bluuebunny/crossref_metadata_2025_split huggingface dataset:
Vertical group formation: A social process perspective
Authors: Shanley Mark , Peteraf Margaret 
Published: September 2004

AbstractVertical groups are a common occurrence that is rarely studied as a group‐level phenomenon. This paper brings attention to the vertical group, as a collective actor, and group formation processes. We define the vertical group and describe, in brief, why they exist. We explore the issue of how they form and the social processes under which different types of groups are likely to emerge. These include trial and error learning, social learning, and social identification. We introduce a framework that shows how variation in two more fundamental enabling conditions affects both the group development process and the characteristics of the emerging groups. Copyright © 2004 John Wiley & Sons, Ltd.
Localized in vivo human 1H MRS at very short echo times
Authors: Zhong Kai , Ernst Thomas 
Published: September 2004

AbstractA new point‐resolved spectroscopy (PRESS) sequence was developed that allows localized human proton MR spectra to be acquired at echo times (TEs) of 10 ms or less. The method was implemented on a 4 Tesla Varian research console and a clinical 3 Tesla Siemens Trio scanner. Human brain spectra acquired in vivo from the prefrontal cortex at TE = 8 ms showed improved signals from coupled resonances (such as glutamate, glutamine, and myo‐inositol) compared to spectra acquired at TE = 30 ms. These improvements should result in more accurate quantitation of these metabolites. Magn Reson Med 52:898–901, 2004. © 2004 Wiley‐Liss, Inc.
Ultrafast 2D NMR spectroscopy using sinusoidal gradients: Principles and ex vivo brain investigations
Authors: Sela Noa , Degani Hadassa , Frydman Lucio 
Published: September 2004

AbstractA new methodology capable of delivering complete 2D NMR spectra within a single scan was recently introduced. The resulting potential gain in time resolution could open new opportunities for in vivo spectroscopy, provided that the technical demands of the methodology are satisfied by the corresponding hardware. Foremost among these demands are the relatively short switching times expected from the applied gradient‐echo trains. These rapid transitions may be particularly difficult to accomplish on imaging systems. As a step toward solving this problem, we assessed the possibility of replacing the square‐wave gradient train currently used during the course of the acquisition by a shaped sinusoidal gradient. Examples of the implementation of this protocol are given, and successful ultrafast acquisitions of 2D NMR spectra with suitable spectral widths on a microimaging probe (for both phantom solutions and ex vivo mouse brains) are demonstrated. Magn Reson Med 52:893–897, 2004. © 2004 Wiley‐Liss, Inc.
In vivo tumor lactate relaxation measurements by selective multiple‐quantum‐coherence (Sel‐MQC) transfer
Authors: Muruganandham Manickam , Koutcher Jason A. , Pizzorno Giuseppe , He Qiuhong 
Published: September 2004

AbstractThe frequency‐selective multiple‐quantum‐coherence (Sel‐MQC) lactate (Lac) filter offers complete lipid and water suppression in a single scan for robust in vivo detection of tumor Lac, even in the presence of abundant lipids. Conversion of the detected signal into accurate tissue concentrations of Lac requires knowledge of in vivo Lac T1 and T2 relaxation times. This work reports modifications to the Sel‐MQC pulse sequence, T1‐ and T2‐Sel‐MQC, that facilitate relaxation measurements of Lac. The T1‐Sel‐MQC sequence combines an inversion prepulse with the Sel‐MQC filter. The T2‐Sel‐MQC sequence incorporates a CH3‐selective 180° pulse during the MQ preparation period to overcome the J‐modulation effects and allow the insertion of variable echo delays. The performance of these sequences was evaluated with the use of phantoms and subcutaneous murine tumor models in vivo. The present approach will allow investigators to correct for the relaxation‐induced Lac signal loss in Sel‐MQC experiments for the quantitative mapping of in vivo tumor Lac distribution. Magn Reson Med 52:902–906, 2004. © 2004 Wiley‐Liss, Inc.
Noise‐adaptive nonlinear diffusion filtering of MR images with spatially varying noise levels
Authors: Samsonov Alexei A. , Johnson Chris R. 
Published: September 2004

