bluuebunny/crossref_metadata_2025_split huggingface dataset:
Extended field‐of‐view imaging with table translation and frequency sweeping
Authors: Zhu Yudong , Dumoulin Charles L. 
Published: May 2003

AbstractA new method for MRI of an extended field of view (FOV) has been developed and validated. The method employs concurrent MR data acquisition and patient table motion. Table motion‐induced image artifacts are minimized by sweeping the frequency of the receiver at a rate matching the table's speed. Multiple regional images are collected and combined to reconstruct the full FOV. The imaging parameters and table speed are chosen to ensure that each regional image of the subject is collected while the corresponding anatomy is in the useable imaging volume of the scanner. Additional strategies are applied to further reduce field inhomogeneity‐induced artifacts, especially distortions due to gradient field nonlinearity. The method is robust and can be easily incorporated into most multislice 2D and volumetric 3D imaging pulse sequences. It is anticipated that this technique will be useful for a variety of applications, including angiographic runoffs, whole‐body screening, and short‐magnet imaging. Magn Reson Med 49:1106–1112, 2003. © 2003 Wiley‐Liss, Inc.
Method to determine in vivo the relaxation time T1 of hyperpolarized xenon in rat brain
Authors: Choquet Philippe , Hyacinthe Jean‐Noël , Duhamel Guillaume , Grillon Emmanuelle , Leviel Jean‐Louis , Constantinesco André , Ziegler Anne 
Published: May 2003

AbstractThe magnetic polarization of the stable 129Xe isotope may be enhanced dramatically by means of optical techniques and, in principle, hyperpolarized 129Xe MRI should allow quantitative mapping of cerebral blood flow with better spatial resolution than scintigraphic techniques. A parameter necessary for this quantitation, and not previously known, is the longitudinal relaxation time (T) of 129Xe in brain tissue in vivo: a method for determining this is reported. The time course of the MR signal in the brain during arterial injection of hyperpolarized 129Xe in a lipid emulsion was analyzed using an extended two‐compartment model. The model uses experimentally determined values of the RF flip angle and the T1 of 129Xe in the lipid emulsion. Measurements on rats, in vivo, at 2.35 T gave T = 3.6 ± 2.1 sec (±SD, n = 6). This method enables quantitative mapping of cerebral blood flow. Magn Reson Med 49:1014–1018, 2003. © 2003 Wiley‐Liss, Inc.
Microvascular BOLD contribution at 4 and 7 T in the human brain: Gradient‐echo and spin‐echo fMRI with suppression of blood effects
Authors: Duong Timothy Q. , Yacoub Essa , Adriany Gregory , Hu Xiaoping , Uǧurbil Kâmil , Kim Seong‐Gi 
Published: May 2003

AbstractThe BOLD signal consists of an intravascular (IV) and an extravascular (EV) component from both small and large vessels. Their relative contributions are dependent on field strength, imaging technique, and echo time. The IV and EV contributions were investigated in the human visual cortex at 4 and 7 T using spin‐echo and gradient‐echo BOLD fMRI with and without suppression of blood effects. Spin‐echo acquisition suppresses EV BOLD from large veins and reflects predominantly blood T2 changes and EV BOLD signal from small blood vessels. At a short echo time (32 ms), diffusion gradient‐based suppression of blood signals resulted in a 75% and 20% decrease in spin‐echo BOLD changes at 4 T and 7 T, respectively. However, at echo times (55–65 ms) approximating tissue T2 typically used for optimal BOLD contrast, these gradients had much smaller effects at both fields, consistent with the decreasing blood T2 with increasing field strength. Gradient‐echo BOLD percent changes, with relatively long echo times at both fields, were virtually unaffected by gradients that attenuated the blood contribution because the EV BOLD surrounding both large and small vessels dominated. These results suggest that spin‐echo BOLD fMRI at 4 and 7 T, with TE approximating tissue T2, significantly reduces nonspecific mapping signals from large vessels and significantly accentuates microvasculature contributions. Magn Reson Med 49:1019–1027, 2003. © 2003 Wiley‐Liss, Inc.
Mapping ischemic risk region and necrosis in the isolated heart using EPR imaging
Authors: Velayutham Murugesan , Li Haiquan , Kuppusamy Periannan , Zweier Jay L. 
Published: May 2003

