bluuebunny/crossref_metadata_2025_split huggingface dataset:
Correction of off resonance‐related distortion in echo‐planar imaging using EPI‐based field maps
Authors: Reber Paul J. , Wong Eric C. , Buxton Richard B. , Frank Lawrence R. 
Published: March 2007

AbstractWe present, here, a simple method for measurement and correction of off‐resonance related geometric distortion in echo‐planar imaging (EP1). This method uses high signal‐to‐noise ratio (SNR) EPI‐based field maps, rapidly acquired using a series of gradient recalled images collected across a range of TE values. This field map is distorted in the same manner as the EPI images to be unwarped, providing a direct look‐up table for the correct location of each pixel of data. This method adds very little scan time and is robust and easy to implement.
Gradient, high‐resolution, magic angle spinning 1H nuclear magnetic resonance spectroscopy of intact cells
Authors: Weybright Patrick , Millis Kevin , Campbell Natalee , Cory David G. , Singer Samuel 
Published: March 2007

AbstractThe application of new gradient, high‐resolution, magic angle spinning (MAS) 1H nuclear magnetic resonance (NMR) spectroscopy to the study of intact undifferentiated and differentiated NIH 3T3 F442A cells demonstrated improved spectral resolution and sensitivity compared with static studies. MAS of cells permits the detection and quantitation of many cellular metabolites that are not clearly resolved in nonspinning measurements and provides an improved visibility of phospholipids. Gradient, MAS enables the use of diffusion weighting for compartment assignment and the determination of mobility for many metabolites which are incompletely resolved using static techniques. The smaller, undifferentiated preadipocytes show no microscopic evidence of cell lysis after 2 h of MAS at 3.5 kHz and 82% of these cells remain viable by trypan blue exclusion. In contrast, 15‐19% of the larger, lipid‐laden differentiated adipocytes were found to suffer some degree of cell lysis with MAS. This new method is an attractive alternative to either nonspinning perfusion or extraction techniques for NMR studies of cells.
Self‐navigated spiral fMRI: Interleaved versus single‐shot
Authors: Glover Gary H. , Lai Song 
Published: March 2007

AbstractThis study compares the measured activation volumes in motor cortex as well as the fluctuation noise and off‐resonance characteristics for 1‐, 2‐, and 4‐shot spiral gradient‐recalled echo blood oxygen level dependent contrast functional magnetic resonance imaging (fMRI) acquisitions, under conditions of constant resolution and scan time and with two readout durations. Reconstructions were made with and without self‐navigator correction. It was found that the navigator correction provided a 50% reduction in image fluctuation noise with 4‐shot acquisitions, and that multishot acquisitions perform as well as single‐shot techniques when self‐navigation is employed. An analysis of blurring showed that off‐resonance Af causes blurring when Δf> 1/(2*Tad), whereTadis the readout duration. Off‐resonance effects were readily corrected during reconstruction with retrospective linear shim, even with the longer readout duration needed for single‐shot methods. With navigator and shim correction, single‐shot and multishot spiral methods are highly effective for fMRI acquisitions.
Correction of B1 inhomogeneities using echo‐planar imaging of water
Authors: Thulborn Keith R. , Boada Fernando E. , Shen Gary X. , Christensen James D. , Reese Timothy G. 
Published: March 2007

AbstractReliable interpretation of the MR signal intensity over the FOV of an image must consider the spatial heterogeneity of instrumental sensitivity. A major source of such variation is the nonuniformity of the B1 magnetic field of the radiofrequency coil. This heterogeneity can be minimized by coil design but is exaggerated by surface coils, which are used to maximize the signal‐to‐noise ratio for some applications. This paper describes a rapid method for mapping the B1 field over the sample of interest, using 1H echo‐planar imaging, to correct for B1 distortions. The method applies to 1H imaging and has been extended to non‐1H imaging by using dual‐frequency coils in which the B., distributions are matched for the 1H frequency and the frequency of interest. The approach is demonstrated in phantoms, animals, and humans and for sodium imaging.
MR‐microscopic visualization of anisotropic internal cartilage structures using the magic angle technique
Authors: Gründer Wilfried , Wagner Manuela , Werner Annett 
Published: March 2007