AbstractAnisotropic diffusion filtering is widely used for MR image enhancement. However, the anisotropic filter is nonoptimal for MR images with spatially varying noise levels, such as images reconstructed from sensitivity‐encoded data and intensity inhomogeneity‐corrected images. In this work, a new method for filtering MR images with spatially varying noise levels is presented. In the new method, a priori information regarding the image noise level spatial distribution is utilized for the local adjustment of the anisotropic diffusion filter. Our new method was validated and compared with the standard filter on simulated and real MRI data. The noise‐adaptive method was demonstrated to outperform the standard anisotropic diffusion filter in both image error reduction and image signal‐to‐noise ratio (SNR) improvement. The method was also applied to inhomogeneity‐corrected and sensitivity encoding (SENSE) images. The new filter was shown to improve segmentation of MR brain images with spatially varying noise levels. Magn Reson Med 52:798–806, 2004. © 2004 Wiley‐Liss, Inc.
Reproducibility of fMRI at 1.5 T in a strictly controlled motor task
Authors: Liu Jing Z. , Zhang Luduan , Brown Robert W. , Yue Guang H. 
Published: September 2004

AbstractReproducibility of functional MRI (fMRI) data has been controversial. This issue was examined in this study by evaluating a strictly controlled voluntary force‐matching handgrip task. Handgrip force, electromyogram (EMG), and fMRI data of brain activity were simultaneously recorded during the task performance. The task was repeated three times in each of the two experimental sessions. While force remained unchanged and EMG showed little variation across trials and sessions, the results revealed that fMRI‐measured brain signals varied significantly among individual trials. However, the averaged fMRI signals over the three trials did not show significant difference between the two sessions. Our data suggest that fMRI is better at defining brain activation qualitatively than quantitatively, i.e., the locations of the activation areas could be reproduced quite reliably while their sizes fluctuated substantially. Magn Reson Med 52:751–760, 2004. © 2004 Wiley‐Liss, Inc.
Automated rectilinear self‐gated cardiac cine imaging
Authors: Crowe Mark E. , Larson Andrew C. , Zhang Qiang , Carr James , White Richard D. , Li Debiao , Simonetti Orlando P. 
Published: September 2004

AbstractECG‐based gating in cardiac MR imaging requires additional patient preparation time, is susceptible to RF and magnetic interference, and is ineffective in a significant percentage of patients. “Wireless” or “self‐gating” techniques have been described using either interleaved central k‐space lines or projection reconstruction to obtain MR signals synchronous with the cardiac cycle. However, the interleaved, central line method results in a doubling of the acquisition time, while radial streak artifacts are encountered with the projection reconstruction method. In this work, a new self‐gating technique is presented to overcome these limitations. A retrospectively gated TrueFISP cine sequence was modified to acquire a short second echo after the readout and phase gradients are rewound. The information obtained from this second echo was used to derive a gating signal. This technique was compared to ECG‐based gating in 10 healthy volunteers and shown to have no significant difference in image quality. The results indicate that this method could serve as an alternative gating strategy without the need for external physiological signal detection. Magn Reson Med 52:782–788, 2004. © 2004 Wiley‐Liss, Inc.
Finite‐difference simulations of 3He diffusion in 3D alveolar ducts: Comparison with the “cylinder model”
Authors: Fichele Stanislao , Paley Martyn N.J. , Woodhouse Neil , Griffiths Paul D. , van Beek Edwin J.R. , Wild Jim M. 
Published: September 2004

AbstractTime‐dependent measurements of 3He diffusion in the lung could provide an accurate method to quantify alveolar length scales and the progression of diseases such as emphysema. However, the apparent diffusion coefficient (ADC) presents a complex problem to model and solve analytically. Here, finite‐difference methods were used to simulate diffusion in 3D alveolar ducts. The results were compared to the only available analytical model—the “cylinder model”—from which it is possible to estimate the average radii of the alveolar ducts from in vivo data. The trend in data observed from simulations was found to agree well with the cylinder model. However, the cylinder model always overestimated the average radii of the simulated alveolar ducts. The simulations also demonstrated that the measurement of the longitudinal ADC (along the alveolar ducts) should be sensitive to early emphysematous changes, whereas the measured radii should be far less sensitive. Magn Reson Med 52:917–920, 2004. © 2004 Wiley‐Liss, Inc.
In vivo safe catheter visualization and slice tracking using an optically detunable resonant marker
Authors: Weiss Steffen , Kuehne Titus , Brinkert Florian , Krombach Gabriele , Katoh Marcus , Schaeffter Tobias , Guenther Rolf W. , Buecker Arno 
Published: September 2004

