Comparison of fast field-cycling magnetic resonance imaging methods and future perspectives

ABSTRACT

Fast field-cycling (FFC) nuclear magnetic resonance relaxometry is a well-established method to determine the relaxation rates as a function of magnetic field strength. This so-called nuclear magnetic relaxation dispersion gives insight into the underlying molecular dynamics of a wide range of complex systems and has gained interest especially in the characterisation of biological tissues and diseases. The combination of FFC techniques with magnetic resonance imaging (MRI) offers a high potential for new types of image contrast more specific to pathological molecular dynamics. This article reviews the progress in FFC-MRI over the last decade and gives an overview of the hardware systems currently in operation. We discuss limitations and error correction strategies specific to FFC-MRI such as field stability and homogeneity, signal-to-noise ratio, eddy currents and acquisition time. We also report potential applications with impact in biology and medicine. Finally, we discuss the challenges and future applications in transferring the underlying molecular dynamics into novel types of image contrast by exploiting the dispersive properties of biological tissue or MRI contrast agents.     

Recent developments in the statistical processing of textual data

Statisticians are accustomed to processing numerical, ordinal or nominal data. In many circumstances, such as socio-economic, epidemiologic sample surveys and documentary data bases, this data is juxtaposed with textual data (for example, responses to open questions in surveys). This article presents a series of language-independent procedures based upon applying multivariate techniques (such as correspondence analysis and clustering) to sets of generalized lexical profiles. The generalized lexical profile of a text is a vector whose components are the frequencies of each word (graphical form) or ‘repeated segment’ (sequence of words appearing with a significant frequency in the text). The processing of such large (and often sparse) vectors and matrices requires special algorithms. The main outputs are the following: (1) printouts of the characteristic words and characteristic responses for each category of respondent (these categories are generally derived from available nominal variables); (2) graphical displays of the proximities between words or segments and categories of respondents; (3) when analysing a combination of several texts: graphical displays of proximities between words or segments and each text, or between words or segments and groupings of texts. The systematic use of ‘repeated segments’ provides a valuable help in interpreting the results from a semantic point of view.     

Transposition des oxirannes-ethanols par l'intermediaire d'alcoxyetains. Influence de la configuration de l'oxiranne

The transposition of oxirane-ethanols, through alkoxytin compounds, into oxetane-2-methanols and/or oxolan-3-ols (tetrahydrofuran-3-ols) is dependent upon the oxirane configuration. Cis configuration is more suitable for the formation of the smallest ring. Steric hindrance is not sufficient enough to explain the results.     

Phosphine-catalyzed enantioselective [3+2] annulations of 2,3-butadienoates with imines

The first systematic screening of chiral phosphines in the cycloaddition reaction between 2,3-butadienoates and arylimines has led to the identification of fairly efficient catalysts. 2-Aryl-3-pyrrolines have been obtained with enantiomeric excesses up to 64%. In one instance, the enantiomeric excess could be increased to 91% ee by combining the enantioselective cyclization reaction with a crystallization step.     

Layer by layer buildup of polysaccharide films: physical chemistry and cellular adhesion aspects

The formation ofpolysaccharide films based on the alternate deposition of chitosan (CHI) and hyaluronan (HA) was investigated by several techniques. The multilayer buildup takes place in two stages: during the first stage, the surface is covered by isolated islets that grow and coalesce as the construction goes on. After several deposition steps, a continuous film is formed and the second stage of the buildup process takes place. The whole process is characterized by an exponential increase of the mass and thickness of the film with the number of deposition steps. This exponential growth mechanism is related to the ability of the polycation to diffuse "in" and "out" of the whole film at each deposition step. Using confocal laser microscopy and fluorescently labeled CHI, we show that such a diffusion behavior, already observed with poly(L-lysine) as a polycation, is also found with CHI, a polycation presenting a large persistence length. We also analyze the effect of the molecular weight (MW) of the diffusing polyelectrolyte (CHI) on the buildup process and observe a faster growth for low MW chitosan. The influence of the salt concentration during buildup is also investigated. Whereas the CHI/HA films grow rapidly at high salt concentration (0.15 M NaCl) with the formation of a uniform film after only a few deposition steps, it is very difficult to build the film at 10(-4) M NaCl. In this latter case, the deposited mass increases linearly with the number of deposition steps and the first deposition stage, where the surface is covered by islets, lasts at least up to 50 bilayer deposition steps. However, even at these low salt concentrations and in the islet configuration, CHI chains seem to diffuse in and out of the CHI/HA complexes. The linear mass increase of the film with the number of deposition steps despite the CHI diffusion is explained by a partial redissolution of the CHI/HA complexes forming the film during different steps of the buildup process. Finally, the uniform films built at high salt concentrations were also found to be chondrocyte resistant and, more interestingly, bacterial resistant. Therefore, the (CHI/HA) films may be used as an antimicrobial coating.