AbstractReperfusion of ischemic tissue is a common event in the treatment of heart attack and stroke. To study disease pathogenesis, methods are required to measure tissue perfusion and area at risk, as well as localized regions of injury. While histology can provide this information, its destructive nature precludes assessment of time course. Thus, there is a critical need for a noninvasive technique to obtain this information. To map myocardial redox state as a possible index of cellular ischemia and viability, electron paramagnetic resonance (EPR) imaging experiments were performed on isolated rat hearts before and after the onset of regional ischemia using nitroxide spin labels. With coronary artery occlusion, the EPR images clearly showed the risk region as a void of lower intensity that reversed upon reperfusion. The extent of risk region in the heart was similar in EPR imaging and histological measurements. The unique information obtained regarding the time course of changes in redox metabolism of the risk region and normal myocardium can provide important insights regarding the mechanisms of myocardial injury during and following ischemia. Magn Reson Med 49:1181–1187, 2003. © 2003 Wiley‐Liss, Inc.
Spectral fitting: The extraction of crucial information from a spectrum and a spectral image
Authors: Mailer Colin , Robinson Bruce H. , Williams Benjamin B. , Halpern Howard J. 
Published: May 2003

AbstractA highly accurate line‐width simulation computer program is used that can account for both high amplitude and frequency of the Zeeman modulation in an electron paramagnetic resonance (EPR) experiment. This allows for the overmodulation of EPR lines to increase signal‐to‐noise ratio (SNR) in EPR spectra and spectroscopic images, without any sacrifice in the determination of the intrinsic line width (1/γ · T2e). The technique was applied to continuous‐wave EPR spectroscopic images of a narrow, single‐line trityl spin probe wherein a full EPR spectrum was extracted from each 3D spatial voxel. Typical improvements are a three‐ to fivefold increase in SNR in the high‐gradient projections in the image and a reduction in the standard deviation (SD), by a factor of 3, of the line widths in the low‐gradient domain. This method is a general one that is also applicable to the analysis of conventional 14N or 15N nitroxide spin probes. Magn Reson Med 49:1175–1180, 2003. © 2003 Wiley‐Liss, Inc.
Selective arterial spin labeling (SASL): Perfusion territory mapping of selected feeding arteries tagged using two‐dimensional radiofrequency pulses
Authors: Davies Nigel P. , Jezzard Peter 
Published: May 2003

AbstractTo date, most perfusion magnetic resonance imaging (MRI) methods using arterial spin labeling (ASL) have employed slab‐selective inversion pulses or continuous labeling within a plane in order to obtain maps derived from all major blood vessels entering the brain. However, there is great potential for gaining additional information on the territories perfused by the major vessels if individual feeding arteries could be tagged. This study demonstrates noninvasive arterial perfusion territory maps obtained using two‐dimensional (2D) selective inversion pulses. This method is designated “selective ASL” (SASL). The SASL method was used to tag the major arteries below the circle of Willis. A combination of 2D selective tagging and multislice readout allows perfusion territories to be clearly visualized, with likely applications to cerebrovascular disease and stroke. Magn Reson Med 49:1133–1142, 2003. © 2003 Wiley‐Liss, Inc.
Fast measurement of relaxation times by steady‐state free precession of 129Xe in carrier agents for hyperpolarized noble gases
Authors: Oregioni Alain , Parizel Nathalie , de Sousa Paulo Loureiro , Grucker Daniel 
Published: May 2003

AbstractHyperpolarized gases (129Xe and 3He) are being used increasingly in both MRI and NMR spectroscopy studies. However, it has been shown that carrier agents are required to preserve the long relaxation times of gases in biological fluids. Optimized gas transport can be achieved through controlled T1 and T2 measurements of 129Xe gas at equilibrium, using the steady‐state free precession method (SSFP). The accuracy of the method was proven with the use of CuSO4‐doped water samples and xenon dissolved in chloroform. The following T1 and T2 values were measured for xenon dissolved in a 30% intralipid emulsion: T1 = 29 ± 3 s; T2 = 1.0 ± 0.1 s. The values obtained in the intralipid emulsion contrast significantly with those obtained in conventional gas NMR experiments, in which it is commonly assumed that T1 = T2. This highlights the importance of obtaining accurate relaxation time measurements for medical applications of hyperpolarized gases. Magn Reson Med 49:1028–1032, 2003. © 2003 Wiley‐Liss, Inc.
Dynamic radial projection MRI of inhaled hyperpolarized 3He gas
Authors: Wild Jim M. , Paley Martyn N.J. , Kasuboski Larry , Swift Andrew , Fichele Stan , Woodhouse Neil , Griffiths Paul D. , van Beek Edwin J.R. 
Published: May 2003