AbstractThe purpose of this study was to test the in vivo feasibility of safe automatic catheter tracking based on an optically detunable resonant marker installed on the catheter tip, and also to test the compatibility of this approach with guidewire materials. The design of the resonant marker and the integration into the real‐time MR environment is described. The catheter was used for real‐time MR‐guided catheterization of the aorta, left ventricle, and carotid in two swine. For in‐plane visualization, the marker was repeatedly detuned. For automatic slice tracking, a projection difference measurement including detuning was interleaved with the imaging sequence. In vitro experiments were conducted to investigate the RF‐safety of the marker and the effect of the guidewires on the signal intensity. For all orientations the marker provided excellent in vivo contrast using a radial steady‐state free‐precession sequence. Flashing of the marker by repetitive tuning/detuning further improved the in‐plane visualization. Automatic slice tracking during real‐time imaging was successfully performed. The plastic guidewires did not interfere with the marker, and detuning by guidewires containing nitinol could be compensated. In conclusion, automatic slice tracking as well as excellent in‐plane visualization can be achieved with this approach and it is safe with respect to RF transmission. Magn Reson Med 52:860–868, 2004. © 2004 Wiley‐Liss, Inc.
Inherent insensitivity to RF inhomogeneity in FLASH imaging
Authors: Wang Danli , Heberlein Keith , LaConte Stephen , Hu Xiaoping 
Published: September 2004

AbstractRadiofrequency (RF) field inhomogeneity is an unavoidable problem in MRI, and it becomes severe at high magnetic fields due to the dependence of B1 on the sample. It leads to nonuniformities in image intensity and contrast, causing difficulties in quantitative interpretation and image segmentation. In this work, it is observed that with the fast low‐angle shot (FLASH) sequence, which is often used for anatomic imaging and morphometric studies, sensitivity to RF inhomogeneity can be substantially reduced when the same coil is used for both transmission and reception, and an appropriate nominal flip angle is employed. This observation can help us understand the signal behavior of FLASH in the presence of RF inhomogeneity, and provide a guide for selecting parameters in FLASH imaging. Magn Reson Med 52:927–931, 2004. © 2004 Wiley‐Liss, Inc.
Human whole‐blood relaxometry at 1.5T: Assessment of diffusion and exchange models
Authors: Stefanovic Bojana , Pike G. Bruce 
Published: September 2004

AbstractHuman whole‐blood relaxometry experiments were performed to allow the prediction of blood signal changes with blood oxygen saturation (Y) and refocusing interval (τ180). Such predictions are particularly relevant for spin‐echo (SE) blood oxygenation level‐dependent (BOLD) experiments and a recently proposed noninvasive fMRI method for measuring cerebral blood volume (CBV). Ensemble fitting of the entire set of T2 estimates, obtained over an extensive range of Y and τ180 values, was performed with the use of both a fast chemical exchange model and a model of diffusion in weak magnetic field inhomogeneities. The diffusion modeling resulted in a large reduction in the residual sum‐of‐squares compared to the fast exchange modeling. The longitudinal relaxation rate decreased linearly with Y, and increased with hematocrit. The results support the application of the recently reported diffusion model to describe deoxyhemoglobin (dHb)‐induced blood transverse relaxation rate enhancement at 1.5 T. Magn Reson Med 52:716–723, 2004. © 2004 Wiley‐Liss, Inc.
Measurement of compartment size in q‐space experiments: Fourier transform of the second derivative
Authors: Kuchel Philip W. , Eykyn Thomas R. , Regan David G. 
Published: September 2004

AbstractRestricted diffusion in compartmentalized systems can lead to spatial coherence phenomena being observed in q‐space plots from pulsed field gradient spin‐echo (PGSE) nuclear magnetic resonance (NMR) experiments. The underlying features observed in these plots contain information on the geometry of the compartments that is otherwise difficult to obtain. A numerical procedure is proposed that accentuates these coherence features: the data are weighted with a bell‐shaped window function, interpolated with a shifting cubic spline, and then the second derivative is taken prior to Fourier transformation. The window function provides apodization of the noisy data at high q values, while it and the second derivative are equivalent to applying a high‐pass filter to remove the zero‐ or low‐frequency components in the echo‐signal attenuation. Using a combination of theory, Monte Carlo simulations, and data from PGSE NMR experiments on human red blood cells, we demonstrate this to be a valuable processing tool for delineating the underlying coherence features. It should prove particularly useful where the coherence features are poorly defined or where more than one pattern is present in a q‐space plot. Magn Reson Med 52:907–912, 2004. © 2004 Wiley‐Liss, Inc.
Quantifying the intra‐ and extravascular contributions to spin‐echo fMRI at 3 T
Authors: Jochimsen Thies H. , Norris David G. , Mildner Toralf , Möller Harald E. 
Published: September 2004

