (Mattsson et al., 1998). To optimize, one has to study how diagnostic accuracy depends on the activity undergone for each particular study, taking i…  相似文献   

2.
By applying the wavefront coding technique to an optical system, the depth of focus can be greatly increased. Several complicated methods, such as Fisher Information based method, have already been taken to optimize for the best pupil phase mask in ideal condition. Here one simple point spread function （PSF） based method with only the standard deviation method used to evaluate the PSF stability over the depth of focus is taken to optimize for the best coefficients of pupil phase mask in practical optical systems. Results of imaging simulations for optical systems with and without pupil phase mask are presented, and the sharpness of image is calculated for comparison. The optimized results showed better and much more stable imaging quality over the original system without changing the position of the image plane.  相似文献   

3.
The lipase activities in the three main Chinese castor bean varieties [ZiBi 05 （ZB 5）,TongBi 06 （TB 6）,JinBi 02 （JB 2）] were investigated.Two lipases were found in extracts from the endosperms of all the three varieties.One lipase is located in the lipid bodies,showing higher activity at pH 4.5 and exhibiting the highest activity at day 1 of germination.The other lipase is located in the particulates,showing higher activity at pH 9.5 and particularly active at day 4 of germination.The lipid bodies from ZB 5 show the highest lipase activity at day 1 and pH 4.5 among the three varieties during the 8 d of germination.The acid lipase located in the lipid bodies of ZB 5 of germination 1 d was purified to homogeneity by DEAE-sepharose CL-6B and butyl-sepharose CL-4B chromatography after isolation,delipidation and solubilization,with a purification factor of 16.1 and overall activity recovery of 17.7%.The characterizations （such as optimum pH,pH stability,optimum temperature,thermal stability,effect of metal ions,organic solvents stability,substrate specificity） of the purified acid lipase were conducted in detail.A combination of the characterized properties may make this enzyme a useful biocatalyst for industrial application.  相似文献   

4.
Variation in the limit dextrinase activity of barley malt, and the relationships between limit dextrinase activity and malt quality parameters were investigated using eight cultivars grown at seven diverse locations in China for two successive years. Limit dextrinase activity varied with genotype and location, with the levels ranging from 0.245 U/g to 0.980 U/g. The results showed that the variation in limit dextrinase activity was more attributable to the environment （location and year） than to the genotype. The response of limit dextrinase activity to the environment differed markedly among cultivars, and was reflected by large difference in coefficient of variation of cultivars across diverse locations. Regression analysis showed that limit dextrinase activity was negatively correlated with malt viscosity （r=-0.52, P〈0.01）, positively correlated with Kolbach index （r=0.38, P〈0.01） and malt extract （r=0.30, P〈0.05）, but had no significant correlation with malt protein content and diastatic power.  相似文献   

5.
The formation of brominated trihalomethanes （THMs-Br） which is proved more carcinogenic than their chlorinated analogues reported was very different at various water qualities. This study was performed to assess the effects of water quality parameters （bromide concentration, pH value and ammonia concentration）, chlorination conditions （chlorine dose, reaction time） and ratios of Br^-/DOC and Br^-/Cl2 consumption on the formation and distribution of THMs-Br in chlorination. The results showed good correlation between the bromine incorporation factor （BIF） n（Br） and Br^-/Cl2 consumption ratio. The formation of total THM （TTHM） was found to＇ decrease with increasing ammonia concentration but to increase with bromide concentration and pH value. The n（Br） trends were significantly affected by the presence of bromide concentration. The effects on the molar yields of THMs were more strongly influenced by bromide concentration and dissolved organic carbon （DOC） concentration than pH value and natural organic matter （NOM） source. High Br^-/DOC and Br^-/Cl2 favor the formation of THMs-Br over chlorinated THMs （THMs-Cl）. The experimental data including the main parameters such as bromide, DOC, ammonia, pH and reaction time were used for developing the predictive model for THMs-Br.  相似文献   

6.
This paper presents a quadratic programming method for optimal multi-degree reduction of Bézier curves with G1-continuity. The L2 and l2 measures of distances between the two curves are used as the objective functions. The two additional parameters, available from the coincidence of the oriented tangents, are constrained to be positive so as to satisfy the solvability condition. Finally, degree reduction is changed to solve a quadratic problem of two parameters with linear constraints. Applica  相似文献   

