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排序方式: 共有860条查询结果,搜索用时 15 毫秒
91.
92.
Stefan Franz Nemec Ursula Nemec Peter C. Brugger Dieter Bettelheim Siegfried Rotmensch John M. Graham Jr. David L. Rimoin Daniela Prayer 《黑龙江环境通报》2012,32(3):205-213
Magnetic resonance imaging (MRI) appears to be increasingly used, in addition to standard ultrasonography for the diagnosis of abnormalities in utero. Previous studies have recently drawn attention to the technical refinement of MRI to visualize the fetal bones and muscles. Beyond commonly used T2-weighted MRI, echoplanar, thick-slab T2-weighted and dynamic sequences, and three-dimensional MRI techniques, are about to provide new imaging insights into the normal and the pathological musculoskeletal system of the fetus. This review emphasizes the potential significance of MRI in the visualization of the fetal musculoskeletal system. © 2012 John Wiley & Sons, Ltd. 相似文献
93.
Stefan Trapp 《Environmental Sciences Europe》2000,12(5):246-255
Phytoremediation is the quite novel technique of cleaning polluted sites through the use of plants. Phytoremediation methods are comparatively cheap, are accepted by the public and are ecologically advantageous, compared to common technological approaches. Until today, there have been only a few examples for successful applications. One reason is that the processes occurring are complex, and a full clean up may require many years. Plants have an influence on the water balance of a site, they change redox potential and pH, and stimulate microbial activity of the soil. These indirect influences may accelerate degradation in the root zone or reduce the leaching of compounds into the groundwater. Compounds taken up into plants may be metabolised, accumulate, or volatilise into air. Based on these processes, several phytoremediation methods have been developed: Phytoextraction, rhizofiltration, phytostabilisation, rhizound phytodegradation, pump and tree, land farming, phytovolatilisation, hydraulic control and even more. Already in use are plants (and here mainly willows, poplars and grasses) for the degradation of petroleum products, aromatic hydrocarbons (BTEX), chlorinated solvents, explosives and cyanides. Phytotoxicity and pollutants mass balance have rarely been documented carefully. Often, the success of the projects was not controlled, and only estimates can be made about the applicability and the potential of phytoremediation. This lack of experience about possibilities and limitations seems to be a hindrance for a broader use of these techniques. 相似文献
94.
95.
Uwe Heiser Stefan Norra Doris Stüben Magnus von Wagner 《Umweltwissenschaften und Schadstoff-Forschung》1999,11(1):15-19
Für die Anwendung der sequentiellen Schwermetallextraktion auf Proben, die nur im Milligramm-Bereich erhalten werden k?nnen
(Mikroproben, wie z. B. Staubniederschl?ge), wurde die Methode vonZeien &Brümmer (1989) weiterentwickelt. Der Extraktionsma?stab wurde um den Faktor 1∶100 verringert und die Reproduzierbarkeit des Verfahrens
mit variablen Probenmengen um 20mg und einem Extraktionsvolumen von 500 μl mit Hilfe von Standardb?den nachgewiesen. M?gliche
Einflüsse variabler Extraktionsverh?ltnisse (Verh?ltnis von Probenmenge zu Extraktionsvolumen) sowie eine aufwendige Vorbehandlung
von Staubproben, die nach der Bergerhoff-Methode (VDI 2119/2, 1972) gesammelt wurden, lagen bei diesen Untersuchungen weitgehend
im Variationsbereich des Verfahrens. Die sequentielle Extraktion kann folglich zur Untersuchung der Mobilisierbarkeit von
Schwermetallen auch aus Mikroproben mit variablen Probenmengen eingesetzt werden.
Mit Hilfe dieses Verfahrens wurden die Mobilisierbarkeiten von Zink, Kupfer und Blei der Schluff- und Tonfraktion von Stra?ensedimenten
mit Staubniederschl?gen an zwei urbanen Standorten mit unterschiedlicher, verkehrsbedingter Schwermetallbelastung verglichen.
Diese Untersuchungen werden in Teil 2 (“Sequentielle Schwermetallextraktion von st?dtischen St?uben”) vorgestellt. 相似文献
96.
97.
