排序方式: 共有48条查询结果,搜索用时 46 毫秒
21.
Lee JT Liau J Murphy P Schroeder ME Sirlin CB Bydder M 《Magnetic resonance imaging》2012,30(4):572-578
The purpose of this study was to investigate the relationship between liver fat fraction (FF) and diffusion parameters derived from the intravoxel incoherent motion (IVIM) model. Thirty-six subjects with suspected nonalcoholic fatty liver disease underwent diffusion-weighted magnetic resonance imaging with 10 b-values and spoiled gradient recalled echo imaging with six echoes for fat quantification. Correlations were measured between FF, transverse relaxivity (R2), diffusivity (D) and perfusion fraction (f). The primary finding was that no significant correlation was obtained for D vs. FF or f vs. FF. Significant correlations were obtained for D vs. R2 (r=-0.490, P=.002) and f vs. D (r=-0.458, P=.005). The conclusion is that hepatic steatosis does not affect measurement of perfusion or diffusion and therefore is unlikely to confound the use of apparent diffusivity to evaluate hepatic fibrosis. 相似文献
22.
Yang JS Yan JL Hwang CY Chiou SY Liau KL Gavin Tsai HH Lee GH Peng SM 《Journal of the American Chemical Society》2006,128(43):14109-14119
The synthesis, crystal structure, and fluorescence behavior of acetylene-bridged pentiptycene dimer (2), trimer (3), and tetramer (4) are reported. For comparison, a phenylene-pentiptycene-phenylene three-ring system (5) is also investigated. As a result of the unique intrachain pentiptycene-pentiptycene interactions in 3 and 4, their twisted conformers are populated in polar solvents and at low temperatures, and the phenomenon of nonequilibration of excited rotational conformers is observed. Twisting of the pi-conjugated backbones leads to blue-shifted absorption and fluorescence spectra and increased fluorescence quantum yields and lifetimes. The fluorescence spectra of 2-4 undergo small red shifts but large intensity variations in the 0-1 vs 0-0 bands on going from solutions to thin solid films, which can be accounted for by the reabsorption effect. However, the reduction in fluorescence quantum yields for 2-4 in films vs solutions is mainly attributed to efficient interchain exciton migration to nonfluorescent energy traps. In contrast, the behavior of nonequilibration of excited rotamers is not observed for 5 in solutions. Compound 5 forms J-type aggregates through terminal phenylene pi-stackings in the solid state, resulting in a new absorption band at 377 nm and large red shifts of the structured fluorescence spectra. 相似文献
23.
Design of survivable IP-over-optical networks 总被引:2,自引:0,他引:2
Sylvie Borne Eric Gourdin Bernard Liau A. Ridha Mahjoub 《Annals of Operations Research》2006,146(1):41-73
In the past years, telecommunications networks have seen an important evolution with the advances in optical technologies
and the explosive growth of the Internet. Several optical systems allow a very large transport capacity, and data traffic
has dramatically increased. Telecommunications networks are now moving towards a model of high-speed routers interconnected
by intelligent optical core networks. Moreover, there is a general consensus that the control plan of the optical networks
should utilize IP-based protocols for dynamic provisioning and restoration of lightpaths. The interaction of the IP routers
with the optical core networks permits to achieve end-to-end connections, and the lightpaths of the optical networks define
the topology of the IP network. This new infrastructure has to be sufficiently survivable, so that network services can be
restored in the event of a catastrophic failure. In this paper we consider a multilayer survivable network design problem
that may be of practical interest for IP-over-optical neworks. We give an integer programming formulation for this problem
and discuss the associated polytope. We describe some valid inequalities and study when these are facet defining. We discuss
separation algorithms for these inequalities and introduce some reduction operations. We develop a Branch-and-Cut algorithm
based on these results and present extensive computational results. 相似文献
24.
