Heavy metal content of the hair of children in industrial cities

Nuchal screening has the following advantages: Non invasive. First trimester procedure. Results are obtained quickly. 80-90% detection rate for Down's Syndrome.     

Fast detection of venous air embolism in Doppler heart sound usingthe wavelet transform

The introduction of air bubbles into the systemic circulation can result in significant morbidity. Real-time monitoring of continuous heart sound in patients detected by precordial Doppler ultrasound is, thus, vital for early detection of venous air embolism (VAE) during surgery. In this study, the multiscale feature of wavelet transforms (WT's) is exploited to examine the embolic Doppler heart sound (DHS) during intravenous air injections in dogs. As both humans and dogs share similar physiological conditions, the authors' methods and results for dogs are expected to be applicable to humans. The WT of DHS at scale 2 ^j(j=1,2) selectively magnified the power of embolic, but not the normal, heart sound. Statistically, the enhanced embolic power was found to be sensitive (P<0.01 at 0.01 ml of injected air) and correlated significantly (P<0.0005, τ=0.83) with the volume of injected air from 0.01 to 0.10 ml. A fast detection algorithm of O(N) complexity with unit complexity constant for VAE was developed (processing speed=8 ms per heartbeat), which confirmed the feasibility of real-time processing for both humans and dogs     

浅谈碾压混凝土的施工技术

介绍碾压混凝土坝发展的历程，简述了碾压混凝土筑坝技术以及施工工艺、特点、程序、质量控制等相关方面的施工经验。     

Effect of grain boundary phase on the thermal conductivity of aluminium nitride ceramics

AIN with high thermal conductivity was fabricated by pressureless sintering with Y₂O₃ as the sintering aid. The thermal conductivity was observed to increase with sintering time (up to 8 h) at 1810 °C. The distribution of the sintering aid was identified as one of the major factors influencing the thermal conductivity in AIN. Non-uniform distribution of the grain boundary phase was found to be associated with a significant amount of porosity, resulting in the enhancement of phonon scattering and thereby lowering the thermal conductivity.     

Modification of hypoxia-induced injury in cultured rat astrocytes by high levels of glucose

BACKGROUND AND PURPOSE: Preexisting hyperglycemia exacerbates central nervous system injury after transient global and focal cerebral ischemia. Increased anaerobic metabolism with resultant lactic acidosis has been shown to cause the hyperglycemic, neuronal injury. The contribution of astrocytes in producing lactic acidosis under hyperglycemic/ischemic conditions is unclear, whereas the protective role of astrocytes in ischemic-induced neuronal injury has been documented. The ability of astrocytes to maintain energy status and ion homeostasis under hyperglycemic conditions could ultimately reduce neuronal injury. Therefore, we determined the effects of increased glucose concentrations on glucose utilization, lactate production, extracellular pH, and adenosine triphosphate concentrations in hypoxia-treated astrocyte cultures. METHODS: Primary astrocytes were prepared from neonatal rat cerebral cortices. After 35 days in vitro, cultures were incubated with 0-60 mmol/L glucose and subjected to hypoxic conditions at 95% N2/5% CO2 for 24 hours. In addition, under high-glucose conditions (30 mmol/L), astrocytes were exposed to up to 72 hours of hypoxia. Determination of lactate dehydrogenase efflux, adenosine triphosphate concentrations, and extracellular lactate concentrations defined astrocyte status. Equiosmolar levels of mannitol were added in place of high glucose concentrations to distinguish hyperosmotic effect. RESULTS: When physiological concentrations of glucose (7.5 mmol/L) or lower concentrations were used, significant cell damage occurred with 24 hours of hypoxia, as determined by increased efflux of lactate dehydrogenase and loss of cell protein. When higher glucose concentrations (15-60 mmol/L) were used, efflux of lactate dehydrogenase was similar to that observed in normoxic cultures, despite an increased utilization of glucose. Lactate concentrations in the media at low or normal glucose concentrations exceeded normoxic levels, but higher glucose concentrations (15-30 mmol/L) failed to increase lactate levels further. Values of adenosine triphosphate for hypoxic astrocytes treated with high glucose concentrations were significantly higher than those of astrocytes with zero or low glucose levels. In cultures exposed to hypoxia and high glucose levels (30 mmol/L), no cellular injury was observed before 48 hours of hypoxia. Lactate concentrations in the media increased during the first 24 hours of hypoxia and reached steady state. The pH of the media decreased to 6.4 after 24 hours and 5.5 at 48 hours. The latter pH was concomitant with a marked increase in extracellular lactate dehydrogenase activity. Hyperosmotic mannitol failed to protect cultured astrocytes against hypoxia. CONCLUSIONS: Hypoxic injury to mature astrocytes was reduced by the presence of 15-60 mmol/L glucose in the medium during 24-30 hours of hypoxia. Injury occurred when the pH of the medium was < 5.5. This protection was not afforded by the hyperosmotic effect of high glucose concentrations, nor was the hypoxic injury at later time periods with 30 mmol/L glucose mediated solely by lactate accumulation.     