7.
This paper presents a quadratic programming method for optimal multi-degree reduction of Bézier curves with G1-continuity. The L2 and l2 measures of distances between the two curves are used as the objective functions. The two additional parameters, available from the coincidence of the oriented tangents, are constrained to be positive so as to satisfy the solvability condition. Finally, degree reduction is changed to solve a quadratic problem of two parameters with linear constraints. Applica- tions of degree reduction of Bézier curves with their parameterizations close to arc-length parameterizations are also discussed.  相似文献   

8.
Fused deposition modelling （FDM） is one of rapid prototyping （RP） technologies which uses an additive fabrication approach.Each commercially available FDM model has different types of process parameters for different applications.Some of the desired parts require excellent surface finish as well as good tolerance.The most common parameters requiring setup are the raster angle,tool path,slice thickness,build orientation,and deposition speed.The purpose of this paper is to discuss the process parameters of FDM Prodigy Plus （Stratasys,Inc.,Eden Prairie,MN,USA）.Various selected parameters were tested and the optimum condition was proposed.The quality of the parts produced was accessed in terms of dimensional accuracy and surface finish.The optimum parameters obtained were then applied in the fabrication of the master pattern prior to silicone rubber moulding （SRM）.These parameters would reduce the post processing time.The dimensional accuracy and surface roughness were analyzed using coordinate measuring machine （CMM） and surface roughness tester,respectively.Based on this study,the recommended parameters will improve the quality of the FDM parts produced in terms of dimensional accuracy and surface roughness for the application of SRM.  相似文献   

9.
Objective： To study the effect of halo substitution on disubstituted aryl semicarbazones on the anticonvulsant potential and model the activity based on quantum mechanics. Methods： A series of twenty-six compounds of N^4-（4-bromo-3-methylphenyl） semicarbazones were synthesized and evaluated for the anticonvulsant activity in the maximal electroshock seizure （MES） and subcutaneous pentylenetetrazole （scPTZ） seizure threshold tests. Some potential compounds were also tested in the subcutaneous strychnine （scSTY） and subcutaneous picrotoxin （scPIC） seizure threshold tests. The synthesized compounds were tested for behavioral impairment and CNS （central nervous system） depression in mice. Quantum mechanical modelling was carried out on these compounds to gain understanding on the structural features essential for activity. Results： Some compounds possessed broad spectrum anticonvulsant activity as indicated by their effect in pentylenetetrazole, strychnine, picrotoxin and maximal electroshock seizures models in resemblance to other aryl semicarbazone derivatives reported earlier. The higher the difference in HOMO （highest occupied molecular orbital） and LUMO （lowest unoccupied molecular orbital） energy levels was, the greater was the activity profile. Conclusion： The pharmacophoric requirements for compounds to exhibit anticonvulsant activity that includes one aryl unit in proximity to a hydrogen donor-acceptor domain and an electron donor have been justified with the molecular orbital surface analysis of the synthesized compounds.  相似文献   

10.
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11.
Hierarchical Bayesian method for estimating the failure probabilityp i under DOOF by taking the quasi-Beta distributionB(p i−1, 1, 1,b) as the prior distribution is proposed in this paper. The weighted Least Squares Estimate method was used to obtain the formula for computing reliability distribution parameters and estimating the reliability characteristic values under DOOF. Taking one type of aerospace electrical connector as an example, the correctness of the above method through statistical analysis of electrical connector accelerated life test data was verified. Project (No. 59975081) supported by the National Natural Science Foundation of China  相似文献   

12.
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13.
The line persistence of a graph G, Pt （ G ） is the minimum number of lines which must be removed to increase the diameter of G. In Ref. [7] （J. Shanghai Univ., 2003,7（4）：352-357）, we gave a characterization of graphs of diameter five with ρ1 （ G ）≥2. In this paper we will show that each of the 8 special graphs Xi （ i = 1,2,3,4,5,6,7,8） listed in condition （2） of Theorem 1 in Ref. [7] can not be deleted. Therefore the results we obtained in Ref. [7] can not in general be improved.  相似文献   

