A national audit of mammography equipment performance, image quality and dose has been conducted in Croatia. Film-processing parameters, optical density (OD), average glandular dose (AGD) to the standard breast, viewing conditions and image quality were examined using TOR(MAM) test object. Average film gradient ranged from 2.6 to 3.7, with a mean of 3.1. Tube voltage used for imaging of the standard 45 mm polymethylmethacrylate phantom ranged from 24 to 34 kV, and OD ranged from 0.75 to 1.94 with a mean of 1.26. AGD to the standard breast ranged from 0.4 to 2.3 mGy with a mean of 1.1 mGy. Besides clinical conditions, the authors have imaged the standard phantom in the referent conditions with 28 kV and OD as close as possible to 1.5. Then, AGD ranged from 0.5 to 2.6 mGy with a mean of 1.3 mGy. Image viewing conditions were generally unsatisfying with ambient light up to 500 lx and most of the viewing boxes with luminance between 1000 and 2000 cd per m(2). TOR(MAM) scoring of images taken in clinical and referent conditions was done by local radiologists in local image viewing conditions and by the referent radiologist in good image viewing conditions. Importance of OD and image viewing conditions for diagnostic information were analysed. The survey showed that the main problem in Croatia is the lack of written quality assurance/quality control (QA/QC) procedures. Consequently, equipment performance, image quality and dose are unstable and activities to improve image quality or to reduce the dose are not evidence-based. This survey also had an educational purpose, introducing in Croatia the QC based on European Commission Guidelines. 相似文献
The characteristics of cell growth, lactic acid production, amino acid release and consumption by single-strain cultures of lactic acid bacteria (isolated from kefir grains), and by a multiple-strain kefir starter prepared from them, were studied. The change in the levels of free amino acids was followed throughout the kefir process: single-strain kefir bacteria and the kefir starter (Lactococcus lactis C15-1%+Lactobacillus helveticus MP12-3%+(Streptococcus thermophilus T15+Lactobacillus bulgaricus HP1 = 1:1)-3%) were cultivated in pasteurized (92 degrees C for 20 min) cow's milk (3% fat content) at 28 degrees C for 5 h (the kefir starter reached pH 4.7) and subsequently grown at 20 degrees C for 16 h; storage was at 4 degrees C for 168 h. The strain L. helveticus MP12 was unrivaled with respect to free amino acid production (53.38 mg (100 g)(-1)) and cell growth (17.8 x 10(8) CFU ml(-1)); however, it manifested the lowest acidification activity. L. bulgaricus HP1 released approximately 3.7 times less amino acids, nearly 5 times lower cell growth, and produced about 1.2 times more lactic acid. S. thermophilus T15 demonstrated dramatically complex amino acid necessities for growth and metabolism. With L. lactis C15, the highest levels of growth and lactic acid synthesis were recorded (18.3 x 10(8) CFU ml(-1) and 7.8 g l(-1) lactic acid at the 21st hour), and as for free amino acid production, it approximated L. bulgaricus HP1 (17.03 mg (100 g)(-1) maximum concentration). In the L. lactis C15 culture, the amino acids were used more actively throughout the first exponential growth phase (by the 10th hour) than during the second growth phase. The unique properties of the L. helveticus MP12 strain to produce amino acids were employed to create a symbiotic bioconsortium kefir culture, which, under conditions of kefir formation, enhanced lactic acid production and shortened the time required to reach pH 4.7; intensified cell growth activity, resulting in a respective 90- and 60-fold increase in the concentration of lactobacilli and cocci in the mixed culture compared to individual cultures; and accumulated free amino acids in the final kefir with higher total concentrations (56.88 mg (100 g)(-1)) and an individual concentration of essential amino acids (1.5 times) greater than that of yogurt. 相似文献
Safe use of welded structures is dependent on fracture mechanics properties of welded joints. In present research, high strength low alloyed HSLA steel in a quenched and tempered condition, corresponding to the grade HT 80, was used. The fluxo cored arc welding process (FCAW), with CO2 as shielding gas, was used and two different tubular wires were selected. The aim of this paper is to analyse fracture behaviour of undermatched welded joints, and also to determine relevant parameters which contribute to higher critical values of fracture toughness. Towards this end three differently undermatched welded joints were analysed using results of testing the composite notched specimens with through thickness crack front positioned partly in the weld metal, partly in heat affected zone (HAZ) and partly in base material (BM).The presence of different microstructures along the pre‐crack fatigue front has an important effect on the critical crack tip opening displacement (CTOD). This value is the relevant parameter for safe service of welded structure. In the case of specimens with through thickness notch partly in the weld metal, partly in the heat affected zone and partly in the base material, i.e. using the composite notched specimen, fracture behaviour strongly depends on a partition of ductile base material, size and distribution of mismatching factor along vicinity of crack front. If local brittle zones occur in the process zone, ductile base metal can not prevent pop‐in instability, but it can reduce it to an insignificant level while the fracture toughness parameter is higher and the weakest link concept can not be applied. 相似文献
Lead-free Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) ceramic powders were synthesized using the sol–gel method. The ceramics thickness was reduced to achieve high-energy storage and large electrocaloric effect in bulk ceramics. Dielectric, ferroelectric, energy storage, and electrocaloric properties were investigated for BCZT ceramic with 400 µm. Here, pure crystalline structure and homogenous microstructure were identified by XRD analysis and SEM measurements, respectively. The dielectric measurements revealed a maximum dielectric constant associated with ferroelectric–paraelectric phase transition. The maximum of \(\varepsilon^{\prime}_{{\text{r}}}\) was 17841, around 352 K. Furthermore, the BCZT ceramic exhibited improved energy storage and electrocaloric properties. A high recoverable energy density Wrec of 0.24 J/cm3 and a total energy density Wtotal of 0.27 J/cm3 with an efficiency coefficient of?~?88% at 423 K under an electric field of 55 kV/cm were obtained. Besides, The maximum value of ΔT?=?2.32 K, the electrocaloric responsivity ζ?=?0.42 K mm/kV, the refrigeration capacity RC?=?4.59 J/kg and the coefficient of performance COP?=?12.38 were achieved around 384 K under 55 kV/cm. The total energy density Wtotal and the temperature change ΔT were also calculated by exploiting the Landau–Ginzburg–Devonshire (LGD) theory. The theoretical results matched the experimental findings. These results suggest that the synthesized BCZT ceramic with reduced thickness could be a promising candidate for energy storage and electrocaloric applications.