AbstractNMR microscopic studies of articular cartilage at 7.1 T are presented. Using a special experimental design, T2‐weighted spin‐echo images of cartilage‐bone plugs were taken under variable angles with respect to the static magnetic field Bo to visualize the angular‐dependent representation of internal matrix structures mediated by the collagen network arrangement. To quantify the observed orientational effect in the MR images, exact measurements of the transverse relaxation time T2 were taken using the CPMG sequence. The NMR experiments show the strong influence of the cartilage orientation with respect to the static magnetic field on the inhomogeneous appearance of the articular cartilage in the MR image. Additionally performed polarization light microscopic investigations demonstrate the direct relation between the oriented collagenous structures and the anisotropic regions observed in the MR images. A simple cartilage matrix model derived from the experimental findings is proposed, and consequences for the clinical assessment of the articular joint are discussed.
Removal of local field gradient artifacts in T2*‐weighted images at high fields by gradient‐echo slice excitation profile imaging
Authors: Yang Qing X. , Williams Gerald D. , Demeure Roger J. , Mosher Timothy J. , Smith Michael B. 
Published: March 2007

AbstractDevelopment of high magnetic field MRI techniques is hampered by the significant artifacts produced by B0 field inhomogeneities in the excited slices. A technique, gradient‐echo slice excitation profile imaging (GESEPI), is presented for recovering the signal lost caused by intravoxel phase dispersion in T2*‐weighted images. This technique superimposes an incremental gradient offset on the slice refocusing gradient to sample Jr‐space over the full range of spatial frequencies of the excitation profile. A third Fourier transform of the initial two‐dimensional image set generates an image set in which the artifacts produced by the low‐order B0 inhomogeneity field gradients in the sample are separated and removed from the high‐order microscopic field gradients responsible for T2* contrast. Application to high field brain imaging, at 3.0 T for human and at 9.4 T for immature rat imaging demonstrates the significant improvement in quality of the T2*‐weighted contrast images.
Quantitation of intrinsic magnetic susceptibility‐related effects in a tissue matrix. Phantom study
Authors: Yablonskiy Dmitriy A. 
Published: March 2007

AbstractA theoretical background and experimental method that allows a separation of intrinsic, tissue‐matrix‐specific magnetic‐field inhomogeneity effects from both macroscopic (large compared with voxel dimensions) and microscopic (on the order of molecular dimensions) inhomogeneities is proposed. Such separation allows one to take full advantage of these tissue‐matrix‐specific magnetic field inhomogeneity effects to extract information about tissue structure. A method to measure the volume fraction occupied by the susceptibility‐perturbing component in a tissue matrix, the R2′ relaxation rate constant, and the susceptibility difference between the bulk component and the susceptibility‐perturbing component in a tissue matrix has been developed and tested on phantoms. This method offers the potential to assess a variety of tissue parameters, including cerebral blood volume, blood volume and blood oxygenation‐level changes in functional MRI, the structure of trabecular bone, and other physiologically important issues.
Quantification of phosphorus metabolites from chemical shift imaging spectra with corrections for point spread effects and B1 inhomogeneity
Authors: Murphy Joseph , Jiang Hong , Stoyanova Radka , Brown Truman R. 
Published: March 2007

AbstractA method is described for quantifying phosphorus metabolites in tissue using spectra localized with surface coils and chemical shift imaging (CSI) and assuming that metabolites are uniformly distributed within a well‐defined volume. An analytical expression is developed that yields a single numerical correction factor that takes into account the excitation and receiver profiles of the coil, T1 saturation, and point spread effects associated with Fourier transformation of CSI data. An external phosphorus standard is used to calibrate instrument gain and the B1 profile of the coil. For spherical samples, point spread effects can modulate the signal intensities of three‐dimensional CSI spectra from ‐32% to +54%, depending on the voxel size. Measurements of phantoms of known concentrations showed systematic variations of ± 10% and random errors of ± 5%. We have used this method to measure the concentration of phosphocreatine in the thigh muscle of normal volunteers.
Boron‐11 NMR of borocaptate: Relaxation and in vivo detection in melanoma‐bearing mice
Authors: Bendel Peter , Frantz Anatol , Zilberstein Judith , Kabalka George W. , Salomon Yoram 
Published: March 2007

