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111.
Crossing fibers, diffractions and nonhomogeneous magnetic field: correction of artifacts by bipolar gradient pulses 总被引:1,自引:0,他引:1
In recent years, diffusion tensor imaging (DTI) and its variants have been used to describe fiber orientations and q-space diffusion MR was proposed as a means to obtain structural information on a micron scale. Therefore, there is an increasing need for complex phantoms with predictable microcharacteristics to challenge different indices extracted from the different diffusion MR techniques used. The present study examines the effect of diffusion pulse sequence on the signal decay and diffraction patterns observed in q-space diffusion MR performed on micron-scale phantoms of different geometries and homogeneities. We evaluated the effect of the pulse gradient stimulated-echo, the longitudinal eddy current delay (LED) and the bipolar LED (BPLED) pulse sequences. Interestingly, in the less homogeneous samples, the expected diffraction patterns were observed only when diffusion was measured with the BPLED sequence. We demonstrated the correction ability of bipolar diffusion gradients and showed that more accurate physical parameters are obtained when such a diffusion gradient scheme is used. These results suggest that bipolar gradient pulses may result in more accurate data if incorporated into conventional diffusion-weighted imaging and DTI. 相似文献
112.
Recently, in [12] a very general class oftruncated Newton methods has been proposed for solving large scale unconstrained optimization problems. In this work we present the results of an extensive numericalexperience obtained by different algorithms which belong to the preceding class. This numerical study, besides investigating which arethe best algorithmic choices of the proposed approach, clarifies some significant points which underlies every truncated Newton based algorithm. 相似文献
113.
罗仕乐 《纯粹数学与应用数学》2008,24(1):54-59
本文利用熊庆来的型函数研究平面上零级K-拟亚纯映射的值分布,给出了零级K-拟亚纯映射在平面上存在充满圆序列及Borel方向的条件. 相似文献
114.
Christopher Barner‐Kowollik Sébastien Perrier 《Journal of polymer science. Part A, Polymer chemistry》2008,46(17):5715-5723
We examine the reversible addition fragmentation chain transfer (RAFT) process with regard to its potential and limits in future industrial applications (including those conducted on a larger scale) as well as materials science. The outlook for the RAFT process is bright: Its unrivaled inherent process simplicity coupled with a wide tolerance to monomer classes and functionalities makes it a prime candidate for the use in large reactors. At the same time, it allows for ready access to complex macromolecular architectures of variable shape and size. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5715–5723, 2008 相似文献
115.
In this paper, by using Ahlfors' theory of covering surfaces, we prove that for an algebroid function w(z) satisfying , there exists at least one Nevanlinna direction dealing with multiple values. 相似文献
116.
本文提出预测复合材料中裂纹方向的比应变能密度准则,并将Tsai-Hill与Norris准则扩展来预测复合材料中的开裂方向.用这三个准则预测了具有各种不同纤维方向的单向纤维复合材料的裂纹扩展方向,预测结果与现有的比正应力准则和应变能密度准则进行了对比. 相似文献
117.
The effects of the direction of transition dipole moment on polarization-dependent phase-conjugate (PC) reflectivity by degenerate four-wave mixing in a saturable-dye-doped film are theoretically investigated. On the basis of a four-energy-level model for the saturable dyes, the pump-intensity dependence of the PC reflectivity is calculated for the two cases of a probe wave being polarized either parallel or perpendicular to pump waves, when the transition dipole moment associated with excited-state absorption tilts from that associated with ground-state one. For the parallel polarization, in the case of two transition dipole moments having different directions, a dip appears in the PC reflectivity curve near a resonant frequency on account of the characteristics of nonlinear absorption, whereas it disappears when the frequency of light is off resonance due to the influence of nonlinear refractive index. 相似文献
118.
119.
相对性原理及其对自然界定律的协变性要求--机械能守恒定律协变性疑难的解答 总被引:1,自引:1,他引:0
阐明狭义相对性原理的准确含义,批出它并不要求自然界每条定律都单独协变,但要求每条定律至少属于一个协变集合,给出最小协变集的求法。表明机械能守恒定律满足狭义相对性原理及其对自然界定律的协变性要求,还指出一切社会科学定律也都满足狭义相对性原理。 相似文献
120.
张洪申 《数学物理学报(A辑)》2009,29(6):1642-1650
设T(r, w)满足:limr →∞lg T(r, w)/lg r =0,limr→∞lg T(r, w)/lg lg r =+ ∞, 则一定存在一条方向arg z=θ0 ,使对任意给定N>0,任意复数 a (至多有2 v个例外值), 有∑i1/(lg|zi(a;?(θ0,δ))|)N=∞.设T(r, w)满足:limr→∞T(r, w)/lgKr =+∞,limr→+∞lg T(r, w) /lg lg r =M, 则一定存在一条方向argz=θ0 ,对任意复数a (至多有2 v个例外值),有∑i1/lg|zi(a;?(θ0,δ))|)σ=∞(σ = M-2或σ = M-2-ε.即使在亚纯函数,这些奇异方向也未见研究. 相似文献