bluuebunny/crossref_metadata_2025_split huggingface dataset:
Pittfalls of MRI measurement of white matter perfusion based on arterial spin labeling
Authors: van Gelderen P. , de Zwart J.A. , Duyn J.H. 
Published: March 2008

AbstractAlthough arterial spin labeling (ASL) MRI has been successfully applied to measure gray matter (GM) perfusion in vivo, accurate detection of white matter (WM) perfusion has proven difficult. Reported literature values are not consistent with each other or with perfusion measured with other modalities. In this work, the cause of these inconsistencies is investigated. The results suggest that WM perfusion values are substantially affected by the limited image resolution and by signal losses caused by the long transit times in WM, which significantly affect the label. From gadolinium diethylenetriamine pentaacetic acid (Gd‐DTPA) bolus‐tracking experiments (N = 6), it is estimated that the transit time can be several seconds long in deep WM. Furthermore, simulations show that even at a spatial resolution of 7 μl voxel size, contamination by the GM signals can exceed 40% of the actual WM signal. From 10‐min long flow‐sensitive alternating inversion recovery ASL (FAIR‐ASL) measurements at 3T in normal subjects (N = 7), using highly sensitive detectors, it is shown that single‐voxel (7 μl) deep WM perfusion values have an signal‐to‐noise ratio (SNR) less than 1. The poor sensitivity and heterogeneous transit time limit the applicability of ASL for measurement of perfusion in WM. Magn Reson Med 59:788–795, 2008. © 2008 Wiley‐Liss, Inc.
Robust MRI brain tissue parameter estimation by multistage outlier rejection
Authors: Manjón José V. , Tohka Jussi , García‐Martí Gracian , Carbonell‐Caballero José , Lull Juan J. , Martí‐Bonmatí Luís , Robles Montserrat 
Published: March 2008

AbstractThis article addresses the problem of the tissue type parameter estimation in brain MRI in the presence of partial volume effects. Automatic MRI brain tissue classification is hampered by partial volume effects that are caused by the finite resolution of the acquisition process. Due to this effect intensity distributions in brain MRI cannot be well modeled by a simple mixture of Gaussians and therefore more complex models have been developed. Unfortunately, these models do not seem to be robust enough for clinical conditions, as the quality of the tissue classification decreases rapidly with the image quality. Also, the application of these methods for pathological images with unmodeled intensities (e.g. MS plaques, tumors, etc.) remains uncertain. In the present work a new robust method for brain tissue characterization is presented, treating the partial volume affected voxels as outliers of the pure tissue distributions. The proposed method estimates the tissue characteristics from a reduced set of intensities belonging to a particular pure tissue class. This reduced set is selected by using a trimming procedure based on local gradient information and distributional data. This feature makes the method highly tolerant of a large amount of unexpected intensities without degrading its performance. The proposed method has been evaluated using both synthetic and real MR data and compared with state‐of‐the‐art methods showing the best results in the comparative. Magn Reson Med 59:866–873, 2008. © 2008 Wiley‐Liss, Inc.
Magnetic resonance imaging of neuronal and glial swelling as an indicator of function in cerebral tissue slices
Authors: Stroman P.W. , Lee A.S. , Pitchers K.K. , Andrew R.D. 
Published: March 2008

AbstractHere we demonstrate a new basis of signal change in magnetic resonance imaging (MRI) related to neuronal function, independent of blood oxygenation or flow. Time series MRI data acquired from living, superfused brain slices of adult rats revealed that the signal intensity reversibly increased with depolarization evoked by briefly elevating extracellular K+. This was presumably a consequence of increased tissue water in the intracellular compartment. Reversible increases in light transmittance (LT) demonstrating a similar time course in response to K+elevation supported cellular swelling as generating the MRI signal intensity changes. This was confirmed by reversibly swelling cells in the slice under hypoosmotic challenge, which increased both MRI and LT signals with an identical time course. Conversely, shrinking cells under hyperosmotic challenge reversibly decreased the MRI and LT signals. We propose that specific MRI of neuronal function (fMRI) signals detected under identical parameters during predominantly proton‐density‐weighted fMRI of the spinal cord can now be explained by neuronal and glial swelling in activated central nervous system (CNS) regions. These observations demonstrate the biophysical basis of the fMRI contrast mechanism that has been termed “signal enhancement by extravascular water protons,” or SEEP. Magn Reson Med 59:700–706, 2008. © 2008 Wiley‐Liss, Inc.
Cross‐validation‐based kernel support selection for improved GRAPPA reconstruction
Authors: Nana Roger , Zhao Tiejun , Heberlein Keith , LaConte Stephen M. , Hu Xiaoping 
Published: March 2008