Comparisons of PWM and one-cycle control for power amplifier with multilevel converter

Based on the concepts of cascade multilevel converters and one-cycle technique, comparisons are conducted to characterize the distortion of the pulsewidth modulation (PWM) and one-cycle control methods that were applied to converters. Simulation results for the different control schemes are obtained in PSIM software initially to see the effect of one-cycle control different from that of PWM control. Through the comparisons, the advantages and disadvantages are identified for each method. The one-cycle scheme is better than PWM control in reducing undesirable harmonics and tracing dynamic waveforms. Simulation and experimental results are also provided to verify the conclusions.     

Numerical simulation of aerosol collection in filters with staggered parallel rectangular fibres

 In this paper, filters with rectangular fibres arranged in a staggered and parallel array and placed transverse to the flow are studied numerically. A two- dimensional flow field is obtained by solving Navier–Stokes equations with the control volume method. Periodic boundary conditions are introduced in the calculation. In order to achieve higher accuracy, a second-order upwind scheme is adopted and a fine mesh is arranged near the fibre and the symmetrical plane of the flow field where large gradients in velocity are expected. Particle trajectories are calculated by solving the corresponding Lagrangian equation of motion to obtain the collection efficiency of a single rectangular fibre, in which positions of the approaching particles on the inlet plane of the flow field are randomly distributed according to the Monte-Carlo principle. The simulation considers all the important mechanisms of particle capture including interception, inertial impaction and Brownian motion. Effects of fibre aspect ratio, filter packing density, particulate size and Reynolds number on the collection efficiency are numerically determined. The volumetric packing density ranges from 0.4 to 4% and the particle diameter is from 0.01 μm to 2 μm. Reynolds number based on the height of computational domain varies from 20 to 100 and the aspect ratio is from 0.1 to 10. Simulations with and without Brownian motion are carried out for different Reynolds numbers, packing densities and aspect ratios and the results show that Brownian effects are significant for particles smaller than 1 μm. Received 25 May 2001     

A circular crossed-field amplifier for in situ measurements, studyof reentrant beam effects, and comparison with numerical simulation

A low frequency, injected beam, circular format crossed-field amplifier has been designed and constructed for the purpose of studying electron-radio frequency wave interaction in reentrant devices. The device has been designed to allow in situ diagnostic probe measurements in the space between the anode and sole. The device has been operated in nonreentrant, fully reentrant, and reentrancy controlled configurations. Details of the design and operating parameters are described. Device characteristics are examined with respect to the amount of circulating charge or degree of reentrancy. A large increase in gain has been achieved from nonreentrant to the fully reentrant format. A gain of 7.2 dB has been obtained for the latter whereas only 3.8 dB has been obtained for the former with 30 mA of injected beam current. A maximum gain of 14.4 dB has been achieved for the fully reentrant configuration. Electron beam and noise measurements versus the degree of reentrancy have also been examined. Results from the nonreentrant amplifier performance have been directly compared with the MASK simulation code and good agreement has been obtained. These experiments will provide the basis for more detailed investigations on the effect of reentrancy on CFA operation and will also allow for the development of more accurate computer models of the reentrant system for numerical simulation of CFA operation     

The relationship between preparation convergence and retention of extracoronal retainers

PURPOSE: Although traditional ideal convergence (the sum of taper of the opposite sides) for crown preparation has been arbitrarily set at 4 degrees to 10 degrees, some believe absolute parallelism yields the highest retention. This study examined the relationship between the degree of convergence of a machined metal die and the retention of its casting. MATERIALS AND METHODS: The method used was that of cementing cast metal crowns onto full crown preparations on brass dies with varying convergence angles, and then recording the force required to remove the crowns from the dies in a vertical direction using a Tate-Emery Testing Machine and Load Indicator. RESULTS: It was found that retention (i.e., the force needed to remove the cemented castings from the die in their common long axis) increases from 0 degree convergence to peak between 6 degrees to 12 degrees convergence. It also seems that a critical film thickness does exist for optimum retention, and that film thicknesses smaller than the critical thickness may be responsible for the phenomenon that we have observed and directly related to the convergence angle itself. CONCLUSIONS: There seems to be experimental data supporting the use of traditionally taught convergence. Our study found that convergence angles between 6 degrees and 12 degrees seem to be optimum for tooth crown preparation when one plans to use zinc phosphate cement. Convergence angles of less than 6 degrees may not be desirable even if they can be clinically achieved. The results of our study indicate that a relationship exists between the convergence angle and the critical cement thickness that is necessary to realize the maximum strength properties of zinc phosphate cement.