bluuebunny/crossref_metadata_2025_split huggingface dataset:
Turbulent Fluctuation Velocity: The Most Significant Determinant of Signal Loss in Stenotic Vessels
Authors: Oshinski John N. , Ku David N. , Pettigrew Roderic I. 
Published: March 2007

AbstractStudies of flow in a 90%‐stenosis phantom were conducted to elucidate the parameters and mechanisms responsible for signal loss in MR angiographic images. The studies independently evaluated the effect of velocity, Reynolds number, turbulent fluctuation velocity, and turbulence intensity on the amount of post‐stenotic signal loss. Results suggested that the magnitude of the turbulent fluctuation velocity, not merely the presence of turbulence or the intensity of turbulence, was the parameter that determined the extent of the signal loss. The study suggests that future flow phantom studies should be conducted with fluids having physiologic velocities and viscosities to obtain accurate levels of turbulent fluctuation velocities and hence reproduce results of in‐vivo signal‐loss patterns. The mechanism for signal loss is that the temporal and spatial variations of the turbulent fluctuation velocity cause a range of phases to be present within a voxel. Examination of the theoretical aspects of fluid turbulence suggest that shortening gradient durations and imaging during diastole may help reduce signal loss.
Dual‐Echo Interleaved Echo‐Planar Imaging of the Brain
Authors: Slavin Glenn S. , Butts Kim , Rydberg John N. , Jack Clifford R. , Riederer Stephen J. 
Published: March 2007

AbstractAn interleaved echo‐planar imaging (EPI) technique is described that provides images from 20 sections of the brain at two echo times (27 and 84 ms) in 1:05. Six echoes per image per repetition are collected in 24 repetitions of the pulse sequence. MR images of the brain obtained from five volunteers using the dual‐echo EPI sequence, fast spin‐echo (FSE), and conventional dual‐echo spin‐echo were evaluated qualitatively for diagnostic use and quantitatively for relative signal‐to‐noise ratio (SNR), contrast, and contrast‐to‐noise ratios (CNR).
Active Acoustic Screening: Reduction of Noise in Gradient Coils by Lorentz Force Balancing
Authors: Mansfield Peter , Chapman Barry L. W. , Bowtell Richard , Glover Paul , Coxon Ron , Harvey Paul R. 
Published: March 2007

AbstractWe have designed and constructed a quiet gradient set with restricted access for the combined purposes of evaluating the principles of active acoustic screening, recently introduced by Mansfield, Glover, and Bowtell, and for EPI studies of the head at 3.0 T. The design utilizes the return paths of the conductors in a closed arc loop arrangement to eliminate net Lorentz forces thereby attenuating acoustic noise especially at low frequency. This design should significantly reduce the dangers to patients of high noise levels, especially in high field magnetic resonance imaging systems.
Hybrid Three Dimensional (1D‐Hadamard, 2D‐Chemical Shift Imaging) Phosphorus Localized Spectroscopy of Phantom and Human Brain
Authors: Gonen Oded , Hu Jiani , Stoyanova Radka , Leigh John S. , Goelman Gadi , Brown Truman R. 
Published: March 2007

AbstractA hybrid of two localized spectroscopy techniques, chemical shift imaging (CSI) and Hadamard spectroscopic imaging (HSI), is used to obtain an array of 16 x 16 x 4 (3 x3 x 3 cm3 voxels) proton‐decoupled phosphorus (31P) spectra of human brain. For equal spatial resolution, this organ's oblate shape requires fewer axial than coronal or sagittal slices. These different spatial requirements are well suited to 1D, 4th order, transverse HSI in the axial direction, combined with 2D 16 x 16 CSI in the other two orientations. The reduced localization matrix (16 x 16 x 4 over just the brain versus a cubic‐16 x 16 x 16 matrix of equal resolution, over the entire head) may proportionally shorten data acquisition if the voxel size is not signal‐to‐noise limited. In addition, the use of Hadamard encoding can improve the intervoxel spectral isolation.
A Single‐Step Method for Estimation of Local Cerebral Blood Volume from Susceptibility Contrast MRI Images
Authors: Bahn Mark M. 
Published: March 2007

