bluuebunny/crossref_metadata_2025_split huggingface dataset:
Diffusion of metabolites in normal and ischemic rat brain measured by localized 1H MRS
Authors: van der Toorn Annette , Dijkhuizen Rick M. , Tulleken Cees A. F. , Nicolay Klaas 
Published: March 2007

AbstractThe apparent diffusion coefficient (ADC) of choline‐containing compounds (Cho), creatine and phosphocreatine (Cre), N‐acetyl‐aspartate (NAA), lactate, and water was measured in normal rat brain, and in the ischemic and contralateral region of rat brain approximately 3 and 24 h after induction of focal cerebral ischemia. After 3 h of ischemia, the ADC of Cre and NAA in the ischemic region had significantly decreased by 29% and 19%, respectively (P < 0.05). Lactate ADC was also obtained in the ischemic region. After 24 h of focal ischemia, no ADC values could be measured for NAA, Cre and Cho in the ischemic region because their concentrations had become too low. The ADCs of lactate and water in the ischemic volume were virtually identical at 3 and 24 h after occlusion. The experiments suggest that the ADC decrease of water after induction of ischemia is partly caused by changes in the diffusion characteristics of the intracellular compartment.
Improved image registration by using fourier interpolation
Authors: Eddyy William F. , Fitzgerald Mark , Noll Douglas C. 
Published: March 2007

AbstractThis paper presents a technique for performing two‐dimensional rigid‐body image registration for functional magnetic resonance images (fMRI). The method provides accurate motion correction without local distortion. The approach is to perform the translation and rotation in the Fourier domain. For images sampled on a grid, such as in echo‐planar imaging (EPI), one potential stumbling block to this approach is the computational burden of reconstruction, since the rotated image will no longer be on the Cartesian grid. A method of approximating rotations via local translations (shearing) is presented, which keeps the data on the Cartesian grid. This can provide quite accurate approximations with only a moderate amount of computation. A mean squared error (MSE) criterion is used for determining the registration parameters. This method is tested on several sets of simulated images and shown to have an accuracy ranging from 0.02 to 0.3 pixels for images with SNRs ranging from 100 to 10, respectively. They techniques have been tested on several sets of images. They are shown to work well on real subjects, for both echo‐planar and spiral data acquisition schemes. The techniques are used in an activation study in which the subject moved his head during image collection. After use of this registration technique, the activation is easily detected.
Curved slice imaging
Authors: Börnert Peter , Schäffter Tobias 
Published: March 2007

AbstractCurved slice imaging based on multidimensional RF pulses is introduced and discussed. This new approach makes it possible to image curved anatomical structures by using MRI. The 2D RF or 3D RF pulses used can be tailored to excite or refocus transverse magnetization of a previously defined arbitrarily curved slice profile in a 3D space. These RF pulses can be integrated into all standard MRI sequences to perform slice selection. The final curved slice image is obtained as a projection of the curved slice magnetization onto a selected imaging plane. The problem of ambiguities arising due to this projection process is addressed. Phantom and in vivo experiments were performed to illustrate the advantages and limitations of this approach.
Effects of polar sampling in k‐space
Authors: Lauzon M. Louis , Rutt Brian K. 
Published: March 2007

AbstractMagnetic resonance imaging allows numerous k‐space sampling schemes such as cartesian, polar, spherical, and other non‐rectilinear trajectories. Non‐rectilinear MR acquisitions permit fast scan times and can suppress motion artifacts. Still, these sampling schemes may adversely affect the image characteristics due to aliasing. Here, the Fourier aliasing effects of uniform polar sampling, i.e., equally spaced radial and azimuthal samples, are explained from the principal point spread function (PSF). The principal PSF is determined by assuming equally spaced concentric ring samples in k‐space. The radial effects such as replication, smearing, truncation artifacts, and sampling requirements, are characterized based on the PSF.
A method for the calculation of currents on shielded gradient coils
Authors: Carlson Joseph W. , Zha Leping 
Published: March 2007

