CHARACTERISING THE EOS SLOT-SCANNING SYSTEM WITH THE EFFECTIVE DETECTIVE QUANTUM EFFICIENCY.

NlmCategory="UNASSIGNED">As opposed to the standard detective quantum efficiency (DQE), effective DQE (eDQE) is a figure of merit that allows comparing the performances of imaging systems in the presence of scatter rejection devices. The geometry of the EOS™ slot-scanning system is such that the detector is self-collimated and rejects scattered radiation. In this study, the EOS system was characterised using the eDQE in imaging conditions similar to those used in clinical practice: with phantoms of different widths placed in the X-ray beam, for various incident air kerma and tube voltages corresponding to the phantom thickness. Scatter fractions in EOS images were extremely low, around 2 % for all configurations. Maximum eDQE values spanned 9-14.8 % for a large range of air kerma at the detector plane from 0.01 to 1.34 µGy. These figures were obtained with non-optimised EOS setting but still over-performed most of the maximum eDQEs recently assessed for various computed radiology and digital radiology systems with antiscatter grids.

Para além das fronteiras da química: relações entre filosofia, psicologia e ensino de química

The concepts of molecule and of molecular structure are so central to understand chemical phenomena that seems to be no doubt about the uniqueness of its meanings. Nevertheless, the idea that the world exhibits a multiform structure and that to different spheres of the world correspond different ways of knowing (Berger & Luckmann, 1967) has received support from different areas of scientific inquiry. Bachelard (1940, 1982) showed that a single philosophical doctrine is not enough to describe all the different ways of thinking when we try to explain a single concept. Wooley's question about the possibility of deducing the concept of molecular structure from quantum theory (Wooley, 1978) strengthened the feasibility of thinking the concept of molecule as a profile that encompasses different meanings. Moreover, research on students' learning of scientific concepts have brought to light that students use several ideas to explain scientific and everyday phenomena which are different from those learned in formal schooling. These ideas are not extinguished or replaced by scientific concepts, despite the efforts to do so in science classes. The common sense and scientific ways of understanding and talking about reality seems to be complementary in the same sense of the Bohr's complementarity (Halliday & Martin, 1993). So, we have to include in our profile of the concept of molecule not only scientific but also common sense zones. Drawing from Bachelard's notion of epistemological profile, from the history of science and from the research on children's ideas in science, we have developed the idea of a conceptual profile and used it to analyse basic scientific concepts, such as the concepts of matter and physical states of matter (Mortimer, 1995) and to investigate new ways to teach them. In the present paper, we will discuss the zones that might constitute a conceptual profile of molecule. The need of complementary views to account for the molecular structure in different contexts bring important issues for understanding and teaching chemistry, which will be discussed further in the article.

Chemistry and light - part 2: light and energy

The conversion of solar energy into more useful forms of energy, such as chemical fuels or electricity, is one of the central problems facing modern science. Progress in photochemistry and chemical synthesis has led to a point where light energy conversion by means of artificial molecular devices can be rationally attempted. In this article, a general approach towards this challenging goal is presented.

Research and training in medicinal chemistry in south and central american countries and sub-saharan Africa

With the proposal to search for universal cooperation in the field of Medicinal Chemistry, the IUPAC group has elaborated a line of work divided into two phases: a- An Awareness of the true situation of Medicinal Chemistry in the different geographic areas of the world; b- A proposal of actions as to achieve more effective cooperation. This first report presents and discusses the actual situation in South and Central America as well as in sub-Saharan Africa.

The chemistry of peroxynitrite, a biological toxin

Oxyradicals play a tole in several diseases. While for several decades the hydroxyl radical - produced via the Fenton reaction - has been considered the species that initiates oxyradical damage, new findings suggest that much of this damage can be ascribed to peroxynitrite, O=NOO-, formed from the reaction of the superoxide anion with nitrogen monoxide near activated macrophages. The rate constant for the reaction of this reaction has been investigated by flash photolysis and was found to be significantly higher than previously described in the literature, 1.9 x 10(10) M-1s-1. Studies of the isomerization to nitrate resulted in the discovery of a complex between peroxynitrite and its protonated form with a stability constant of 1 x 10(4) M-1. Some of the harmful reaction of peroxynitrous acid have been ascribed to the hydroxyl radical as a product of homolysis of the O-O bond during the conversion to nitrate. Kinetics of the isomerization reaction as a function of pressure show that the activation volume is only +1.5+1.0 ml mol-1, which is inconsistent with homolysis. Instead, an intermediate, possibly a distorted trans-isomer of O=NOOH could be responsible for the harmful reactions of peroxynitrite.