AbstractA new single‐step method is described for estimation of local cerebral blood volume (CBV) from multiple rapidly acquired T2‐ or T2*‐weighted MRI images after bolus administration of a susceptibility contrast agent. This method improves on existing methods in three ways. First, the method includes the baseline scan intensity value as a fitted parameter. Second, the model is fitted directly to the original scan intensity values instead of to transformed concentration curves, avoiding the propagation of errors that occurs in the transformation. Third, the equations are reparameterized with CBV fitted directly as a model parameter. Parameter estimation methods are compared by Monte Carlo simulation. The direct method described here yields more precise parameter estimates, with smaller mean absolute deviations from the true parameter values, than the existing method when compared using simulated data. Implementation is discussed and numerical evaluation of the Digamma or Psi function is described.
Continuous Update with Random Encoding (CURE): A New Strategy for Dynamic Imaging
Authors: Parrish Todd , Hu Xiaoping 
Published: March 2007

AbstractAlthough dynamic imaging is presently used for various applications, it is still limited by the temporal resolution. In this paper, we present a new technique that uses a random phase‐encoding strategy to facilitate faster and smoother update of images and to improve the temporal resolution in dynamic studies. The technique was implemented on a conventional clinical scanner and demonstrated with various in vivo studies. Technical details, simulations, and experimental results are described. Images from experimental studies indicate that the new technique is robust in generating dynamic images and can be potentially utilized for clinical applications.
Time Resolved Flow Quantification with MRI Using Phase Methods: A Linear Systems Approach
Authors: Peeters Frank , Luypaert Robert , Eisendrath Henri , Osteaux Michel 
Published: March 2007

AbstractPhase‐related unsteady (pulsatile) flow effects in MRI have been studied by means of linear response theory. These flow effects can be described in the frequency domain: the influence of the gradients on the phase shift is described by a transfer function, the spectrum of the gradient being the determining factor. An analysis of this transfer function is shown to provide information about the process of flow encoding: instant of encoding, induced distortions and how they are related to the gradient waveform. The connection with the traditional description in terms of the gradient moment expansion has also been investigated and clarified. This approach was applied to study the response of two time‐resolved flow quantification techniques (Fourier flow method and phase mapping) by analyzing their amplitude and phase transfer functions. By simulation it is shown that a better interpretation of the measured velocity waveform is obtained and that Fourier analysis in combination with a correction by the inverse transfer function results in an accurate reconstruction of the velocity waveform studied.
Increase of GPC Levels in Cultured Mammalian Cells during Acidosis. A 31P MR Spectroscopy Study Using a Continuous Bioreactor System
Authors: Galons Jean‐Phillipe , Job Constantin , Gillies Robert J. 
Published: March 2007

AbstractThe purpose of this study was to study the metabolic events during a slow acidosis in three different cell lines by combining 31P magnetic resonance spectroscopy and hollow fiber bioreactor technology. The rate of change in intracellular pH, glycerophosphorylcholine (GPC), phophorylcholine (PCho), and nucleoside‐triphosphate (NTP) levels were measured during 8 h of acidosis and 16 h of recovery in EPO, EAT, and RN1a cells, three cultured mammalians cell lines. Our results show a significant increase in GPC levels to 330 ± 21, 540 ± 25, and 220 ± 21% of their initial value correlated to a decrease of PCho levels to 57 ± 14, 58 ± 17 and 45 ± 15% of their initial value in EAT, RN1a, and EPO cells, respectively. These changes are discussed in terms of perturbation of energetic metabolism in cells undergoing a slow acidosis.
Three‐Dimensional Spectroscopic Imaging with Time‐Varying Gradients
Authors: Adalsteinsson Elfar , Irarrazabal Pablo , Spielman Daniel M. , Macovski Albert 
Published: March 2007

AbstractA spectroscopic imaging sequence with a time‐varying readout gradient in the slice selection direction is used to image multiple contiguous slices. For a given voxel size, the imaging time and signal‐to‐noise ratio of the three‐dimensional spectroscopic sequence are the same as for a single slice acquisition without the oscillating readout gradient. The data reconstruction employs a gridding algorithm in two dimensions to interpolate the nonuniformly sampled data onto a Cartesian grid, and a fast Fourier transform in four dimensions: three spatial dimensions and the spectral dimension. The method is demonstrated by in vivo imaging of NAA in human brain at 1.5 T with 10 slices of 16 x 16 pixels spectroscopic images acquired in a total scan time of 17 min.
Interpretation of 31P MRS Spectra in Determining Intracellular Free Magnesium and Potassium Ion Concentrations
Authors: Golding Elke M. , Golding Raymund M. 
Published: March 2007