AbstractAlternate expressions for the current density on the shielding surface of a gradient coil are derived for cylindrical, planar, and hyperbolic gradient coils. For the planar and hyperbolic geometries, these expressions allow more rapid high‐precision calculations of current densities than the conventional solution.
TmDOTP5−: A substance for NMR temperature measurements in vivo
Authors: Zuo Chun S. , Bowers John L. , Metz Kenneth R. , Nosaka Toshihisa , Sherry A. Dean , Clouse Melvin E. 
Published: March 2007

AbstractThe chemical shifts of 31P and 1H in thulium 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetrakis(methylene phosphonate) (TmDOTP5−) are approximately two orders of magnitude more sensitive to temperature than are water proton and 19F shifts. In the physiologically relevant pH range, the 31P and 1H chemical shifts of TmDOTP5− are linear functions of temperature between 25 and 47°C. The results indicate that using TmDOTP5− can provide measurements of temperature in vivo that are significantly more accurate than methods based on water and fluorocarbon chemical shifts.
Correction for distortion of echo‐planar images used to calculate the apparent diffusion coefficient
Authors: Haselgrove John C. , Moore James R. 
Published: March 2007

AbstractAn algorithm for correcting the distortions that occur in diffusion‐weighted echo‐planar images due to the strong diffusion‐sensitizing gradients is presented. The dominant distortions may be considered to be only changes of scale coupled with a shear and linear translation in the phase‐encoding direction. It is then possible to correct for them by using an algorithm in which each line of the image in the phase‐encoding direction is considered in turn, with only one parameter (the scale) to be found by searching.
Reversible, reproducible reduction of brain water apparent diffusion coefficient by cortical electroshocks
Authors: Zhong Jianhui , Petroff Ognen A. C. , Pleban Lisa A. , Gore John C. , Prichard James W. 
Published: March 2007

AbstractRat brains were imaged after cortical electroshock pulse trains (1 ms pulses at 100 Hz) of varying durations (0.1–10 s), with diffusion‐weighted echo planar imaging sequences at 2.0 T. The apparent water diffusion coefficient (ADC) decreased after either single or repeat electroshock trains. ADC reductions were observed within 6 s after the first shock. The size of the affected area of the brain increased in subsequent images during the 1st min after a 10‐pulse (0.1 s) train, and also increased with the duration of electroshock trains. ADC reduction was reproducible in extent and time course after single 10‐shock trains and was reversible. In the affected pixels the mean ADC reduction was 4% for a single shock train (0.1 s), and 7–8% for trains repeated once a minute, independent of electroshock train duration. The results indicate that neuronal activity associated with electrostimulation may be monitored with water diffusion measurements, and they may be useful for measuring the severity of seizure activity in patients with medically intractable epilepsy.
Characterization and prediction of gradient acoustic noise in MR imagers
Authors: Hedeen Robert A. , Edelstein William A 
Published: March 2007

AbstractGradient acoustic noise has been measured and characterized for an epoxy‐potted, shielded gradient assembly in a 1.5 T MRI system. Noise levels vary by 10 dB or more as a function of longitudinal position in the scanner and reflect the pattern of forces applied to the gradient assembly. The noise level increases slightly (1–3 dB) with a patient in the scanner. The spectrum of the noise is similar (but not identical) to the spectrum of the input signal. A gradient‐pulse‐to‐acoustic‐noise transfer function was obtained by using a white noise voltage input to the gradient system. The transfer function enabled us to accurately predict acoustic noise output for a pulse sequence consisting of a series of trapezoidal pulses on a single axis and for a clinical fast spin echo sequence with gradients present on all three axes.
Multinuclear solid‐state nmr spectroscopy of envelopes from virgin and explanted silicone breast prostheses: An exploratory study
Authors: Picard Frédéric , Alikacem Nadir , Guidoin Robert , Auger Michèle 
Published: March 2007

AbstractThe structure and dynamics of silicone envelopes from virgin and explanted breast prostheses have been studied by 1H, 13C, and 29Si high‐resolution solid‐state NMR spectroscopy. The results indicate that this combination of techniques can be used to unambiguously identify the presence of methyltri‐fluoropropylsiloxane units and diphenylsiloxane units at concentrations of about 2.0% and 3.5%, respectively, in some of the envelopes. The presence of about 10% silica also was detected and a trace of lipids was found in the explanted silicone envelopes. We have also measured the proton T2 and T1 relaxation times of the envelopes to characterize the dynamics of the silicone in the envelopes.
Temporal lobe epilepsy: Correlation of proton magnetic resonance spectroscopy and 18F‐fluorodeoxyglucose positron emission tomography
Authors: Lu Dongfeng , Margouleff Claude , Rubin Eric , Labar Douglas , Schaul Neil , Ishikawa Tatsuya , Kazumata Ken , Antonini Angelo , Dhawan Vijay , Hyman Roger A. , Eidelberg David 
Published: March 2007

