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991.
992.
Functional MRI of the motor cortex using a conventional gradient system: comparison of FLASH and EPI techniques 总被引:1,自引:0,他引:1
Claudia Fellner Jürgen Schlaier Edgar Müller Franz Fellner 《Magnetic resonance imaging》1998,16(10):1171-1180
Gradient echo (GE) and echo planar imaging (EPI) techniques are two different approaches to functional MRI (fMRI). In contrast to GE sequences, the ultra short EPI technique facilitates fMRI experiments with high spatial and temporal resolution or mapping of the whole brain. Although it has become the method of choice for fMRI, EPI is generally restricted to modern scanners with a strong gradient system. The aim of our study was to evaluate the applicability of EPI for fMRI of the motor cortex using a 1.5 T scanner with a conventional gradient system of 10 mT/m (rise time: 1 ms). Therefore, EPI was compared with a well-established high resolution fast low angle shot (FLASH) technique (matrix size 1282). The FLASH technique was applied additionally with a 642 matrix size to exclude influences caused by different spatial resolution, because the EPI sequence was restricted to a 642 matrix size. A total of 35 healthy volunteers were included in this study. The task consisted of clenching and spreading of the right hand. FLASH and EPI techniques were compared regarding geometric distortions as well as qualitative and quantitative fMRI criteria: Mean signal increase between activation and rest and the area of activation were measured within the contralateral, ipsilateral, and supplementary motor cortex. The quality of subtraction images between activation and rest, as well as the quality of z-maps and time course within activated regions of interest, was evaluated visually. EPI revealed significant distortions of the anterior and postior brain margins; lateral distortions (relevant for the motor cortex) could be neglected in most cases. The mean signal increase was significantly higher using FLASH 1282 compared to FLASH 642 and EPI 642, whereas the activated areas proved to be smaller in FLASH 1282 functional images. Both results can be explained by well-documented partial volume effects, caused by different voxel size. Similar quality of the subtraction images and of the time courses in different regions of interest were found for all techniques under investigation, but slightly reduced quality of z-map in FLASH 1282. Within the limits of reproducibility and measurement accuracy, the location of contralateral activation was similar using FLASH and EPI sequences. In conclusion, EPI proved to be a reliable technique for fMRI of the motor cortex, even on an MR scanner with a conventional gradient system. 相似文献
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Lorenzo Mejias Matthias H. Reihmann Silvia Sepulveda‐Boza Helmut Ritter 《Macromolecular bioscience》2002,2(1):24-32
The flavonol quercetin, its glycoside rutin, the flavanol catechin, the isoflavone daidzein and 5,6,4′‐trihydroxyisoflavone can be oxidatively polymerized via a radical polyrecombination mechanism catalyzed by horseradish or soybean peroxidase in a mixture of phosphate buffer (pH 7) and 1,4‐dioxane. The average molecular weights of the polymers were found to be in the range 4 000–12 000 g/mol. Daidzein derivatives with a methoxy (formononetin) or nitro group or a hydrogen atom at C‐4′ could not be polymerized under these conditions. These results and molecular modeling studies suggest that polymerization preferentially occurs via the electron‐rich ring B of the isoflavonoids. The polymers were characterized by means of FT‐IR and UV spectroscopy, and their redox behavior was analyzed using cyclic voltammetry. 相似文献
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Gerald Schwarzenbacher Marion S. Gangl Marian Goriup Martin Winter Matthias Grunert Franz Renz Wolfgang Linert Robert Saf 《Monatshefte für Chemie / Chemical Monthly》2001,7(1):519-529
11-(4H-1,2,4-Triazol-4-yl)-undecylmethacrylate (1), a new ligand for Fe(II) spin-crossover (SCO) complexes containing a polymerizable group, was synthesized and characterized.
The complex [Fe·1
3](BF4)2 (2) was obtained by reaction of 1 with Fe(BF4)2·6H2O (molar ratio 1/Fe(II) = 3/1) in THF. Complex 2 showed a gradual spin-crossover between 80 and 230 K. The methacrylate units in the ligands of complex 2 could be oligomerized radically in solution (initiator: azoisobutyronitrile) without loss of the spin-crossover behaviour. 相似文献
1000.
Franz Auer 《Calculus of Variations and Partial Differential Equations》1999,9(3):269-275
We construct a Riemannian metric on the 3-torus such that no closed surface minimizing area in its homology class is incompressible, i.e., each such surface is of genus
greater than one. In particular, for such a Riemannian metric, the homotopically area minimizing 2-tori constructed in [5]
do not minimize area in their homology classes. The example is easily generalized to arbitrary 3-manifolds. The constructed
Riemannian metric can be chosen to be conformally equivalent to any arbitrary given one.
Received September 4, 1998 / Accepted October 23, 1998 相似文献