AbstractA radial projection sliding‐window sequence has been developed for imaging the rapid flow of 3He gas in human lungs. The short echo time (TE) of the radial sequence lends itself to fast repetition times, and thus allows a rapid update in the image when it is reconstructed with a sliding window. Oversampling in the radial direction combined with angular undersampling can further reduce the time needed to acquire a complete image data set, without significantly compromising spatial resolution. Controlled flow phantom experiments using hyperpolarized 3He gas exemplify the temporal resolution of the method. In vivo studies on three healthy volunteers, one patient with chronic obstructive pulmonary disease (COPD), and one patient with hemiparalysis of the right diaphragm demonstrate that it is possible to accurately resolve the passage of gas down the trachea and bronchi and into the peripheral lung. Magn Reson Med 49:991–997, 2003. © 2003 Wiley‐Liss, Inc.
Error reduction and parameter optimization of the TAPIR method for fast T1 mapping
Authors: Zaitsev M. , Steinhoff S. , Shah N.J. 
Published: May 2003

AbstractA methodology is presented for the reduction of both systematic and random errors in T1 determination using TAPIR, a Look‐Locker‐based fast T1 mapping technique. The relations between various sequence parameters were carefully investigated in order to develop recipes for choosing optimal sequence parameters. Theoretical predictions for the optimal flip angle were verified experimentally. Inversion pulse imperfections were identified as the main source of systematic errors in T1 determination with TAPIR. An effective remedy is demonstrated which includes extension of the measurement protocol to include a special sequence for mapping the inversion efficiency itself. Magn Reson Med 49:1121–1132, 2003. © 2003 Wiley‐Liss, Inc.
Macrophage infiltration into the rat knee detected by MRI in a model of antigen‐induced arthritis
Authors: Beckmann Nicolau , Falk Regina , Zurbrügg Stefan , Dawson Janet , Engelhardt Petra 
Published: May 2003

AbstractThree‐dimensional (3D) MR images were obtained from the knees of rats in a model of antigen‐induced arthritis, elicited by the intraarticular administration of methylated bovine serum albumin (mBSA) to previously immunized rats. Superparamagnetic particles of iron oxide (SPIO) were administered i.v. 24 hr before each imaging session. Starting 4 days postantigen injection, images from arthritic knees exhibited distinctive signal attenuation in the synovium. This signal attenuation was significantly smaller in knees from animals treated with dexamethasone, a glucocorticosteroid, and completely absent in contralateral knees that had been challenged with vehicle. A significant negative correlation was found between the MRI signal intensity in the synovium and the histologically determined iron content in macrophages located in the same region. These results suggest the feasibility of detecting macrophage infiltration into the knee synovium in this model of antigen‐induced arthritis by labeling the cells with SPIO. This readout could provide an early marker of disease progression, before more aggressive changes like cartilage and bone erosion take place. Monitoring early changes associated with arthritis can have an impact in preclinical studies by shortening the duration of the experimental period and by facilitating the investigation of novel immunomodulatory therapies acting on macrophages. Also, the approach can be potentially adapted to clinical studies. Magn Reson Med 49:1047–1055, 2003. © 2003 Wiley‐Liss, Inc.
Circular spectrum mapping for intravoxel fiber structures based on high angular resolution apparent diffusion coefficients
Authors: Zhan Wang , Gu Hong , Xu Su , Silbersweig David A. , Stern Emily , Yang Yihong 
Published: May 2003