A gram-scale enantiospecific synthesis of the A'B'-subunit of angelmicin B is reported. The synthesis involves a Lewis acid catalyzed contrasteric Diels-Alder reaction and a tandem silyl zincate 1,6-addition/enolate oxidation sequence. 相似文献
25.
Pao-Kuei Liau 《Linear algebra and its applications》2011,435(1):67-76
We characterize generalized Lie derivations on skew elements of prime algebras A with involution, provided that A does not satisfy polynomial identities of low degree. The analogous result for matrix algebras is also described. 相似文献
26.
Joy Liau Masoud Shiehmorteza Olivier M. Girard Claude B. Sirlin Mark Bydder 《Magnetic resonance imaging》2013
This study evaluates the robustness of a magnetic resonance (MR) fat quantification method to changes in R2* caused by an intravenous infusion of superparamagnetic iron oxide (SPIO) contrast agent. The R2* and proton density fat fraction (PDFF) were measured in liver and spine in 14 subjects using an investigational sequence (IDEAL IQ) provided by the MR scanner vendor. Measurements were made before and after SPIO infusion. Results showed SPIO significantly increased R2* in both liver (p = 8.8 × 10− 8) and spine (p =1.3 × 10− 2) but PDFFs were not significantly different in either the liver (p = 5.5 × 10− 1) or the spine (p = 5.6 × 10− 1). These results confirm that the IDEAL IQ method of fat quantification is robust to changes in R2*. 相似文献
27.
Yongho Park Yumi Koga Cindy Su Amanda L. Waterbury Christopher L. Johnny Brian B. Liau 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(16):5441-5445
Cycloheximide (CHX) is an inhibitor of eukaryotic translation elongation that has played an essential role in the study of protein synthesis. Despite its ubiquity, few studies have been directed towards accessing synthetic CHX derivatives, even though such efforts may lead to protein synthesis inhibitors with improved or alternate properties. Described here is the total synthesis of CHX and analogues, and the establishment of structure–activity relationships (SAR) responsible for translation inhibition. The SAR studies aided the design of more potent compounds, one of which irreversibly blocks ribosomal elongation, preserves polysome profiles, and may be a broadly useful tool for investigating protein synthesis. 相似文献
28.
The photochemical behavior of trans-4-(N-arylamino)stilbene (1, aryl = 4-substituted phenyl) in solvents more polar than THF is strongly dependent on the substituent in the N-aryl group. This is attributed to the formation of a twisted intramolecular charge transfer (TICT) state for those with a methoxy (1OM), methoxycarbonyl (1CO), or cyano (1CN) substituent but not for those with a methyl (1Me), hydrogen (1H), chloro (1Cl), or trifluoromethyl (1CF) substituent. On the basis of the ring-bridged model compounds 3-6, the TICT states for 1CN and 1CO result from the twisting of the anilino-benzonitrilo C-N bond, but for 1OM it is from the twisting of the stilbenyl-anilino C-N bond, both of which are distinct from the TICT states previously proposed for N,N-dimethylaminostilbenes. 相似文献
29.
The optical response of an atomic vapor can be coherently manipulated by tunable quantum interference occurring in atomic transition processes. A periodic layered medium whose unit cells consist of a dielectric and an EIT (electromagnetically induced transparency) atomic vapor is designed for light propagation manipulation. Such an EIT‐based periodic layered medium exhibits a flexible frequency‐sensitive optical response, where a very small change in probe frequency can lead to a drastic variation of reflectance and transmittance. As the destructive quantum interference relevant to two‐photon resonance arises in EIT atoms interacting with both control and probe fields, the controllable optical processes that depend sensitively on the external control field will take place in this EIT‐based periodic layered medium. Such a frequency‐sensitive and field‐controlled optical behavior of reflection and transmission in the EIT photonic crystal can be applicable to designs of new devices such as photonic switches, photonic logic gates and photonic transistors, where one laser field can be controlled by the other one, and would have potential applications in the areas of integrated optical circuits and other related techniques (e.g., all‐optical instrumentations). 相似文献
30.