AbstractProton magnetic resonance spectroscopy (MRS) has demonstrated reduction of N‐acetylaspartate (NAA) in the epileptogenic temporal lobe. However, the correlation of NAA reduction with cerebral metabolic abnormalities is unknown in temporal lobe epilepsy (TLE). Proton MRS and 18F‐fluorodeoxyglucose positron emission tomography (FDG/PET) were used to study 12 unilateral TLE patients with medically intractable seizures and 26 age‐matched healthy volunteers. The epileptogenic temporal lobe of each patient was determined by both electroencephalography and FDG/PET. The NAA/choline‐plus‐creatine (NAA/(Cho+Cr)) ratio correlated significantly with the interictal glucose metabolism (r = 0.54, P 0.01) in 12 TLE patients. The mean NAA/(Cho + Cr) ratio in the epileptogenic temporal lobe was significantly less than that in the contralateral side (P < 0.01), and less than that in normal control temporal lobes (P < 0.0001). These results suggest that quantitative MRS abnormalities reflect underlying metabolic pathology in TLE.
Histological and 1H magnetic resonance spectroscopic imaging analysis of quinolinic acid‐induced damage to the rat striatum
Authors: Strauss Ian , Fernandez Erik J. , Williamson John M. , Bertram Edward H. , Lothmant Eric W. 
Published: March 2007

AbstractNAA has been described as a neuron‐specific compound. NAA levels as determined by magnetic resonance spectroscopic imaging (MRSI) have been used to determine degree of neuronal loss in several neurological diseases, but there has been limited work to document the accuracy and reliability of this technique. This study addresses this question quantitatively with histological analysis of cell viability and tissue shrinkage in quinolinic acid (QA)‐induced damage of the rat striatum compared with 1H MRSI measurement of N‐acetyl aspartate (NAA) as a noninvasive measure of neuronal loss. Both 'H MRSI and histology detect damage to the lesioned striatum; however, there are differences in the degree of damage as assessed by the two methods. Although partial‐volume effects and tissue shrinkage may decrease the sensitivity of MR to such damage, the sparing of axons by QA may be another important factor in the differences in assessment. These results indicate that further studies of NAA metabolism and its distribution within neurons are warranted.
In vivo measurement of T2 distributions and water contents in normal human brain
Authors: Whittall Kenneth P , Mackay Alex L. , Graeb Douglas A. , Nugent Robert A. , Li David K. B. , Paty Donald W. 
Published: March 2007

AbstractUsing a 32‐echo imaging pulse sequence, T2 relaxation decay curves were acquired from five white‐ and six gray‐matter brain structures outlined in 12 normal volunteers. The water contents of white and gray matter were 0.71 (0.01) and 0.83 (0.03) g/ml, respectively. All white‐matter structures had significantly higher myelin water percentages (signal percentage with T2 between 10 and 50 ms) than all gray‐matter structures. The range in geometric mean T2 of the main peak for both white and gray matter was from 70 to 86 ms. T2 distributions from the posterior internal capsules and splenium of the corpus callosum were significantly wider (width is related to water environment inhomogeneity) than those from any other white‐ or gray‐matter structures. Thus, quantitative measurement and analysis of T2 relaxation reveals differences in brain tissue water environments not discernible on conventional MR images. These differences may make short T2 components reliable markers for normal myelin.
Diffusion of solutes in agarose and alginate gels: 1H and 23Na PFGSE and 23Na TQF NMR studies
Authors: Lundberg Peter , Kuchel Philip W 
Published: March 2007

