The Experts below are selected from a list of 324540 Experts worldwide ranked by ideXlab platform
Dmitriy A Yablonskiy - One of the best experts on this subject based on the ideXlab platform.
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lung morphometry with hyperpolarized 129xe Theoretical Background
Magnetic Resonance in Medicine, 2012Co-Authors: A L Sukstanskii, Dmitriy A YablonskiyAbstract:The 3He lung morphometry technique, based on MRI measurements of hyperpolarized 3He gas diffusion in lung airspaces, provides unique information on the lung microstructure at the alveolar level. In vivo 3D tomographic images of standard morphological parameters (airspace chord length, lung parenchyma surface-to-volume ratio, and number of alveoli per unit volume) can be generated from a rather short (several seconds) MRI scan. The technique is based on a theory of gas diffusion in lung acinar airways and experimental measurements of diffusion-attenuated MRI signal. The present work aims at developing the Theoretical Background of a similar technique based on hyperpolarized 129Xe gas. As the diffusion coefficient and gyromagnetic ratio of 129Xe gas are substantially different from those of 3He gas, the specific details of the theory and experimental measurements with 129Xe should be amended. We establish phenomenological relationships between acinar airway geometrical parameters and the diffusion-attenuated MR signal for human and small animal lungs, both normal lungs and lungs with mild emphysema. Optimal diffusion times are shown to be about 5 ms for human and 1.3 ms for small animals. The expected uncertainties in measuring main morphometrical parameters of the lungs are estimated in the framework of Bayesian probability theory. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
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in vivo lung morphometry with hyperpolarized 3he diffusion mri Theoretical Background
Journal of Magnetic Resonance, 2008Co-Authors: A L Sukstanskii, Dmitriy A YablonskiyAbstract:MRI-based study of (3)He gas diffusion in lungs may provide important information on lung microstructure. Lung acinar airways can be described in terms of cylinders covered with alveolar sleeve [Haefeli-Bleuer, Weibel, Anat. Rec. 220 (1988) 401]. For relatively short diffusion times (on the order of a few ms) this geometry allows description of the (3)He diffusion attenuated MR signal in lungs in terms of two diffusion coefficients-longitudinal (D(L)) and transverse (D(T)) with respect to the individual acinar airway axis [Yablonskiy et al., PNAS 99 (2002) 3111]. In this paper, empirical relationships between D(L) and D(T) and the geometrical parameters of airways and alveoli are found by means of computer Monte Carlo simulations. The effects of non-Gaussian signal behavior (dependence of D(L) and D(T) on b-value) are also taken into account. The results obtained are quantitatively valid in the physiologically important range of airway parameters characteristic of healthy lungs and lungs with mild emphysema. In lungs with advanced emphysema, the results provide only "apparent" characteristics but still could potentially be used to evaluate emphysema progression. This creates a basis for in vivo lung morphometry-evaluation of the geometrical parameters of acinar airways from hyperpolarized (3)He diffusion MRI, despite the airways being too small to be resolved by direct imaging. These results also predict a rather substantial dependence of (3)He ADC on the experimentally-controllable diffusion time, Delta. If Delta is decreased from 3 ms to 1 ms, the ADC in normal human lungs may increase by almost 50%. This effect should be taken into account when comparing experimental data obtained with different pulse sequences.
Martin Schafer - One of the best experts on this subject based on the ideXlab platform.