AbstractThe current paper develops a simple mathematical model for the determination of the concentration of ionic species using the chemical shifts of ATP from phosphorus magnetic resonance spectroscopy (31P MRS) spectra. An iterative method is used to evaluate the equilibrium constants for a range of Mg/ATP equilibria. Initially, an expression for free magnesium concentration is determined by consideration of four equilibria. We then demonstrate how any number of equilibria may be included using this simplified analysis, by incorporation of pseudo equilibrium constants. This approach may be applied to the interpretation of both ex vivo and, more importantly, in vivo MRS spectra. Furthermore, the authors suggest that the commonly used equation for determining free intracellular magnesium concentration as presented by Gupta and colleagues is an oversimplification, particularly under conditions of fluctuating pH.
Action of Compression and Cations on the Proton and Deuterium Relaxation in Cartilage
Authors: Lüsse Steffen , Knauss Robert , Werner Annett , Gründer Wilfried , Arnold Klaus 
Published: March 2007

AbstractIn this paper, investigations are described on the influence of osmotic pressures and of varying cation concentrations on water relaxation times in cartilage (pig articular cartilage and bovine nasal cartilage). Both water content and relaxation times decrease strongly with increasing osmotic pressure. This relaxation behavior can be explained in terms of a fast chemical exchange between unbound and bound water. Na+ does not influence water content or relaxation times, whereas Ca2+ causes a small reduction in these parameters.
Magnetization Transfer and T2 Relaxation Components in Tissue
Authors: Harrison Rodney , Bronskill Michael J. , Mark Henkelman R. 
Published: March 2007

AbstractT2 relaxation makes an important contribution to tissue contrast in magnetic resonance (MR) imaging. Many tissues are known to exhibit multicomponent T2 relaxation that suggests some compartmental segregation of mobile protons on a T2 timescale. Magnetization transfer (MT) is another relaxation mechanism that can be used to produce tissue contrast in MR imaging. The MT process depends strongly on water‐macromolecular interactions. To investigate the relationship between multicomponent T2 relaxation and the MT process, multiecho T2 measurements have been combined with MT measurements for freshly excised samples of cardiac muscle, striated muscle, and white matter. For muscle, short T2 components show greater MT than long T2 components, consistent with the belief that they represent distinct water environments. For white matter, quantitative MT measurements were identical for the two major T2 components, apparently because of exchange between the T2 compartments on a timescale characteristic of the MT experiment. Implications for accurate modeling of MT in tissue and the use of MT for MR image contrast are discussed.
An Algorithm for MR Angiography Image Enhancement
Authors: Chen Haiguang , Hale James 
Published: March 2007

AbstractThis study describes a three‐dimensional (3D) filtering algorithm for magnetic resonance angiography (MRA) image enhancement. Based on the properties of 3D MRA data, the algorithm uses the dispersion range of the 13 outputs of a 3D directional low‐pass filter bank to identify the vessel structure and control the processing of MRA data. Vascular structures in the data are preserved while the components of static tissues and isolated noise impulses are reduced by the processing. As a result, the small vessels have been enhanced and visibility of vessels in the projected image has been improved.
Improved Assessment of Significant Activation in Functional Magnetic Resonance Imaging (fMRI): Use of a Cluster‐Size Threshold
Authors: Forman Steven D. , Cohen Jonathan D. , Fitzgerald Mark , Eddy William F. , Mintun Mark A. , Noll Douglas C. 
Published: March 2007