AbstractThe extended version of the generalized autocalibrating partially parallel acquisition (GRAPPA) technique incorporates multiple lines and multiple columns of measuredk‐space data to estimate missing data. For a given accelerated dataset, the selection of the measured data points for fitting a missing datum (i.e., the kernel support) that provides optimal reconstruction depends on coil array configuration, noise level in the acquired data, imaging configuration, and number and position of autocalibrating signal lines. In this work, cross‐validation is used to select the kernel support that best balances the conflicting demands of fit accuracy and stability in GRAPPA reconstruction. The result is an optimized tradeoff between artifacts and noise. As demonstrated with experimental data, the method improves image reconstruction with GRAPPA. Because the method is simple and applied in postprocessing, it can be used with GRAPPA routinely. Magn Reson Med 59:819–825, 2008. © 2008 Wiley‐Liss, Inc.
Water chemical shift in 1H NMR of red cells: Effects of pH when transmembrane magnetic susceptibility differences are low
Authors: Larkin Timothy J. , Bubb William A. , Kuchel Philip W. 
Published: March 2008

AbstractThe 1H magic angle spinning (MAS) NMR spectrum of water in erythrocyte suspensions shows peaks from each of the intracellular and extracellular water pools. The splitting is a true chemical shift and is brought about by the elimination of water exchange under MAS conditions due to physical separation of the two water populations. The size of the chemical shift difference is determined by the concentration of intracellular protein affecting the average extent of hydrogen bonding of water. We present here a model of the chemical shift behavior for water in erythrocytes under normal high‐resolution NMR conditions based on results from MAS experiments on these cells exposed to different pH and osmotic conditions. The model accurately predicts the chemical shift of water for a static sample, and the results demonstrate that in high‐resolution NMR experiments the chemical shift of water will appear to be invariant if differences in magnetic susceptibility across the cell membrane are minimal (<10% of the magnetic susceptibility of water). Thus, changes in the shape and chemical shift of the water resonance are not due to pH changes in the physiological range. The findings are fundamental to an interpretation of the mechanism of chemical shift effects on the water resonance that may occur in functional MRI. Magn Reson Med 59:707–711, 2008. © 2008 Wiley‐Liss, Inc.
Self‐calibrating GRAPPA operator gridding for radial and spiral trajectories
Authors: Seiberlich Nicole , Breuer Felix , Blaimer Martin , Jakob Peter , Griswold Mark 
Published: March 2008

AbstractSelf‐calibrating GRAPPA operator gridding (GROG) is a method by which non‐Cartesian MRI data can be gridded using spatial information from a multichannel coil array without the need for an additional calibration dataset. Using self‐calibrating GROG, the non‐Cartesian datapoints are shifted to nearby k‐space locations using parallel imaging weight sets determined from the datapoints themselves. GROG employs the GRAPPA Operator, a special formulation of the general reconstruction method GRAPPA, to perform these shifts. Although GROG can be used to grid undersampled datasets, it is important to note that this method uses parallel imaging only for gridding, and not to reconstruct artifact‐free images from undersampled data. The innovation introduced here, namely, self‐calibrating GROG, allows the shift operators to be calculated directly out of the non‐Cartesian data themselves. This eliminates the need for an additional calibration dataset, which reduces the imaging time and also makes the GROG reconstruction more robust by removing possible inconsistencies between the calibration and non‐Cartesian datasets. Simulated and in vivo examples of radial and spiral datasets gridded using self‐calibrating GROG are compared to images gridded using the standard method of convolution gridding. Magn Reson Med 59:930–935, 2008. © 2008 Wiley‐Liss, Inc.
Quantitative mapping of ventilation‐perfusion ratios in lungs by 19F MR imaging of T1 of inert fluorinated gases
Authors: Adolphi Natalie L. , Kuethe Dean O. 
Published: March 2008