Let W be a simply connected region in , analytic in W and γ a positively oriented Jordan curve in W that does not pass through any zero of f. We present an algorithm for computing all the zeros of f that lie in the interior of γ. It proceeds by evaluating certain integrals along γ numerically and is based on the theory
of formal orthogonal polynomials. The algorithm requires only f and not its first derivative f'. We have found that it gives accurate approximations for the zeros. Moreover, it is self-starting in the sense that it does
not require initial approximations. The algorithm works for simple zeros as well as multiple zeros, although it is unable
to compute the multiplicity of a zero explicitly. Numerical examples illustrate the effectiveness of our approach.
Received: November 2, 1998; revised March 30, 1999 相似文献
This paper reports on the experience gained in solving large combinatorial problems by using the Outer-Approximation /Equality-Relaxation (OA/ER) algorithm in two multilevel MINLP strategies. The first one is a Linked Multilevel Hierarchical Strategy (LMHS) and the second one is a Reduced Integer Space (RIS) strategy. Both strategies are used to decompose the original MINLP problem in a hierarchical manner into several MINLP levels that are much easier to solve than the original one. While the first LMHS strategy can be applied to problems that contain only simple mixed-integer constraints, e.g. standard dimensions, the RIS strategy can be used to solve problems with more complex mixed-integer constraints, e.g. different design equations for alternative units. The LMHS strategy is rigorous and can solve convex problems to global optimal solutions. On the other hand, when the RIS strategy is applied for the solution of large combinatorial problems, the global optimality cannot be guaranteed, but very good solutions can be obtained. The synthesis problem of a roller steel gate for a hydroelectric power station with 19623 binary variables is presented to illustrate the LMHS strategy, whilst the synthesis problem of a heat exchanger network comprising different types of exchangers with 1782 binary variables is presented to present the RIS strategy. 相似文献
The generalized coordinates partitioning is a well-known procedure that can be applied in the framework of a numerical integration
of the DAE systems. However, although the procedure proves to be a very useful tool, it is known that an optimization algorithm
for the coordinates partitioning is needed to obtain the best performance. In the paper, the optimized partitioning of the
generalized coordinates is revisited in the context of a numerical forward dynamics of the holonomic and non-holonomic multibody
systems. After a short presentation of the geometric background of the optimized coordinates partitioning, a structure of
the optimally partitioned vectors is discussed on the basis of a gradient analysis of the separate constraint sub-manifolds
at the configuration and the velocity levels when holonomic and non-holonomic constraints are present in the system. It is
shown that, for holonomic systems, the vectors of optimally partitioned coordinates have the same structure for the generalized
positions and velocities. On the contrary, in the case of non-holonomic systems, the optimally partitioned coordinates generally
differ at the configuration and the velocity levels. The conclusions of the paper are illustrated within the framework of
the presented numerical example. 相似文献
During numerical forward dynamics of constrained multibody systems, a numerical violation of system kinematical constraints
is the important issue that has to be properly treated. In this paper, the stabilized time-integration procedure, whose constraint
stabilization step is based on the projection of integration results to underlying constraint manifold via post-integration
correction of the selected coordinates is discussed. A selection of the coordinates is based on the optimization algorithm
for coordinates partitioning. After discussing geometric background of the optimization algorithm, new formulae for optimized
partitioning of the generalized coordinates are derived. Beside in the framework of the proposed stabilization algorithm,
the new formulae can be used for other integration applications where coordinates partitioning is needed. Holonomic and non-holonomic
systems are analyzed and optimal partitioning at the position and velocity level are considered further. By comparing the
proposed stabilization method to other projective algorithms reported in the literature, the geometric and stabilization issues
of the method are addressed. A numerical example that illustrates application of the method to constraint violation stabilization
of non-holonomic multibody system is reported.
An erratum to this article can be found at 相似文献