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151.
Condition number as a measure of noise performance of diffusion tensor data acquisition schemes with MRI 总被引:10,自引:0,他引:10
Skare S Hedehus M Moseley ME Li TQ 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2000,147(2):340-352
Diffusion tensor mapping with MRI can noninvasively track neural connectivity and has great potential for neural scientific research and clinical applications. For each diffusion tensor imaging (DTI) data acquisition scheme, the diffusion tensor is related to the measured apparent diffusion coefficients (ADC) by a transformation matrix. With theoretical analysis we demonstrate that the noise performance of a DTI scheme is dependent on the condition number of the transformation matrix. To test the theoretical framework, we compared the noise performances of different DTI schemes using Monte-Carlo computer simulations and experimental DTI measurements. Both the simulation and the experimental results confirmed that the noise performances of different DTI schemes are significantly correlated with the condition number of the associated transformation matrices. We therefore applied numerical algorithms to optimize a DTI scheme by minimizing the condition number, hence improving the robustness to experimental noise. In the determination of anisotropic diffusion tensors with different orientations, MRI data acquisitions using a single optimum b value based on the mean diffusivity can produce ADC maps with regional differences in noise level. This will give rise to rotational variances of eigenvalues and anisotropy when diffusion tensor mapping is performed using a DTI scheme with a limited number of diffusion-weighting gradient directions. To reduce this type of artifact, a DTI scheme with not only a small condition number but also a large number of evenly distributed diffusion-weighting gradients in 3D is preferable. 相似文献
152.
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155.
研究了对称双势阱玻色-爱因斯坦凝聚体系(BEC)存在均匀噪声或高斯噪声时的自俘获现象.结果发现噪声的存在破坏了自俘获现象的临界行为特征,使得原来约瑟夫森振荡和自俘获之间的临界点变成了一个过渡区域,而且噪声强度越大,这个过渡区域展得越宽.同时发现,对于确定的相互作用强度,当噪声强度增大到一定程度时,相平面会出现混乱,如果这时固定噪声强度增大相互作用强度,相平面中的轨道会重新出现.对纯量子系统加噪声后,自俘获同样不存在临界值,而是存在一个临界区域,且随噪声的增强临界区域会展宽.与平均场近似情况下不同的是,纯量子情况下噪声促进自俘获的产生,且噪声越强自俘获越明显.
关键词:
玻色-爱因斯坦凝聚
自俘获
双势阱
噪声 相似文献
156.
157.
E. Scalas U. Garibaldi S. Donadio 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,60(2):271-272
This erratum corrects a mistake in reference [E. Scalas, U. Garibaldi, S. Donadio, Eur. Phys. J. B 53, 267 (2006)].
In that paper,
we needed an aperiodic version of the BDY game, but, in formula (1),
we incorrectly presented
a periodic transition matrix of period 2 in the special case of g = 2 agents.
Here, we present the right aperiodic version. 相似文献
158.
B. Dybiec L. Schimansky-Geier 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,57(3):313-320
We study the stationary probability density of a Brownian particle in a potential with a single-well subject to the purely
additive thermal and dichotomous noise sources. We find situations where bimodality of stationary densities emerges due to
presence of dichotomous noise. The solutions are constructed using stochastic dynamics (Langevin equation) or by discretization
of the corresponding Fokker-Planck equations. We find that in models with both noises being additive the potential has to
grow faster than |x| in order to obtain bimodality. For potentials ∝|x| stationary solutions are always of the double exponential
form. 相似文献
159.
A.-J. Li C.-L. Wang L. Wang J.-H. Wu J.-Y. Gao 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,42(3):373-379
We investigate the spontaneously generated coherence (SGC) in a Doppler broadened four-level atomic system driven by two coherent
fields. We plot
the spontaneous emission spectra with different parameters and discuss how
the initial atomic conditions and parameters of both fields change the
number of peaks and dark lines of spontaneous emission spectra. Furthermore, we also show how the spontaneous emission spectrum
is modified by
Doppler effects in the viewed direction. Our results have important
references to the experimental observation of SGC in hot atomic vapors. 相似文献
160.