AbstractThe typical functional magnetic resonance (fMRI) study presents a formidable problem of multiple statistical comparisons (i.e, > 10,000 in a 128 x 128 image). To protect against false positives, investigators have typically relied on decreasing the per pixel false positive probability. This approach incurs an inevitable loss of power to detect statistically significant activity. An alternative approach, which relies on the assumption that areas of true neural activity will tend to stimulate signal changes over contiguous pixels, is presented. If one knows the probability distribution of such cluster sizes as a function of per pixel false positive probability, one can use cluster‐size thresholds independently to reject false positives. Both Monte Carlo simulations and fMRI studies of human subjects have been used to verify that this approach can improve statistical power by as much as fivefold over techniques that rely solely on adjusting per pixel false positive probabilities.
Navigated Diffusion Imaging of Normal and Ischemic Human Brain
Authors: De Crespigny Alex J. , Marks Michael P. , Enzmann Dieter R. , Moseley Michael E. 
Published: March 2007

AbstractThe principal barrier to clinical application of diffusion‐weighted MR imaging is the severe image degradation caused by patient motion. One way to compensate for motion effects is the use of a “navigator echo” phase correction scheme. In this work, a modification of this technique is introduced, in which the phase correction step is performed in the frequency domain (i.e., after the readout Fourier transform). This significantly improves the robustness of the navigator echo approach and, when combined with cardiac gating, allows diagnostic quality diffusion‐weighted images of the brain to be routinely obtained on standard clinical scanner hardware. The technique was evaluated in phantom studies and in 23 humans (3 normal volunteers and 20 patients). Diffusion anisotropy and apparent diffusion coefficient maps were generated from the image data and showed decreased apparent diffusion in acute stroke lesions and, in several cases, increased apparent diffusion in chronic stroke lesions.
Functional Mapping of the Human Somatosensory Cortex with Echo‐Planar MRI
Authors: Sakai Kuniyoshi , Watanabe Eiju , Onodera Yukari , Itagaki Hiroyuki , Yamamoto Etsuji , Koizumi Hideaki , Miyashita Yasushi 
Published: March 2007

AbstractThe somatotopical organization of the human somatosensory cortex was analyzed with echo‐planar imaging at 1.5 Tesla, utilizing deoxyhemoglobin as an endogenous contrast medium. Scrubbing stimulation at a frequency of 3 Hz was applied to one of three cutaneous areas: toes, fingertips, and tongue tip. Parasagittal echo‐planar slices were obtained every 2 s. We found focal bands of increased signal intensity (4% on average) during the stimulation, with a rise time of 2–6 s. These activated bands were located on the contralateral postcentral gyrus. The cortical responses from the three stimulation sites were anatomically distinct and organized medially‐to‐laterally in the order of toes, fingertips, and tongue tip.
1H MR Spectroscopy in Patients with Metastatic Brain Tumors: A Multicenter Study
Authors: Sijens Paul E. , Knopp Michael V. , Brunetti Arturo , Wicklow Karsten , Alfano Bruno , Bachert Peter , Sanders John A. , Stillman Arthur E. , Kett Hans , Sauter Rolf , Oudkerk Matthijs 
Published: March 2007

AbstractIn a cooperative study involving six clinical MR centers, localized 1H MR spectroscopy was used to characterize untreated metastatic brain tumors (40 cases, 45 lesions). Cubic volumes (3.4 or 8 cm3) filled for more than 50% by metastatic brain tissue were examined by single‐voxel double spin echo MRS, by using chemical shift selective imaging (CHESS) pulses for water suppression and TE = 135 ms. Choline (Cho), creatine (Cr) and N‐acetyl aspartate (NAA) levels in brain metastases of mammary carcinoma (n = 13), lung cancer (n = 11) and melanoma (n = 10) were similar. Metastasis NAA/Cho signal intensity ratio varied between 0.00 and 1.17, compared with 2.68 ± 0.56 (SD) in lobus occipitalis and 1.94 ± 0.63 in corpus nuclei caudati region (P < 0.0001, both). 1H MR spectroscopy, although not suited to recognize the primary tumor of metastases, could serve as a clinical test for excluding (metastatic) tumor as cause of solitary focal brain disorders that are hard to diagnose with current imaging methods.
Increased MR Image Contrast by Correlation of Tissue Relaxation Properties
Authors: Mastikhin Igor V. , Lukzen Nikita N. , Zelentzov Eugeny L. , Sagdeev Renad Z. 
Published: March 2007