AbstractA new method is presented for quantitative mapping of ventilation‐to‐perfusion ratios (VA/Q) in the lung: MRI of the 19F longitudinal relaxation time (T1) of an inert fluorinated gas at thermal polarization. The method takes advantage of the dependence of the 19F T1 on the local SF6 partial pressure, which depends on the local value of VA/Q. In contrast to hyperpolarized noble gases, with very long T1s, the T1 of SF6 in mammal lungs is 0.8–1.3 ms. Thus, rapid signal averaging overcomes the low thermal equilibrium polarization. T1 imaging of a phantom consisting of four different SF6/air mixtures with known T1 values validates the modified Look‐Locker T1 imaging sequence. To demonstrate the method in vivo, partial obstruction of the left bronchus was attempted in three rats; 3D free induction decay (FID)‐projection T1 images (2 mm isotropic resolution) revealed obstructed ventilation in two of the animals. In those images, ≈1700 lung voxels contained sufficient SF6 for analysis and T1 was determined in each voxel with a standard error of 8–10%. For comparison, independent VA/Q images of the same animals were obtained using a previously described SF6 MRI technique, and good agreement between the two techniques was obtained. Relative to the previous technique the resolution achieved using the T1 method is lower (for similar VA/Q precision and imaging time); however, the T1 method offers the potential advantages of eliminating the need for image coregistration and allowing patients with impaired lung function to breathe a 70% O2 gas mixture during the entire imaging procedure. Magn Reson Med 59:739–746, 2008. © 2008 Wiley‐Liss, Inc.
A new model of resorbable device degradation and drug release ‐ part I: zero order model
Authors: Arosio Paolo , Busini Valentina , Perale Giuseppe , Moscatelli Davide , Masi Maurizio 
Published: March 2008

AbstractBACKGROUND: Within the field of resorbable devices, recent years have seen an increased demand for better reliability from drug delivery systems and resorbable polymer degradation control, causing researchers to abandon trial‐and‐error approaches towards model‐based methods. In this context, we developed a lumped‐parameters zero‐order model for the degradation of resorbable polymeric drug release systems. Such a model is thought to be applicable in the design of specific devices based on the expected degradation time and drug delivery rates, as it is based on a ‘shrinking core’ approach and takes into account the main physicochemical parameters involved in polymer degradation, in drug release and in mechanical strength prediction, all independently estimated.RESULTS: The model, based on conservation equations, leads to numerically solved ordinary differential equations, the predictions of which were verified through literature data. Data of different authors and various systems were satisfactorily matched by model predictions, thus confirming the reliability of parameter estimation procedures.CONCLUSION: The present model is among the very few that completely address all aspects involved in device degradation and drug delivery altogether, and thus represents a step ahead with respect to current available solutions, firmly enforcing itself into the ongoing debate on the modelling of degradation and release behaviour. Copyright © 2008 Society of Chemical Industry
Facile syntheses of 4‐vinyl‐1,2,3‐triazole monomers by click azide/acetylene coupling
Authors: Takizawa Kenichi , Nulwala Hunaid , Thibault Raymond J. , Lowenhielm Peter , Yoshinaga Kohji , Wooley Karen L. , Hawker Craig J. 
Published: March 2008

AbstractSynthetic strategies for the preparation of a new family of vinyl monomers, 4‐vinyl‐1,2,3‐triazoles, have been developed. These monomers are noteworthy as they combine the stability and aromaticity of styrenics with the polarity of vinylpyridines and the structural versatility of acrylate/methacrylate derivatives. To enable the wide adoption of these unique monomers, new methodologies for their synthesis have been elaborated which rely on Cu‐catalyzed azide/acetylene cycloaddition reactions—“click chemistry”—as the key step, with the vinyl substituent being formed by either elimination or Wittig‐type reactions. In addition, one‐pot “click” reactions have been developed from alkyl halides, which allow for monomer synthesis without isolation of the intermediate organic azides. The high yield and facile nature of these procedures has allowed a library of new monomers including the parent compound, 1‐H‐4‐vinyl‐1,2,3‐triazole, to be prepared on large scales. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2897–2912, 2008
Alternating copolymerization of carbon dioxide and cyclohexene oxide catalyzed by silicon dioxide/ZnCoIII double metal cyanide complex hybrid catalysts with a nanolamellar structure
Authors: Sun Xue‐Ke , Zhang Xing‐Hong , Liu Fei , Chen Shang , Du Bin‐Yang , Wang Qi , Fan Zhi‐Qiang , Qi Guo‐Rong 
Published: March 2008