AbstractCells immobilized in gels experience potential metabolic restrictions in the form of reduced diffusion rates of metabolites and ions and their possible selective adsorption on the gel matrix. Diffusion and relaxation characteristics of common solutes in agarose and barium alginate gels were investigated at 37°C by using 1H PFGSE and 23Na TQF NMR spectroscopy. Glucose, glycine, alanine, lactate, sodium ions, and HDO were studied. There were no selective interactions between any of the metabolites and the gel materials but the diffusion coefficients were uniformly reduced. The effects of metabolite diffusion and utilization, in gel beads and threads containing cells, were simulated by using a reaction diffusion model incorporating the measured diffusion coefficients. Metabolism is expected to be very significantly limited by diffusion of solutes to and from the cells that are centrally located within gel threads or spheres of radius ∼2.0 mm, which is a cornmonly used size.
T1ρ of protein solutions at very low fields: Dependence on molecular weight, concentration, and structure
Authors: Virta Anette , Komu Markku , Kormano Martti 
Published: March 2007

AbstractThe effect of molecular weight, concentration, and structure on 1/T1ρ, the rotating frame relaxation rate, was investigated for several proteins using the on‐resonance spin‐lock technique, for locking fields B1 < 200 μT. The measured values of 1/T1ρ, were fitted to a simple theoretical model to obtain the dispersion curves 1/T1ρ(ω1) and the relaxation rate at zero B1 field, 1/T1ρ,(O). 1/T1ρ, was highly sensitive to the molecular weight, concentration, and structure of the protein. The amount of intra‐ and intermolecular hydrogen and disulfide bonds especially contributed to 1/T1ρ. In all samples, 1/T1ρ(O) was equal to 1/T2ρ measured at the main magnetic field Bo = 0.1 T, but at higher locking fields the dispersion curves mono‐tonically decreased. The results of this work indicate that a model considering the effective correlation time of molecular motions as the main determinant for 1/T1ρ relaxation in protein solutions is not valid at very low B1 fields. The underlying mechanism for the relaxation rate 1/T1ρ at B1 fields below 200 μT is discussed.
Estimation of water extraction fractions in rat brain using magnetic resonance measurement of perfusion with arterial spin labeling
Authors: Silva Afonso C. , Zhang Weiguo , Williams Donald S. , Koretsky Alan P , Koretsky Alan P 
Published: March 2007

AbstractThe model used for calculating perfusion by MRI techniques that use endogenous water as a tracer assumes that arterial water is a freely diffusible tracer. Evidence shows that this assumption is not valid in the brain at high blood flow rates, at which movement of water into and out of the microvasculature becomes limited by diffusion across the blood‐brain barrier. In this work, the arterial spin‐labeling technique is used to show that fraction of arterial water that is dependent on blood flow rate remains in the vasculature and does not exchange with brain tissue water. By using perfusion measurements without and with magnetization transfer (MT) effects, one can distinguish arterial label that exchanges into tissue because blood has much smaller MT than brain tissue. Using this technique, the extraction fraction for water is measured in the rat brain at various cerebral blood flow rates. At high flow rates (‐5 ml/g/min), the extraction fraction for water is found to be about 45% in rat brain. Disruption of the blood‐brain barrier with D‐mannitol caused an increase in the extraction fraction for water. It was possible to form an image related to the extraction fraction for water. The ability to estimate the amount of vascular water exchanging with tissue water by MRI may represent a noninvasive approach to detect the integrity of the blood‐brain barrier.
Measurement of strain exerted on blood vessel walls by double‐quantum‐filtered 2H NMR
Authors: Sharf Yehuda , Akselrod Solange , Navon Gil 
Published: March 2007

AbstractIn this study 2H double quantum‐filtered (DQF) NMR spectros‐copy of water molecules inside the blood vessel wall was used to monitor structural changes after the application of strain. This method was applied to a variety of bovine large blood vessels, including aorta, coronary artery, carotid artery, and vena cava. Inducing strain in coronary arteries by inflating them by air or water revealed dramatic changes in the 2H DQF spectral lineshapes. Uniaxial extensions of longitudinal and circumferential strips led to distinct spectral responses for the different blood vessels. While the spectral lineshapes for the coronary and the carotid arteries were very sensitive to longitudinal strain, they were not affected by circumferential stretching. For the aorta there were no changes in the line‐shape upon strain. For the vena cava a large effect of strain was found that was independent of the choice of elongation axis. The effect of elongation on the spectra was assigned to the outer layer, the adventitia, whereas spectral lineshapes that originated from inner layers, media and intima, were almost insensitive to strain. Angular dependence experiments proved the presence of macroscopic order in the strained state superimposed on a broad distribution of the local directors. Our results demonstrate the sensitivity of 2H DQF NMR as a probe for dynamic processes occurring on the microscopic scale in a molecular network. Key words: blood vessels; strain; double‐quantum; 2H NMR.
An fem approach for the characterization of the RF field homogeneity at high field
Authors: Guclu Cecil , Kashmar George , Nalcioglu Orhan , Hacinliyan Avadis 
Published: March 2007