AbstractA method is presented for mapping intravoxel fiber structures using spectral decomposition onto a circular distribution of measured apparent diffusion coefficients (ADCs). The zeroth‐, second‐, and fourth‐order harmonic components of the ADC distribution on the circle spanned by the major and median eigenvectors of the diffusion tensor can be used to provide quantitative indices for isotropic, linear, and fiber‐crossing diffusion, respectively. A diffusion‐weighted MRI technique with 90 encoding orientations was implemented to estimate the circular ADC distribution and calculate the circular spectrum. A digital phantom was used to simulate various diffusion patterns. Comparisons were made between the circular spectrum and regular DTI‐based index maps. The results indicated that the zeroth‐ and second‐order circular spectrum maps exhibited a strong consistency with the DTI‐based mean diffusivity and linear indices, respectively, and the fourth‐order circular spectrum map was able to identify the fiber crossings. MRI experiments were performed on seven healthy human brains using a 3T scanner. The in vivo fourth‐order maps showed significantly higher densities in several brain regions, including the corpus callosum, cingulum bundle, superior longitudinal fasciculus, corticospinal tract, and middle cerebellar peduncle, which indicated the existence of fiber crossings in these regions. Magn Reson Med 49:1077–1088, 2003. © 2003 Wiley‐Liss, Inc.
Reduction of spectral ghost artifacts in high‐resolution echo‐planar spectroscopic imaging of water and fat resonances
Authors: Du Weiliang , Du Yiping P. , Fan Xiaobing , Zamora Marta A. , Karczmar Gregory S. 
Published: May 2003

AbstractEcho‐planar spectroscopic imaging (EPSI) can be used for fast spectroscopic imaging of water and fat resonances at high resolution to improve structural and functional imaging. Because of the use of oscillating gradients during the free induction decay (FID), spectra obtained with EPSI are often degraded by Nyquist ghost artifacts arising from the inconsistency between the odd and even echoes. The presence of the spectral ghost lines causes errors in the evaluation of the true spectral lines, and this degrades images derived from high‐resolution EPSI data. A technique is described for reducing the spectral ghost artifacts in EPSI of water and fat resonances, using echo shift and zero‐order phase corrections. These corrections are applied during the data postprocessing. This technique is demonstrated with EPSI data acquired from human brains and breasts at 1.5 Tesla and from a water phantom at 4.7 Tesla. Experimental results indicate that the present approach significantly reduces the intensities of spectral ghosts. This technique is most useful in conjunction with high‐resolution EPSI of water and fat resonances, but is less applicable to EPSI of metabolites due to the complexity of the spectra. Magn Reson Med 49:1113–1120, 2003. © 2003 Wiley‐Liss, Inc.
Fast measurement of intracardiac pressure differences with 2D breath‐hold phase‐contrast MRI
Authors: Thompson Richard B. , McVeigh Elliot R. 
Published: May 2003

AbstractIntracardiovascular blood pressure differences can be derived from velocity images acquired with phase‐contrast (PC) MRI by evaluating the Navier‐Stokes equations. Pressure differences within a slice of interest can be calculated using only the in‐plane velocity components from that slice. This rapid exam is proposed as an alternative to the lengthy 3D velocity imaging exams. Despite their good spatial coverage, the 3D exams are prone to artifacts and errors from respiratory motion and insufficient temporal resolution, and are unattractive in the clinical setting due to their excessive scan times (>10 min of free breathing). The proposed single‐slice approach requires only one or two breath‐holds of acquisition time, and the velocity data can be processed for the calculation of pressure differences online with immediate feedback. The impact of reducing the pressure difference calculation to two dimensions is quantified by comparison with 3D data sets for the case of blood flow within the cardiac chambers. The calculated pressure differences are validated using high‐fidelity pressure transducers both in a pulsatile flow phantom and in vivo in a dog model. There was excellent agreement between the transducer and PC‐MRI results in all of the studies. Magn Reson Med 49:1056–1066, 2003. Published 2003 Wiley‐Liss, Inc.
Changes in the proton T2 relaxation times of cerebral water and metabolites during forebrain ischemia in rat at 9.4 T
Authors: Lei Hao , Zhang Yi , Zhu Xiao‐Hong , Chen Wei 
Published: May 2003