AbstractAir‐stable hybrid catalysts of silicon dioxide/double metal cyanide complexes (Si‐DMCCs) based on Zn3[Co(CN)6]2 (ZHCC) were prepared by an in situ sol–gel method. The Si‐DMCCs showed low crystallinity and a nanolamellar structure with a thickness of ∼40–60 nm. In particular, a lamellar structure of regular hexagonal shape was observed for Si‐DMCCs with low SiO2 content. These catalysts had very high catalytic activity for alternating copolymerization of cyclohexene oxide (CHO) and carbon dioxide. A turnover number of 11,444, turnover frequency of 3815 h−1, and apparent efficiency of 7.5 kg polymer/g ZHCC (∼24.0 kg polymer/g Zn) were achieved at 3.8 MPa and 100 °C. The poly(cyclohexenylene carbonate) (PCHC) polymers obtained were completely atactic with a molecular weight (Mn) of ∼10 kg/mol and polydispersity of 2.0–3.0. The PCHCs had a structure of nearly alternating CHO and CO2 units, with a molar fraction of carbonate units of 0.44–0.47. Preliminary investigations of the mechanism suggest that nucleophilic attack by neighboring oxygen atoms is involved in copolymerization initiation with ZnCoIII DMCCs. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3128–3139, 2008
Effect of interaction between poly(ethylene terephthalate) and carbon nanotubes on the morphology and properties of their nanocomposites
Authors: Yoo Hye Jin , Jung Yong Chae , Cho Jae Whan 
Published: March 2008

AbstractPoly(ethylene terephthalate) (PET) nanocomposites were prepared by melt‐extruding mixtures of PET and functionalized multiwalled carbon nanotubes (MWNTs) with some interaction with PET molecules. For the functionalization of MWNTs, benzyl isocyanate and phenyl isocyanate with different molecular flexibility were employed on the surface of the MWNTs via chemical modification, respectively. The reaction for functionalization of MWNTs was confirmed by FTIR and transmission electron microscopy (TEM) measurements. TEM observations indicated that both benzyl and phenyl isocyanate groups covered the surface of the MWNTs after functionalization. The PET nanocomposites containing isocyanate groups showed improved mechanical properties, including the tensile strength and tensile modulus, compared with those with pristine and acid‐treated nanotubes. These improvements were ascribed to π–π interactions between the aromatic rings of PET molecules and the isocyanate group in MWNTs. The functionalized MWNTs showed a better dispersion of carbon nanotubes in the matrix polymer and a different fractured cross‐section morphology in scanning electron microscope measurements relative to the pristine MWNTs. The crystallinity of the functionalized MWNT‐PET nanocomposites was significantly higher than that of the pristine and acid‐treated MWNTs. FTIR results indicated that the presence of carbon nanotubes induced trans‐conformation of PET chains, and trans conformation was particularly dominant in PET composites incorporating MWNT‐phenyl. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 900–910, 2008
Crystallization and melting behaviors of isotactic polypropylene nucleated with compound nucleating agents
Authors: Zhang Yue‐Fei 
Published: March 2008

AbstractCrystallization and melting behaviors of isotactic polypropylene (iPP) nucleated with compound nucleating agents of sodium 2,2′‐methylene‐bis (4,6‐di‐tert‐butylphenyl) phosphate (hereinafter called as NA40)/dicyclohexylterephthalamide (hereinafter called as NABW) (weight ratio of NA40 to NABW is 1:1) were studied by differential scanning calorimetry and wide‐angle X‐ray diffraction (WAXD), the relative β‐amount of iPP nucleated with these compound nucleating agents was also calculated in Turner‐Jones equation by using wide‐angle X‐ray diffraction data. Under isothermal crystallization, there exists a temperature range favorable for formation of β‐iPP. When the concentration of compound nucleating agents is 0.2 wt %, the temperature range is from 100 to 140 °C. While in nonisothermal crystallization, lower cooling rate is favorable for form of β‐iPP and the relative β‐amount of iPP increases with the decreasing of cooling rate in crystallization process. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 911–916, 2008
Efficacy of five selected acaricides against Tetranychus urticae (Acari: Tetranychidae) and their side effects on relevant natural enemies occurring in citrus orchards
Authors: Urbaneja Alberto , Pascual‐Ruiz Sara , Pina Tatiana , Abad‐Moyano Raquel , Vanaclocha Pilar , Montón Helga , Dembilio Oscar , Castañera Pedro , Jacas Josep A. 
Published: March 2008