Brown shrimp (Crangon crangon, L.) are subjected to a huge annual temperature range, and certain thermal conditions during winter have been identified to affect the brown shrimp population. Despite that, little is known about its thermal biology with regard to critically low temperatures. In the present study, we determined the critical thermal minima (CTmin) and the critical lethal minima (CLmin) of male and female brown shrimp of different body sizes in laboratory-based experiments. For the CTmin trials, shrimp were acclimated to 4.0, 9.0, and 14.0 °C and exposed to a cooling rate of ?0.2 °C min?1. In the CLmin trials, brown shrimp were exposed to a cooling rate of ?1.0 °C day?1 without prior thermal acclimation. Acclimation temperature significantly affected the temperature tolerance of brown shrimp (p < 0.001). CTmin among the experimental groups just varied slightly, and no clear effect of gender or body size was observed. In the CLmin trials, brown shrimp even tolerated the coldest temperature of ?1.7 °C that could be established in the experimental setup. However, we observed a negative relationship between temperature and reactivity within the range of 7.0 and 1.0 °C that was determined by means of the flicking response. This relationship suddenly broke between 1.0 and 0.0 °C where an abrupt drop in the reactivity of the shrimp became apparent. The results of this study revealed that brown shrimp hold a wider thermal range as originally reported and that it can cope with subzero temperatures. Implications of low-temperature tolerance are discussed in the context of the brown shrimp’s ecology as well as stock assessment. 相似文献
98.
The structure of turbulence near a tall forest edge: the backward-facing step flow analogy revisited 总被引:5,自引:0,他引:5
Matteo Detto Gabriel G Katul Mario Siqueira Jehn-Yih Juang Paul Stoy 《Ecological applications》2008,18(6):1420-1435
Flow disturbances near tall forest edges are receiving significant attention in diverse disciplines including ecology, forest management, meteorology, and fluid mechanics. Current theories suggest that near a forest edge, when the flow originates from a forest into a large clearing, the flow retains its forest canopy turbulence structure at the exit point. Here, we propose that this framework is not sufficiently general for dense forested edges and suggest that the flow shares several attributes with backward-facing step (BFS) flow. Similar analogies, such as rotor-like circulations, have been proposed by a number of investigators, though the consequences of such circulations on the primary terms in the mean momentum balance at the forest clearing edge have rarely been studied in the field. Using an array of three triaxial sonic anemometers positioned to measure horizontal and vertical gradients of the velocity statistics near a forest edge, we show that the flow structure is more consistent with an intermittent recirculation pattern, rather than a continuous rotor, whose genesis resembles the BFS flow. We also show that the lateral velocity variance, v'2, is the moment that adjusts most slowly with downwind distance as the flow exits from the forest into the clearing. Surprisingly, the longitudinal and vertical velocity variances (u'2 and w'2) at the forest edge were comparable in magnitude to their respective values at the center of a large grass-covered forest clearing, suggesting rapid adjustment at the edge. Discussions on how the forest edge modifies the spectra and co-spectra of momentum fluxes, effective mixing length, and static pressure are also presented. 相似文献
99.
Kyrre Sundseth Jozef M. Pacyna Elisabeth G. Pacyna John Munthe Mohammed Belhaj Stefan Astrom 《Journal of Cleaner Production》2010,18(4):386-394
Anthropogenic processes have increased the exposure of humans and wildlife to toxic methyl mercury (MeHg). Mercury emissions will increase by about 25% between 2005 and 2020, if the present trajectory is maintained. A global assessment of societal damages caused by the ingestion of methyl mercury, based merely on loss of IQ (Intelligence Quotient), suggests that the annual cost will be approximately US$3.7 billion (2005 dollars) in 2020. The corresponding cost of damages resulting from the inhalation of methyl mercury is estimated at US$2.9 million (2005 dollars). Under a higher degree of emission control such as in the case of the Extended Emission Control (EXEC) and the Maximum Feasible Technological Reduction (MFTR) scenarios, total emissions could decrease in the period 2005–2020 by about 50–60%. The corresponding annual benefits in 2020 are estimated to be about US$1.8–2.2 billion (2005 dollars). Large economic benefits can be achieved by reducing global mercury emissions. 相似文献
100.