Cumulant expansion with localization: a new representation of the diffusion MRI signal

Diffusion MR is sensitive to the microstructural features of a sample. Fine-scale characteristics can be probed by employing strong diffusion gradients while the low b-value regime is determined by the cumulants of the distribution of particle displacements. A signal representation based on the cumulants, however, suffers from a finite convergence radius and cannot represent the ‘localization regime' characterized by a stretched exponential decay that emerges at large gradient strengths. Here, we propose a new representation for the diffusion MR signal. Our method provides not only a robust estimate of the first three cumulants but also a meaningful extrapolation of the entire signal decay

Cumulant expansion with localization: a new representation of the diffusion MRI signal

Diffusion MR is sensitive to the microstructural features of a sample. Fine-scale characteristics can be probed by employing strong diffusion gradients while the low b-value regime is determined by the cumulants of the distribution of particle displacements. A signal representation based on the cumulants, however, suffers from a finite convergence radius and cannot represent the ‘localization regime' characterized by a stretched exponential decay that emerges at large gradient strengths. Here, we propose a new representation for the diffusion MR signal. Our method provides not only a robust estimate of the first three cumulants but also a meaningful extrapolation of the entire signal decay

Studying neuroanatomy using MRI

The study of neuroanatomy using imaging enables key insights into how our brains function, are shaped by genes and environment, and change with development, aging, and disease. Developments in MRI acquisition, image processing, and data modelling have been key to these advances. However, MRI provides an indirect measurement of the biological signals we aim to investigate. Thus, artifacts and key questions of correct interpretation can confound the readouts provided by anatomical MRI. In this review we provide an overview of the methods for measuring macro- and mesoscopic structure and inferring microstructural properties; we also describe key artefacts and confounds that can lead to incorrect conclusions. Ultimately, we believe that, though methods need to improve and caution is required in its interpretation, structural MRI continues to have great promise in furthering our understanding of how the brain works

The role of hydrogen and fuel cells in the global energy system

Hydrogen technologies have experienced cycles of excessive expectations followed by disillusion. Nonetheless, a growing body of evidence suggests these technologies form an attractive option for the deep decarb onisation of global energy systems, and that recent improvements in their cost and performance point towards economic viability as well. This paper is a comprehensive review of the potential role that hydrogen could play in the provision of electricity, h eat, industry, transport and energy storage in a low - carbon energy system, and an assessment of the status of hydrogen in being able to fulfil that potential. The picture that emerges is one of qualified promise: hydrogen is well established in certain nic hes such as forklift trucks, while mainstream applications are now forthcoming. Hydrogen vehicles are available commercially in several countries, and 225,000 fuel cell home heating systems have been sold. This represents a step change from the situation of only five years ago. This review shows that challenges around cost and performance remain, and considerable improvements are still required for hydrogen to become truly competitive. But such competitiveness in the medium - term future no longer seems an unrealistic prospect, which fully justifies the growing interest and policy support for these technologies around the world

Hypotension and intractable vomiting in the first trimester of pregnancy

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A new tensor model for the measurement of diffusional anisotropy due to restricted diffusion

Diffusion Tensor Imaging (DTI, Basser et al 1994) is the most widely used technique to measure the diffusion properties of the human brain tissues in-vivo. Although datasets comprising 30 samples, all but one of which acquired at a single b-value, are enough to estimate the diffusion tensor, in recent yearsnumerous acquisition schemes featuring more general diffusion encodings have been proposed. In order to vary the b-value, it is necessary to change either the gradient strength (G) or the duration or separation of the diffusion pulses. In the case of unrestricted diffusion (e.g. Gaussian), the b-value is theonly parameter that determines the diffusion-weighting. Thus, the same level of diffusion sensitization, hence the same signal attenuation, can be obtained by varying the gradient strength or the effective diffusion time (tau). In more realistic cases of diffusion, e.g., within restricted media (such as cells)changing G and tau have different effects on the signal. In this work, we compared the effect of changing the tau and the gradient strength G on the reconstruction error, using two tensor models: DTI, which is appropriate only for unrestricted diffusion, and the Diffusion Imaging with Confinement Tensor(DICT, Yolcu et al., Afzali et al 2015), which is applicable to both restricted and unrestricted diffusion scenarios. singthe either model, it is possible to estimate clinically important features, such as the Mean Diffusivity, and the Fractional Anisotropy (Afzali et al 2015, Pierpaoli and Basser 1996)

A theoretical study of electrocyclizations of dienylketenes to cyclohexadienones: an AM1 study

Cyclizations of various dienylketenes into 2,4-cyclohexadienones were theoretically investigated by using AM I method at the restricted HF level. For most of the dienylketenes, cyclization into 2,4-cyclohexadienone was calculated to be a favored and exothermic process. For the dienylketenes in which terminal double bond is embedded into a benzenoid type aryl moiety, partial or complete loss of aromaticity caused an increase in the activation barrier and made the reaction less exothermic or even endothermic. The effect of aromaticity was less pronounced for the dienylketenes carrying five-membered heterocyclic aromatic substituents instead of a terminal vinyl group. Slightly distorted planar transition structures were located for these types of cyclizations. The forming bond length in transition structures ranges from 1.897 to 2.099 Angstrom, which is shorter than those of typical pericyclic reactions

Engineering geological appraisal of the rock masses and preliminary support design, Dorukhan Tunnel, Zonguldak, Turkey

This paper presents the results of engineering geological studies of the rock masses along a road tunnel. Rock mass qualities of the rock units along the tunnel were determined by means of Rock Mass Rating (RMR), Geomechanic Classification (Q) system, Geological Strength Index (GSI), Rock Mass Index (RMi) and New Australian Tunneling Method (NATM). In order to determine tunnel stability, necessary support types and categories RMR, Q, RMi and NATM systems were employed as empirical tunnel support design methods. However, these empirical design guidelines for tunnel support based on rock mass classification systems failed to analyze the support performance. The performances of the proposed support systems were analyzed by means of numerical analysis, described in this paper. A 2D finite element analysis program was used as numerical method. The necessary rock mass parameters were obtained by means of rock mass classification systems. © 2007 Elsevier B.V. All rights reserved