AbstractHigh field magnetic resonance offers new opportunities because of its high SNR and better spectral resolution for MRI and MRS. However, new problems also emerge at high field. As the field strength increases, the wavelength in the tissue becomes shorter and comparable with the body dimensions. This perturbs the field and also causes standing waves within the patient as a result of the impedance mismatching at the tissue interfaces. Due to the Complexity of the boundary conditions and the solution of Maxwell's equations, an exact analytical calculation for a loaded RF resonator has not been possible. In this paper, we present a birdcage coil simulation study based on a 3D finite element method (FEM) model for the characterization of the field within the tissue. First, the accuracy of the FEM solutions is validated by the 2D analytical solutions at 64 and 223 MHz. In these solutions, the frequency dependence of the conductivity and permittivity is also taken into account. Then, a more realistic 3D model is studied. The results are compared with the experimental measurements. It is shown that the 3D model makes it possible to explore the effects of the end rings in the presence of a tissue sample inside the coil.
Unsupervised, automated segmentation of the normal brain using a multispectral relaxometric magnetic resonance approach
Authors: Alfano Bruno , Brunetti Arturo , Covelli Eugenio M. , Quarantelli Mario , Panico Maria Rosaria , Ciarmiello Andrea , Salvatore Marco 
Published: March 2007

AbstractThe purpose of this study was the development and testing of a method for unsupervised, automated brain segmentation. Two spin‐echo sequences were used to obtain relaxation rates and proton‐density maps from 1.5 T MR studies, with two axial data sets including the entire brain. Fifty normal subjects (age range, 16 to 76 years) were studied. A Three‐dimensional (3D) spectrum of the tissue voxels was used for automatic segmentation of gray matter (GM), white matter (WM), and cerebrospinal fluid (CSF) and for calculation of their volumes. Accuracy and reproducibility were tested with a three‐compartment phantom simulating GM, WM, and CSF. In the normal subjects, a significant decrease of GM fractional volume and increased CSF volume with age were observed (P < 0.0001), with no significant changes in WM. This multi‐spectral segmentation method permits reproducible, operator‐independent volumetric measurements.
A dual approach for minimizing false lesion classifications on magnetic resonance images
Authors: Bedell Barry J. , Narayana Ponnada A , Wolinsky Jerry S. 
Published: March 2007

AbstractSegmentation methods based on dual‐echo MR images are generally prone to significant false lesion classifications. We have minimized these false classifications by (1) improving the lesion‐to‐tissue contrast on MR images by developing a fast spin‐echo sequence that incorporates both cerebrospinal fluid signal attenuation and magnetization transfer contrast and (2) including information from MR flow images. Studies on patients with multiple sclerosis indicate that this dual approach to tissue segmentation reduces the volume of false lesion classifications by an average of 87%.
An analytical model of restricted diffusion in bovine optic nerve
Authors: Stanisz Greg J. , Wright Graham A. , Henkelman R. Mark , Szafer Aaron 
Published: March 2007

AbstractAn analytical model of restricted diffusion in bovine optic nerve is presented. The nerve tissue model is composed of two different objects: prolate ellipsoids (axons) and spheres (glial cells) surrounded by partially permeable membranes. The free diffusion coefficients of intracellular and extra cellular water may differ. Analytical formulas for signal loss due to diffusion in the pulsed gradient spin echo (PGSE) experiment for this tissue model are derived. The model is fitted to experimental data for bovine optic nerve. The obtained model parameters are shown to be reasonable. The model describes all of the characteristics of the PGSE data: anisotropy, upward curvature of decay curves, and diffusion time dependence. The validity and sensitivity of the model are also discussed.