AbstractLongitudinal and transverse relaxation rates for the 11B resonances in sodium borocaptate (BSH) at varying concentrations were measured in undiluted horse serum in a 4.7 Tesla field. The results could be fit by a model that assumes fast exchange of the BSH molecule between a free and a bound state, using values of 0.77 ± 0.7 MHz for the 11B quadrupole coupling constant and (6.3 ± 0.9) × 10−9 s for the rotational correlation time in the bound state. These results were used as a basis for assessing the requirements and limitations of quantitative determination of BSH concentrations in vivo, using 11B NMR. Surface coil 11B NMR spectroscopy was performed on a total of 14 mice injected with BSH. Some of the animals (n = 9) had implanted M2R melanoma tumors grown to various sizes in the rear thigh, in which case the surface coil was placed against the tumor, whereas for the other animals (without tumor), the coil was placed against the rear thigh muscle. NMR spectra were acquired under fully relaxed conditions. The spectra were quantitated by peak integration; apparent absolute BSH concentrations were derived by comparison with spectra from a phantom with known BSH concentration, using extrapolation of the time‐domain data to zero preacquisition delay. The results indicate significantly higher 11B BSH signal intensities in tumors, compared with muscle tissue, whereas the uptake and clearance kinetics were similar.
Spiral scanning with anisotropic field of view
Authors: King Kevin F. 
Published: March 2007

AbstractSpiral scanning has been used to achieve much shorter scan times than conventional techniques for a wide range of applications. The major drawback with spiral scans is blurring from off‐resonant spins, which is proportional to the readout time. Blurring limits maximal spatial resolution and minimal scan time potentially achievable with spiral scanning. Anisotropic field of view is used in conventional scanning to improve image quality by matching /c‐space trajectory to object characteristics. Anisotropic field of view improves spatial resolution in spiral scanning without increasing scan time or blurring. The resolution improvement results from increased maximal k‐space radius allowed by the lower field of view. A field of view reduction by a factor of 2 in one direction provides up to 60% resolution improvement in that direction. Reduced SNR also results from non‐uniform /c‐space sampling.
Ultimate intrinsic signal‐to‐noise ratio in MRI
Authors: Ocali Ogan , Atalar Ergin 
Published: March 2007

AbstractA method to calculate the ultimate intrinsic signal‐to‐noise ratio (SNR) in a magnetic resonance experiment for a point inside an arbitrarily shaped object is presented. The ultimate intrinsic SNR is determined by body noise. A solution is obtained by optimizing the electromagnetic field to minimize total power deposition while maintaining a constant right‐hand circularly polarized component of the magnetic field at the point of interest. A numerical approximation for the optimal field is found by assuming a superposition of a large number of plane waves. This simulation allowed estimation of the ultimate intrinsic SNR attainable in a human torso model. The performance of six coil configurations was evaluated by comparing the SNR of images obtained by the coils with the ultimate values. In addition, the behavior of ultimate intrinsic SNR was investigated as a function of main field strength. It was found that the ultimate intrinsic SNR increases better than linearly with the main magnetic field up to 10 T for our model. It was observed that for field strengths of 4 T or higher, focusing is required to reach the ultimate intrinsic SNR.
Experimental hypoxemic hypoxia: Changes in R2* of brain parenchyma accurately reflect the combined effects of changes in arterial and cerebral venous oxygen saturation
Authors: Lin Weili , Paczynski Richard P. , Celik Azim , Kuppusamy Karthikeyan , Hsu Chung Y. , Powers William J. 
Published: March 2007

AbstractA two‐dimensionalT2*‐weighted gradient‐echo sequence was used to image the rat brain before and during graded hypoxemia. Changes inR2* (δR2*) with respect to the control state were calculated for brain parenchyma and were compared with changes in hemoglobin saturation measured from both arterial and jugular venous blood samples. δR2* was first correlated with the changes in arterial (δYa) and venous (δYv) hemoglobin saturations individually. Although a general trend toward a linear relationship with δR2* was observed for both δYa and δYv, neither alone was strong (correlation coefficientsr= 0.71 and 0.75 for δYa and δYv, respectively, and standard errors of the regression (SER) = 0.52 and 0.48 for δYa and δYv, respectively). However, when an “effective” cerebral blood hemoglobin saturation change (δYb) was constructed that takes into account the approximate weighting of the contributions from the arterial and venous phases of the circulation (δYb = 0.75 × δYv + 0.25 × δYa), a stronger correlation with δR2* was obtained and there was less variance (r= 0.87 and SER = 0.35). It is concluded that an appropriate weighting of the contributions of arterial and venous phases of the circulation must be taken into account in modeling the volume susceptibility effects of deoxyhemoglobin onR2* of brain parenchyma. In this way, a more accurate relationship between δR2* and δYb can be obtained.
Elasticity reconstructive imaging by means of stimulated echo MRI
Authors: Chenevert Thomas L. , Skovoroda Andrei R. , O′donnell Matthew , Emelianov Stanislav Y. 
Published: March 2007