AbstractFunctional MRI (fMRI) by means of spin‐echo (SE) techniques provides an interesting alternative to gradient‐echo methods because the contrast is based primarily on dynamic averaging associated with the blood oxygenation level‐dependent (BOLD) effect. In this article the contributions from different brain compartments to BOLD signal changes in SE echo planar imaging (EPI) are investigated. To gain a better understanding of the underlying mechanisms that cause the fMRI contrast, two experiments are presented: First, the intravascular contribution is decomposed into two fractions with different regimes of flow by means of diffusion‐weighting gradient schemes which are either flow‐compensated, or will maximally dephase moving spins. Second, contributions from the intra‐ and extravascular space are selectively suppressed by combining flow‐weighting with additional refocusing pulses. The results indicate two qualitatively different components of flowing blood which contribute to the BOLD contrast and a nearly equal share in functional signal from the intra‐ and extravascular compartments at TE ≈ 80 ms and 3 T. Combining these results, there is evidence that at least one‐half of the functional signal originates from the parenchyma in SE fMRI at 3 T. The authors suggest the use of flow‐compensated diffusion weighting for SE fMRI to improve the sensitivity to the parenchyma. Magn Reson Med 52:724–732, 2004. © 2004 Wiley‐Liss, Inc.
Pharmacokinetics of a triarylmethyl‐type paramagnetic spin probe used in EPR oximetry
Authors: Matsumoto Ken‐ichiro , English Sean , Yoo John , Yamada Ken‐ichi , Devasahayam Nallathamby , Cook John A. , Mitchell James B. , Subramanian Sankaran , Krishna Murali C. 
Published: September 2004

AbstractThe paramagnetic spin probe Oxo63 is used in oximetric imaging studies based on electron paramagnetic resonance (EPR) methods by monitoring the oxygen‐dependent linewidth while minimizing the contributions from self‐broadening seen at high probe concentrations. Therefore, it is necessary to determine a suitable dose of Oxo63 for EPR‐based oxygen mapping where the self‐broadening effects are minimized while signal intensity adequate for imaging can be realized. A constant tissue concentration of spin probe would be useful to image a subject and assess changes in pO2 over time; accumulation or elimination of the compound in specific anatomical regions could translate to and be mistaken for changes in local pO2, especially in OMRI‐based oximetry. The in vivo pharmacokinetics of the spin probe, Oxo63, after bolus and/or continuous intravenous infusion was investigated in mice using a novel approach with X‐band EPR spectroscopy. The results show that the half‐life in blood was 17–21 min and the clearance by excretion was 0.033–0.040 min−1. Continuous infusion following a bolus injection of the probe was found to be effective to obtain stable plasma concentration as well as image intensity to permit reliable pO2 estimates. Magn Reson Med 52:885–892, 2004. Published 2004 Wiley‐Liss, Inc.
Assessment of axonal fiber tract architecture in excised rat spinal cord by localized NMR q‐space imaging: Simulations and experimental studies
Authors: Chin Chih‐Liang , Wehrli Felix W. , Fan Yingli , Hwang Scott N. , Schwartz Eric D. , Nissanov Jonathan , Hackney David B. 
Published: September 2004