14.
The seed method is used for solving multiple linear systems A^(i) x^(i) = b^(i) for l≤ i≤ s , where the coefficient matrix A^(i) and the right-hand side b^(i) are different in general. It is known that the CG method is an effective method for symmetric coefficient matrices A^(i) . In this paper, the FOM method is employed to solve multiple linear systems when coefficient matrices are non-symmetric matrices. One of the systems is selected as the seed system which generates a Krylov subspace, then the residuals of other systems are projected onto the generated Krylov subspace to get the approximate solutions for the unsolved ones. The whole process is repeated until all the systems are solved.  相似文献   

15.
In this cross-national study, observed process quality in preschool classrooms was compared across five countries—Austria (n = 37), Germany (n=103), Portugal (n=88), Spain (n=80), and the U.S.A. (n=390). Process quality was assessed using the Early Childhood Environment Rating Scale (ECERS) and the Caregiver Interaction Scale (CIS). Both instruments had comparable properties when used in the different countries. A MANOVA found overall ECERS differences between countries, with Austria and Germany scoring higher than Spain, and Austria higher than the U.S.A. CIS Sensitivity scale differences showed Austria and Spain with higher scores than the U.S.A. Tests of homogeneity of variance showed U.S.A. with the highest standard deviations. A discriminant analysis, using countries as groups and ECERS items as criterion variables, revealed differences between countries on two discriminant functions: Personalized Care and Availability and Use of Space and Play Materials. Results are discussed in terms of the early childhood traditions and infrastructures found in the five countries.  相似文献   

16.
The superconductor sample had periodic distribution of pinned region (with length ofL p) and unpinned region (with length ofL−L p) along the driving force direction. Numerical study on the influence of the distribution of pinning centers on pinning of the two-dimensional vortex system showed that the critical depinning forceF c, beyond which the vortex system begins to depin, increases with increase ofL p, indicating that the homogeneity of pinning centers helps to enhance the critical electric current of a superconductor. We found that the critical depinning forceF c depends logarithmically onL/L p. Project supported by the Ministry of Science and Technology of China (NKBRSF-G19990646) and Zhejiang Proviace Foundation (199031).  相似文献   

17.
In this paper, we consider some polynomial (α,β)-metrics, and discuss the sufficient and necessary conditions for a Finsler metric in the form F=α a1β a2β2/α a4β4/α3 to be projectively flat, where ai (i=1,2,4) are constants with a1≠0, α is a Riemannian metric and β is a 1-form. By analyzing the geodesic coefficients and the divisibility of certain polynomials, we obtain that there are only five projectively flat cases for metrics of this type. This gives a classification for such kind of Finsler metrics.  相似文献   

18.
In this paper we study the scaling behavior of nucleotide cluster in 11 chromosomes of Encephalitozoon cuniculi Genome. The statistical distribution of nucleotide clusters for 11 chromosomes is characterized by the scaling behavior of P（S） ∝ e^-αs＇ where S represents nucleotide cluster size. The cluster-size distribution P（S1＋S2） with the total size of sequential C-G cluster and A-T cluster S1＋S2 were also studied. P（S1＋S2） follows exponential decay. There does not exist the case of large C-G cluster following large A-T cluster or large A-T cluster following large C-G cluster. We also discuss the relatively random walk length function L（n） and the local compositional complexity of nucleotide sequences based on a new model. These investigations may provide some insight into nucleotide cluster of DNA sequence.  相似文献   

19.
This article reports first-principles band structure calculations forRMn6Sn6 (R=Tb, Dy). The calculation uses the linear muffin-tin orbitals (LMTO) method in the atomic-sphere-approximation (ASA), and yields results showing that both TbMn6Sn6 and DyMn6Sn6 are ferrimagnetic compounds with antiparallel aligned moments ofR and Mn atoms. In this research the 4f states ofR atoms are treated as localized states, i.e., the hybridization of 4f states with other valence electrons is neglected. The moments of Mn in both compounds were determined to be 2.43 μB and 2.38 μB, respectively. The considerably small additional moments for Mn from the spin-orbit coupling indicates that the spin-orbital coupling is not dominated for Mn atoms. The total moments of Tb and Dy atoms are 10.28 μB and 11.20 μB. All the calculation findings accorded well with experimental results. Project supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry of China.  相似文献   

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