AbstractBACKGROUND: Three groups of natural enemies are fundamental in citrus IPM in Spain: coccinellid and phytoseiid predators and hymenopteran parasitoids. Tetranychus urticae Koch is an important pest affecting citrus, for which biological control has not yet been achieved; therefore, acaricides are commonly used to control it. The goal of this study was to measure the efficacy of different acaricides on this mite and their side effects on three natural enemies relevant for citrus (Cryptolaemus montrouzieri Mulsant, Neoseiulus californicus McGregor and Aphidius colemani Viereck).RESULTS: Some products proved highly effective against T. urticae and harmless to A. colemani (mineral oil, tebufenpyrad, clofentezine and fenazaquin). However, almost all products tested were slightly harmful for both the predators considered. Fenazaquin was even moderately harmful for N. californicus.CONCLUSION: Further studies, like that presented here, are necessary to gain a better understanding of integrating biological and chemical controls. When considering both efficacy and side effects on beneficial arthropods, the best options would seem to be mineral oil, tebufenpyrad and clofentezine. However, it is urgent to complete testing of the side effects of the acaricides used in citrus. This question is crucial if the fact that two recently introduced Tetranychidae are being controlled in citrus by chemical means exclusively is considered. Copyright © 2008 Society of Chemical Industry
Simulating trends in reaction path geometry as a function of external fields. A generalized electronic diabatic model for two‐dimensional energy surfaces
Authors: Arteca Gustavo A. , Rank Jean Pierre , Tapia Orlando 
Published: March 2008

AbstractWe introduce a protocol to represent quantum states as a linear superposition of model electronic diabatic basis states coupled in an external (static) electric field. By considering an entire family of these models, we uncover trends in reaction‐path geometry and the topology of potential‐energy surfaces, including all those that can be realized in a two‐dimensional configurational space. Our approach can be used as a tool to model the key parameters (e.g., diabatic basis states, external field intensity) that yield desired geometrical characteristics in an actual potential energy surface. In this work, external agents such as laser fields, or a group of neighboring charges, are regarded as essential requirements to prompt, or trigger, the occurrence of a chemical process. In these cases, reaction path geometry can be modulated externally so as to yield processes that would appear to occur far from gas‐phase geometries. This phenomenology is intrinsically nonadiabatic. Our present approach accounts for the possibility of such features, i.e., the occurrence of quantum states whose electronic structures resemble products, while at geometries that are more similar to those of reactants. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008
A generic method to detect electrophilic intermediates using isotopic pattern triggered data‐dependent high‐resolution accurate mass spectrometry
Authors: Lim Heng‐Keang , Chen Jie , Cook Kevin , Sensenhauser Carlo , Silva Jose , Evans David C. 
Published: March 2008

AbstractA need still exists for a liquid chromatography/tandem mass spectrometry (LC/MS/MS) method that can detect broad classes of glutathione (GSH) conjugates and provide characterization of their structures. We now describe the development of a method that multiplexes high‐resolution accurate mass analysis with isotope pattern triggered data‐dependent product ion scans, for simultaneous detection and structural elucidation of GSH conjugates within a single analysis using a LTQ/Orbitrap. This method was initially developed to detect GSH conjugates generated from incubating 10 µM test compound with pooled human liver microsomes fortified with NADPH‐regenerating system and a 2:1 ratio of 5 mM glutathione and [13CN‐Gly]glutathione. The GSH conjugates were detected by isotope search of mass defect filtered and control subtracted full scan accurate MS data using MetWorks software. This was followed by elucidation of reactive intermediate structures using chemical formulae for both protonated molecules and their product ions from accurate masses in a single analysis. The mass accuracies measured for the precursor and product ions by the Orbitrap were <2 ppm in external mass calibration mode. Successful detection and characterization of GSH conjugates of acetaminophen, tienilic acid, clozapine, ticlopidine and mifepristone validated this method. In each case, the detected GSH conjugates were within the top five hits by isotope search. This method also has a broader detection capability since it is independent of the collision‐induced dissociation behavior of the GSH conjugates. Furthermore, this method is amenable to a broad class of reactive intermediate trapping agents as exemplified by the simultaneous detection and structural elucidation of the cyano‐N‐methylene iminium ion conjugates of verapamil and its O‐desmethyl metabolites, which we report for the first time. In addition to the chemically tagged reactive intermediates, this method also provides information on stable metabolites from the full scan accurate MS data. Copyright © 2008 John Wiley & Sons, Ltd.