AbstractNMR q‐space imaging is a method designed to obtain information from porous materials where diffusion‐diffraction phenomena were observed from which pore size was derived. Recently, the technique has been applied to the study of biological structures as well. Although diffusive diffraction has so far not been observed in multicellular systems, displacement profiles have been used with some success as a means to estimate structure size. However, there have been no quantitative correlations of the retrieved structure sizes with histology. Clearly, the complexity of tissue architecture poses significant challenges to the interpretation of q‐space data. In this work, simulations were first performed on a two‐compartment model to demonstrate the effects of interference of the diffraction patterns arising from intra and extra‐axonal compartments and finite boundary permeability on q‐space data. Second, q‐space echo attenuation was simulated on the basis of histologic images of various rat spinal cord fiber tracts and the information obtained from the displacement profiles were compared with structural parameters computed from the histologic images. The results show that calculated mean displacements and kurtosis parallel mean axon size and axonal density. Finally, spatially localized q‐space measurements were carried out at the locations where simulations had previously been performed, resulting in displacement data that support those obtained by simulation. The data suggest the NMR q‐space approach has potential for nondestructive analysis of the axonal architecture in the mammalian spinal cord. Magn Reson Med 52:733–740, 2004. © 2004 Wiley‐Liss, Inc.
Projection imaging of the right coronary artery with an intravenous injection of contrast agent
Authors: Green Jordin D. , Schirf Brian E. , Omary Reed A. , McCarthy Richard M. , Carr James C. , Li Debiao 
Published: September 2004

AbstractContrast‐enhanced (CE) MR angiography of the right coronary artery (RCA) was performed using 2D thick‐slice projection imaging with a small (8 mL) intravenous injection of contrast agent in six volunteers. With a tight contrast bolus injection, the RCA was enhanced for a few seconds after the contrast bolus was washed out of the right ventricle. This allowed data to be acquired when only the RCA was enhanced. Using 2D thick‐slice magnetization prepared steady‐state free precession (SSFP) imaging, background signal was suppressed and a complete data set was acquired in three heartbeats. A mean vessel length of 7.1 ± 0.9 cm was depicted with a signal‐to‐noise ratio of 11.8 ± 0.7 and contrast‐to‐noise ratio of 6.1 ± 0.6. Thick‐slice 2D projection CE SSFP is a promising method to depict the RCA. Magn Reson Med 52:699–703, 2004. © 2004 Wiley‐Liss, Inc.
Large field‐of‐view real‐time MRI with a 32‐channel system
Authors: Hardy Christopher J. , Darrow Robert D. , Saranathan Manojkumar , Giaquinto Randy O. , Zhu Yudong , Dumoulin Charles L. , Bottomley Paul A. 
Published: September 2004

AbstractThe emergence of parallel MRI techniques and new applications for real‐time interactive MRI underscores the need to evaluate performance gained by increasing the capability of MRI phased‐array systems beyond the standard four to eight high‐bandwidth channels. Therefore, to explore the advantages of highly parallel MRI a 32‐channel 1.5 T MRI system and 32‐element torso phased arrays were designed and constructed for real‐time interactive MRI. The system was assembled from multiple synchronized scanner‐receiver subsystems. Software was developed to coordinate across subsystems the real‐time acquisition, reconstruction, and display of 32‐channel images. Real‐time, large field‐of‐view (FOV) body‐survey imaging was performed using interleaved echo‐planar and single‐shot fast‐spin‐echo pulse sequences. A new method is demonstrated for augmenting parallel image acquisition by independently offsetting the frequency of different array elements (FASSET) to variably shift their FOV. When combined with conventional parallel imaging techniques, image acceleration factors of up to 4 were investigated. The use of a large number of coils allowed the FOV to be doubled in two dimensions during rapid imaging, with no degradation of imaging time or spatial resolution. The system provides a platform for evaluating the applications of many‐channel real‐time MRI, and for understanding the factors that optimize the choice of array size. Magn Reson Med 52:878–884, 2004. © 2004 Wiley‐Liss, Inc.
Coil‐based artifact reduction
Authors: Atkinson David , Larkman David J. , Batchelor Philipp G. , Hill Derek L.G. , Hajnal Joseph V. 
Published: September 2004

AbstractMultiple MRI receiver coils provide extra information and can enable the reconstruction of multiple images using data from different combinations of coils. Comparison of these images shows that artifacts due to motion or flowing blood appear with different intensities due to the differing coil sensitivities. Typically, the artifact appears amplified in regions of low coil sensitivity. An optimization routine was developed to correct for the artifact by comparing reconstructions from various coil combinations and favoring a self‐consistent solution. It is demonstrated that images artifacted by blood flowing in the aorta, or translational motion of the head, can be improved. Magn Reson Med 52:825–830, 2004. © 2004 Wiley‐Liss, Inc.