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line source simulation for shallow seismic data part 1 Theoretical Background
Geophysical Journal International, 2014Co-Authors: Thomas Forbriger, Lisa Groos, Martin SchaferAbstract:Equivalent line-source seismograms can be obtained from shallow seismic field recordings by (1) convolving the waveforms with sqrt(1/t), (2) applying a sqrt(1/t) time-domain taper, where t is traveltime and (3) scaling the waveform with r_offset*sqrt(2), where roffset is source-to-receiver offset. We require such a procedure when applying algorithms of 2-D adjoint full-waveform inversion (FWI) to shallow-seismic data. Although derived from solutions for acoustic waves in homogeneous full space this simple procedure performs surprisingly well when applied to vertical and radial components of shallow-seismic recordings from hammer blows or explosions. This is the case even in the near field of the force, although the procedure is derived from a far-field approximation. Similar approximative procedures recommended in literature are optimized for reflected waves and do not convert the amplitudes of all shallow seismic wavefield constituents equally well. We demonstrate the suitability of the proposed method for the viscoelastic case by numerical examples as well as analytical considerations. In contrast to the proposed single-trace procedure, integral-transform approaches are exact for all viscoelastic wavefield constituents of the near- and far-field in unknown 1-D-heterogeneous structure. Unfortunately, integral-transform approaches suffer from artefacts in applications to data sampled on 2-D structures. Here, we use the Fourier–Bessel integral transformation as a reference in 1-D heterogeneous structures. We unroll the wave-Theoretical Background of both approaches in order to demonstrate, why the simplistic single-trace simulation approach derived from the asymptotic acoustic case can perform so well when applied to the shallow elastic wavefield. Further we give recommendations for practical implementation and application to field data of the proposed simulation method and compare to the results of alternative conversion rules. The performance of the conversion procedure to data recorded on 2-D heterogeneous structures is presented in a companion study by FWI reconstruction tests.
A L Sukstanskii - One of the best experts on this subject based on the ideXlab platform.
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lung morphometry with hyperpolarized 129xe Theoretical Background
Magnetic Resonance in Medicine, 2012Co-Authors: A L Sukstanskii, Dmitriy A YablonskiyAbstract:The 3He lung morphometry technique, based on MRI measurements of hyperpolarized 3He gas diffusion in lung airspaces, provides unique information on the lung microstructure at the alveolar level. In vivo 3D tomographic images of standard morphological parameters (airspace chord length, lung parenchyma surface-to-volume ratio, and number of alveoli per unit volume) can be generated from a rather short (several seconds) MRI scan. The technique is based on a theory of gas diffusion in lung acinar airways and experimental measurements of diffusion-attenuated MRI signal. The present work aims at developing the Theoretical Background of a similar technique based on hyperpolarized 129Xe gas. As the diffusion coefficient and gyromagnetic ratio of 129Xe gas are substantially different from those of 3He gas, the specific details of the theory and experimental measurements with 129Xe should be amended. We establish phenomenological relationships between acinar airway geometrical parameters and the diffusion-attenuated MR signal for human and small animal lungs, both normal lungs and lungs with mild emphysema. Optimal diffusion times are shown to be about 5 ms for human and 1.3 ms for small animals. The expected uncertainties in measuring main morphometrical parameters of the lungs are estimated in the framework of Bayesian probability theory. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
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in vivo lung morphometry with hyperpolarized 3he diffusion mri Theoretical Background
Journal of Magnetic Resonance, 2008Co-Authors: A L Sukstanskii, Dmitriy A YablonskiyAbstract:MRI-based study of (3)He gas diffusion in lungs may provide important information on lung microstructure. Lung acinar airways can be described in terms of cylinders covered with alveolar sleeve [Haefeli-Bleuer, Weibel, Anat. Rec. 220 (1988) 401]. For relatively short diffusion times (on the order of a few ms) this geometry allows description of the (3)He diffusion attenuated MR signal in lungs in terms of two diffusion coefficients-longitudinal (D(L)) and transverse (D(T)) with respect to the individual acinar airway axis [Yablonskiy et al., PNAS 99 (2002) 3111]. In this paper, empirical relationships between D(L) and D(T) and the geometrical parameters of airways and alveoli are found by means of computer Monte Carlo simulations. The effects of non-Gaussian signal behavior (dependence of D(L) and D(T) on b-value) are also taken into account. The results obtained are quantitatively valid in the physiologically important range of airway parameters characteristic of healthy lungs and lungs with mild emphysema. In lungs with advanced emphysema, the results provide only "apparent" characteristics but still could potentially be used to evaluate emphysema progression. This creates a basis for in vivo lung morphometry-evaluation of the geometrical parameters of acinar airways from hyperpolarized (3)He diffusion MRI, despite the airways being too small to be resolved by direct imaging. These results also predict a rather substantial dependence of (3)He ADC on the experimentally-controllable diffusion time, Delta. If Delta is decreased from 3 ms to 1 ms, the ADC in normal human lungs may increase by almost 50%. This effect should be taken into account when comparing experimental data obtained with different pulse sequences.