AbstractA method is introduced to measure internal mechanical displacement and strain by means of MRI. Such measurements are needed to reconstruct an image of the elastic Young's modulus. A stimulated echo acquisition sequence with additional gradient pulses encodes internal displacements in response to an externally applied differential deformation. The sequence provides an accurate measure of static displacement by limiting the mechanical transitions to the mixing period of the simulated echo. Elasticity reconstruction involves definition of a region of interest having uniform Young's modulus along its boundary and subsequent solution of the discretized elasticity equilibrium equations. Data acquisition and reconstruction were performed on a urethane rubber phantom of known elastic properties and an ex vivo canine kidney phantom using <2% differential deformation. Regional elastic properties are well represented on Young's modulus images. The long‐term objective of this work is to provide a means for remote palpation and elasticity quantitation in deep tissues otherwise inaccessible to manual palpation.
Reproducibility of intracranial volume measurement by unsupervised multispectral brain segmentation
Authors: Alfano Bruno , Quarantelli Mario , Brunetti Arturo , Larobina Michele , Covelli Eugenio M. , Tedeschi Enrico , Salvatore Marco 
Published: March 2007

AbstractTo assess the inter‐study variability of a recently published unsupervised segmentation method (Magn. Reson. Med. 1997;37:84–93), 14 brain MR studies were performed in five normal subjects. Standard deviations for absolute and fractional volumes of intracranial compartments, which reflect the experimental variability, were smaller than 16.5 ml and 1.1%, respectively. By comparing the experimental component of the variability with the variability observed in our reference database, an estimate of the biological variability of the intracranial fractional volumes in the database population was obtained.
Imaging asynchronous mechanical activation of the paced heart with tagged MRI
Authors: McVeigh Elliot R. , Prinzen Frits W. , Wyman Bradley T. , Tsitlik Joshua E. , Halperin Henry R. , Hunter William C. 
Published: March 2007

AbstractA method for imaging the rapid temporal‐spatial evolution of myocardial deformations in the paced heart is proposed. High time resolution‐tagged MR images were obtained after stimulation of the myocardium with an MR‐compatible pacing system. The images were analyzed to reconstruct dynamic models of local 3D strains over the entire left ventricle during systole. Normal canine hearts were studied in vivo with pacing sites on the right atrium, left ventricular free wall and right ventricular apex. This method clearly resolved local variations in myocardial contraction patterns caused by ventricular pacing. Potential applications are noninvasive determination of electrical conduction abnormalities and the evaluation of new pacing therapies.
Sound generation in gradient coil structures for MRI
Authors: Mansfield Peter , Glover Paul M. , Beaumont Joanna 
Published: March 2007

AbstractWhen supporting plates of plastic material are subjected to alternating transverse Lorentz forces while in a strong magnetic field normal to the plate surface, compressional waves within the solid produce a modulation of the plate surface that launches an acoustic wave in air along the magnetic field axis. We have extended our previous theory describing this process to include a detailed description of the formation of an acoustic interference pattern in air described by Fraunhofer diffraction at a distance from the plate surface. The extended theory predicts that the observed acoustic signal midpoint and normal to the plate surface gives a variation with frequency in approximate agreement with our previous measurements. The acoustic output off axis shows acoustic blazing that produces two main diffraction peaks with a splitting inversely proportional to the velocity of sound in the plate material. The new results could have important ramifications for the minimization of sound output in gradient coil design for MRI. A new arrangement of coils is proposed to ameliorate the acoustic output problem centrally and normal to the plate by extending the frequency response of the supporting plates to much higher frequencies. Also presented are estimates of the compressional wave velocities deduced from frequency response data recorded at the center‐point of a number of different plates.
Ultra‐fast velocity imaging in stenotically produced turbulent jets using RUFIS
Authors: Madio David P. , Gach H. Michael , Lowe Irving J. 
Published: March 2007