AbstractProton T2 relaxation times of cerebral water and metabolites were measured before, during, and after transient forebrain ischemia in rat at 9.4 T using localized proton magnetic resonance spectroscopy (1H‐MRS) with Hahn echoes formed at different echo times (TEs). It was found that the T2 values of water and N‐acetyl aspartate (NAA) methyl, but not total creatine (tCr) methyl, decrease significantly (∼10%) during ischemia, and this T2 reduction is reversed by reperfusion. The T2 reduction observed for NAA was most likely caused by the extravascular component of the blood oxygenation level‐dependent (BOLD) effect induced by a drastically increased deoxyhemoglobin content during ischemia. The absence of T2 changes for tCr can probably be explained by the fact that the BOLD‐related T2 decrease was counterbalanced by the conversion of phosphocreatine (PCr) to creatine (Cr), which has a longer T2 than PCr, during ischemia. The changes in T2 should be taken into account for the quantification of metabolite concentrations during ischemia. Magn Reson Med 49:979–984, 2003. © 2003 Wiley‐Liss, Inc.
Anisotropic noise propagation in diffusion tensor MRI sampling schemes
Authors: Batchelor P.G. , Atkinson D. , Hill D.L.G. , Calamante F. , Connelly A. 
Published: May 2003

AbstractThe subject of this study is the controversial choice of directions in diffusion tensor MRI (DT‐MRI); specifically, the numerical algebra related to this choice. In DT‐MRI, apparent diffusivities are sampled in six or more directions and a least‐squares equation is solved to reconstruct the diffusion tensor. Numerical characteristics of the system are considered, in particular the condition number and normal matrix, and are shown to be dependent on the relative orientation of the tensor with respect to the laboratory frame. As a consequence, noise propagation can be anisotropic. However, the class of icosahedral direction schemes is an exception, and icosahedral directions have the same condition number and normal matrix for direction encoding as the ideal scheme with an infinite number of directions. This normal matrix and its condition number are rotationally invariant. Numerical simulations show that for icosahedral schemes with 30 directions the standard deviation of the fractional anisotropy is both low and nearly independent of fiber orientation. The recommended choice of directions for a DT‐MRI experiment is therefore the icosahedral set of directions with the highest number of directions achievable in the available time. Magn Reson Med 49:1143–1151, 2003. © 2003 Wiley‐Liss, Inc.
Reinforcing mechanism of anchors in slopes: a numerical comparison of results of LEM and FEM
Authors: Cai Fei , Ugai Keizo 
Published: May 2003

AbstractThis paper reports the limitation of the conventional Bishop's simplified method to calculate the safety factor of slopes stabilized with anchors, and proposes a new approach to considering the reinforcing effect of anchors on the safety factor. The reinforcing effect of anchors can be explained using an additional shearing resistance on the slip surface. A three‐dimensional shear strength reduction finite element method (SSRFEM), where soil–anchor interactions were simulated by three‐dimensional zero‐thickness elasto‐plastic interface elements, was used to calculate the safety factor of slopes stabilized with anchors to verify the reinforcing mechanism of anchors. The results of SSRFEM were compared with those of the conventional and proposed approaches for Bishop's simplified method for various orientations, positions, and spacings of anchors, and shear strengths of soil–grouted body interfaces. For the safety factor, the proposed approach compared better with SSRFEM than the conventional approach. The additional shearing resistance can explain the influence of the orientation, position, and spacing of anchors, and the shear strength of soil–grouted body interfaces on the safety factor of slopes stabilized with anchors. Copyright © 2003 John Wiley & Sons, Ltd.
Numerical solutions for flow in porous media
Authors: Wang J.G. , Leung C.F. , Chow Y.K. 
Published: May 2003

AbstractA numerical approach is proposed to model the flow in porous media using homogenization theory. The proposed concept involves the analyses of micro‐true flow at pore‐level and macro‐seepage flow at macro‐level. Macro‐seepage and microscopic characteristic flow equations are first derived from the Navier–Stokes equation at low Reynolds number through a two‐scale homogenization method. This homogenization method adopts an asymptotic expansion of velocity and pressure through the micro‐structures of porous media. A slightly compressible condition is introduced to express the characteristic flow through only characteristic velocity. This characteristic flow is then numerically solved using a penalty FEM scheme. Reduced integration technique is introduced for the volumetric term to avoid mesh locking. Finally, the numerical model is examined using two sets of permeability test data on clay and one set of permeability test data on sand. The numerical predictions agree well with the experimental data if constraint water film is considered for clay and two‐dimensional cross‐connection effect is included for sand. Copyright © 2003 John Wiley & Sons, Ltd.