Markus Ganzera - One of the best experts on this subject based on the ideXlab platform.
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Supercritical Fluid Chromatography--Theoretical Background and Applications on Natural Products.
Planta medica, 2015Co-Authors: Anja Hartmann, Markus GanzeraAbstract:The use of supercritical fluid chromatography for natural product analysis as well as underlying Theoretical mechanisms and instrumental requirements are summarized in this review. A short introduction focusing on the historical development of this interesting separation technique is followed by remarks on the current instrumental design, also describing possible detection modes and useable stationary phases. The overview on relevant applications is grouped based on their basic intention, may it be (semi)preparative or purely analytical. They indicate that supercritical fluid chromatography is still primarily considered for the analysis of nonpolar analytes like carotenoids, fatty acids, or terpenes. The low polarity of supercritical carbon dioxide, which is used with modifiers almost exclusively as a mobile phase today, combined with high efficiency and fast separations might explain the popularity of supercritical fluid chromatography for the analysis of these compounds. Yet, it has been shown that more polar natural products (e.g., xanthones, flavonoids, alkaloids) are separable too, with the same (if not superior) selectivity and reproducibility than established approaches like HPLC or GC.
Mastura Jaafar - One of the best experts on this subject based on the ideXlab platform.
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city development strategy Theoretical Background themes and building blocks
The International Journal of Urban Sciences, 2016Co-Authors: Mostafa S Rasoolimanesh, Nurwati Badarulzaman, Mastura JaafarAbstract:ABSTRACTCity development strategy (CDS) has transformed numerous cities in developing countries to cope with the unprecedented challenges of the twenty-first century. However, the CDS literature indicates that the needs, culture, and planning Background of a city determine its CDS themes and methodologies. Drawing upon various CDS experiences, this paper provides an overview of the theory underpinning CDS, its themes, and building blocks to elucidate a more comprehensive understanding of CDS as it applies to real-world cities that have employed CDS and those who plan to do so in the future to confront their challenges and achieve their goals. This paper conducts a comprehensive systematic review, inclusive of journal articles and international CDS implementation literature as previously reported by international agencies and local governments. From this review, a number of recommendations have emerged for cities wishing to apply CDS. This paper makes a significant contribution to the CDS literature by cla...
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city development strategy Theoretical Background themes and building blocks
Social Science Research Network, 2016Co-Authors: Mostafa S Rasoolimanesh, Nurwati Badarulzaman, Mastura JaafarAbstract:City development strategy (CDS) has transformed numerous cities in developing countries to cope with the unprecedented challenges of the twenty-first century. However, the CDS literature indicates that the needs, culture, and planning Background of a city determine its CDS themes and methodologies. Drawing upon various CDS experiences, this paper provides an overview of the theory underpinning CDS, its themes, and building blocks to elucidate a more comprehensive understanding of CDS as it applies to realworld cities that have employed CDS and those who plan to do so in the future to confront their challenges and achieve their goals. This paper conducts a comprehensive systematic review, inclusive of journal articles and international CDS implementation literature as previously reported by international agencies and local governments. From this review, a number of recommendations have emerged for cities wishing to apply CDS. This paper makes a significant contribution to the CDS literature by clarifying the Theoretical Background, themes, and building blocks that are features of successful CDS implementation.