AbstractA method for rapidly producing velocity images is presented. This sequence combines a modified bipolar gradient pulse to magnitude encode the velocity with the rotating ultra‐fast imaging sequence (RUFIS) to image the encoded spins. Velocity encoding is done in 3 msec, and RUFIS acquires 32 projections in 8 msec. The method is applied to turbulent jets associated with a 75% stenosis in a 15‐mm inner diameter glass pipe. Data is acquired upstream and downstream from the stenosis for Reynolds numbers from 560 to 3750. In addition, a robust method of reconstructing the unobserved short time region of a free induction decay is presented and incorporated into the image processing.
Gradient characterization using a Fourier‐transform technique
Authors: Alley Marcus T. , Glover Gary H. , Pelc Norbert J. 
Published: March 2007

AbstractThis paper describes a technique for characterizing the gradient subsystem of a magnetic resonance (MR) system. The technique uses a Fourier‐transform analysis to directly measure the k‐space trajectory produced by an arbitrary gradient waveform. In addition, the method can be easily extended to multiple dimensions and can be adapted to measuring residual gradient effects such as eddy currents. Several examples of gradient waveform and eddy‐current measurements are presented. Also, it is demonstrated how the eddy‐current measurements can be parameterized with an impulse‐response formalism for later use in system tuning. When compared to a peak‐fitting analysis, this technique provides a more direct extraction of the k‐space measurements, which reduces the possibility of analysis error. This approach also has several advantages as compared to the conventional eddy‐current measurement technique, including the ability to measure very short time constant effects.
Dynamic functional imaging of relative cerebral blood volume during rat forepaw stimulation
Authors: Mandeville Joseph B. , Marota John J. A. , Kosofsky Barry E. , Keltner John R. , Weissleder Ralph , Rosen Bruce R. , Weisskoff Robert M. 
Published: March 2007

AbstractDynamic measurements of regional changes in cerebral blood volume (CBV) were performed in rat models of hypercarbia and focal neuronal activation using T2‐weighted imaging after injection of an intravascular contrast agent with a very long blood half‐life. Calculated percent CBV change during hypercarbia was consistent with literature results from other non‐invasive modalities. Equivalent percent CBV increases were found using spin‐ and gradient‐echo images, suggesting proportional changes in blood volume for capillaries and small veins. During electrical stimulation of rat forepaw, focal CBV response to stimulation (24 ± 4%) was significantly delayed relative to blood oxygen level dependent (BOLD) signal after both onset and cessation of stimulation. Poststimulus CBV decay was temporally consistent with the BOLD poststimulus undershoot. The use of exogenous agent increased the functional contrast‐to‐noise ratio relative to BOLD imaging by 5.7 ± 1.3 at a magnetic field strength of 2 Tesla and 1.5 ± 0.2 at 4.7 Tesla.
On the nature and reduction of plaque‐mimicking flow artifacts in black blood MRI of the carotid bifurcation
Authors: Steinman David A. , Rutt Brian K. 
Published: March 2007

AbstractCardiac‐gated black blood MRI of the carotid artery bifurcation in normal human subjects shows signal within the lumen suggesting wall thickening or atherosclerotic plaque. This signal was believed to be artifactual, arising from complex flow patterns present at the carotid bifurcation. Computer simulation of the hemodynamics and black blood multislice image acquisition in a model of the carotid bifurcation showed that these artifacts arise from spins recovering their signal within the slow, recirculating flow of the carotid bulb. The computed hemodynamics also suggested that these artifacts could be minimized or eliminated entirely by gating the acquisition of slices in the most artifact‐prone region of the carotid bulb within a 250‐ms window after peak systole. Application of these predictions to studies of normal volunteers showed that, in most cases, these flow artifacts in black blood MRI can be eliminated simply by altering the phase of the cardiac cycle to which the image acquisition is gated. The observation that the size and placement of the saturation slabs had little effect on these artifacts suggested that, in those cases in which recirculation persists throughout the cardiac cycle, either inversion‐recovery or presaturation within the bulb itself would be required to suppress them.