AbstractAn image postprocessing technique that is based on the relaxation properties of tissues and that can produce MR images with increased contrast is proposed. The technique involves no a priori assumptions concerning the form of the relaxation decay. An arbitrary number of postprocessed images, each emphasizing a selected tissue type, is obtained from the original images of a multiecho acquisition. It is shown with examples that the technique allows more complete utilization of relaxation information for tissue differentiation.
The intravascular contribution to fmri signal change: monte carlo modeling and diffusion‐weighted studies in vivo
Authors: Boxerman Jerrold L. , Bandettini Peter A. , Kwong Kenneth K. , Baker John R. , Davis Timothy L. , Rosen Bruce R. , Weisskoff Robert M. 
Published: March 2007

AbstractUnderstanding the relationship between fMRI signal changes and activated cortex is paramount to successful mapping of neuronal activity. To this end, the relative extravascular and intravascular contribution to fMRI signal change from capillaries (localized), venules (less localized) and macrovessels (remote, draining veins) must be determined. In this work, the authors assessed both the extravascular and intravascular contribution to blood oxygenation level‐dependent gradient echo signal change at 1.5 T by using a Monte Carlo model for susceptibility‐based contrast in conjunction with a physiological model for neuronal activation‐induced changes in oxygenation and vascular volume fraction. The authors compared our Model results with experimental fMRI signal changes with and without velocity sensitization via bipolar gradients to null the intravascular signal. The model and experimental results are in agreement and suggest that the intravascular spins account for the majority of fMRI signal change on T2*‐weighted images at 1.5 T.
A robust single‐shot partial sampling scheme
Authors: Glover Paul , Tokarczuk Pawel F. , Bowtell Richard W. 
Published: March 2007

AbstractA method of reducing the amount of data required to reconstruct an image is described. In this scheme, fully sampled low spatial frequency data are acquired up to a given cutoff frequency and above this point, only alternate lines are sampled. Two images are produced, one of low definition and one of high definition but aliased. The proposed algorithm unwraps the aliased data, which are then used to enhance the low pass image, yielding a best estimate of the true image. The reduced sampling technique is shown to afford biological images that are almost indistinguishable from those obtained from a complete data set.
A phased array coil for human cardiac imaging
Authors: Constantinides Chris D. , Westgate Charles R. , O'Dell Walter G. , Zerhouni Elias A. , Mcveigh Elliot R. 
Published: March 2007

AbstractA prototype cardiac phased array receiver coil was constructed that comprised a cylindrical array and a separate planar array. Both arrays had two coil loops with the same coil dimensions. Data acquisition with the cylindrical array placed on the human chest, and the planar array placed under the back, yielded an overall enhancement of the signal‐to‐noise ratio (SNR) over the entire heart by a factor of 1.1–2.85 over a commercially available flexible coil and a commercially available four‐loop planar phased array coil. This improvement in SNR can be exploited in cardiac imaging to increase the spatial resolution and reduce the image acquisition time.
Optimized isotropic diffusion weighting
Authors: Wong Eric C. , Cox Robert W. , Song Allen W. 
Published: March 2007

AbstractThe authors introduce several sets of time‐efficient gradient waveforms for applying isotropic diffusion weighting in NMR experiments. This creates signal attenuation that depends on the trace of the diffusion tensor and is therefore rotationally invariant. Numerical methods for the calculation of such gradient sets are outlined, and results are shown for isotropic and anisotropic gradient hardware and first order flow moment nulled diffusion weighting gradients. Preliminary experimental results from the human brain validate this new technique.
NMR relaxation rates and blood oxygenation level
Authors: Meyer M.‐E , Yu Olivier , Eclancher Bernard , Grucker Daniel , Chambron Jacques 
Published: March 2007

AbstractA linear relationship between the proton relaxation rates and the fraction of deoxyhemoglobin of circulating whole blood (fHb) has been established in vitro at 4.7 T. These results have been interpreted on the basis of the additivity of relaxation rates. The slope of the lines of transverse relaxation rates (R2) versus fHb was found to increase with interpulse delay in Carr‐Purcell Meiboom‐Gill (CPMG) experiments. The Luz‐Mei‐boom relation applied to this interpulse delay dependence of R2 suggests a two‐site chemical exchange rather than a diffusion mechanism. The 1‐ms water proton exchange time derived from these observations has been interpreted in terms of exchange between exchangeable protons close to the paramagnetic center of hemoglobin and protons of bulk water.