bluuebunny/crossref_metadata_2025_split huggingface dataset:
Gradient moment smoothing: A new flow compensation technique for multi‐shot echo‐planar imaging
Authors: Slavin Glenn S. , Riederer Stephen J. 
Published: March 2007

AbstractThis work identifies an additional source of phase error across ky in multi‐shot echo‐planar imaging resulting from flow or motion along the phase‐encoding direction. A velocity‐independent flow compensation technique, gradient moment smoothing, is presented that corrects this error by forcing the phase to have smooth quadratic behavior. The correction is implemented, without compromising scan time, by changing the first moment of a bipolar prephaser pulse on a shot‐by‐shot basis. In phantom and in vivo experiments, gradient moment smoothing effectively eliminates ghosting and signal loss due to phase‐encoding flow. When used in conjunction with a “flyback” echo‐planar readout, which compensates for flow in the frequency‐encoding direction, gradient moment smoothing renders multi‐shot echo‐planar imaging relatively insensitive to in‐plane flow. This can make multi‐shot echo‐planar imaging a viable technique for accurately imaging in‐plane flow and may desensitize it to the otherwise serious problem of in‐plane motion.
Calculations and experimental studies of the lineshape of the lactate doublet in PRESS‐localized 1H MRS
Authors: Marshall Ian , Wild James M. 
Published: March 2007

AbstractAccurate quantification of NMR metabolites by spectral modeling requires assumed lineshape functions. For singlet resonances, a combination of Lorentzian and Gaussian lineshapes is sufficient, but for weakly J‐coupled resonances such as lactate, more complex lineshapes are necessary. In this work, the lactate lineshape is calculated for the PRESS sequence using standard RF pulses, and compared with experimental measurements. A similar comparison is made for PRESS‐localized spectroscopic imaging, in which the lineshape varies from voxel to voxel across the field of view. These observations have important implications for the quantification of lactate in experimental and human studies.
In Vivo1H‐NMR microimaging with respiratory triggering for monitoring adoptive immunotherapy of metastatic mouse lymphoma
Authors: Fichtner Klaus‐Peter , Schirrmacher Volker , Griesbach Andreais , Hull William E. 
Published: March 2007

AbstractThe metastatic ESb‐MP murine lymphoma in DBN2 mice has been used as a model for investigating metastatic disease and its cure by adoptive immunotherapy (ADI) as monitored by in vivo multislice spin‐echo 1H NMR microimaging at 7 T. Isoflurane inhalation anesthesia facilitated long measurement sessions, and respiratory gating with a fiber‐optic sensor greatly reduced motional artifacts. With T2 weighting (TR = 2 s, TE = 30 ms) mean signal‐to‐noise ratios of 30 and 15 for kidney and liver, respectively, were achieved in 20 min (100‐μm pixels, 1‐mm slices, 25‐mm field of view). Without the use of contrast agents, metastases with diameters > 0.3 mm in the imaged plane could be detected as hyperintense lesions in kidney (contrast ratio ca. 1.4) and liver (contrast ratio ca. 2) with a confidence level of > 98%. For the first time the complete eradication of late‐stage macroscopic metastases by ADI could be demonstrated noninvasively by MRI.
Diffusion‐weighted imaging of the human optic nerve: A New approach to evaluate optic neuritis in multiple sclerosis
Authors: Iwasawa Tae , Matoba Hiroko , Ogi Akiko , Kurihara Hiroaki , Saito Kimihiko , Yoshida Tetsuo , Matsubara Sho , Nozaki Atsushi 
Published: March 2007

AbstractThe apparent diffusion coefficient (ADC) in the optic nerve was measured from diffusion‐weighted magnetic resonance imaging using an intravoxel incoherent motion (IVIM) sequence. The subjects were seven normal volunteers and eight patients with multiple sclerosis (MS) with a total of four optic nerves with acute neuritis and nine nerves with chronic neuritis. The mean ADC (4.18 ± 1.13 × 10−3 mm2/s, n = 9) in the optic nerves with chronic neuritis was significantly higher than that in normal volunteers (1.56 ± 0.675 × 10−3 mm2/s, n = 14) and that in the nerves with acute neuritis (0.94 ± 0.43 × 10−3 mm2/s n = 4) (P < 0.001). The ADC is useful in assessing MS foci in the optic nerves.
A Method to measure arbitrary k‐space trajectories for rapid MR imaging
Authors: Mason Graeme F. , Harshbarger Todd , Hetherington Hoby P. , Zhang Yantian , Pohost Gerald M. , Twieg Donald B. 
Published: March 2007

AbstractA method to measure arbitrary k‐space trajectories was developed to compensate for nonideal gradient performance during rapid magnetic resonance (MR) imaging with actively or nonactively shielded gradients at a magnetic field strength of 4.1 T. Accurate MR image reconstruction requires knowledge of the k‐trajectory produced by the gradient waveforms during k‐space sampling. Even with shielded gradients, residual eddy currents and imperfections in gradient amplifier performance can cause the true k‐space trajectory to deviate from the ideal trajectory. The k‐space determination was used for spiral gradient‐echo imaging of the human brain. While individual calibrations are needed for new pulse sequences, the method of k‐space determination can be used for any sequence of preparation pulses and readout gradient waveforms and should prove useful for other trajectories, including the rastered lines of echo‐planar imaging.
Phase insensitive preparation of single‐shot RARE: Application to diffusion imaging in humans
Authors: Alsop David C. 
Published: March 2007

AbstractAlthough RARE and GRASE can produce single‐shot images of excellent quality, their utility has been restricted because preparation of the magnetization with interesting contrast before imaging can cause severe artifacts. These artifacts relate to the strong sensitivity of multiple spin echo sequences to the phase of the prepared magnetization. Modifications of the RARE sequence to eliminate these artifacts are discussed, and an approach that eliminates the artifact producing signals from the very first echo is presented. The approach is applied to diffusion imaging of the human brain in normal volunteers and one patient.
Increase of creatine kinase activity in the visual cortex of human brain during visual stimulation: A 31p NMR magnetization transfer study
Authors: Chen Wei , Zhu Xiao‐Hong , Adriany Gregor , Uǧurbil Kǎmil 
Published: March 2007

Abstract31P magnetic resonance spectroscopy was used to investigate changes in high energy phosphate levels and creatine kinase (CK) kinetics induced in the human visual cortex during photic stimulation. CK kinetics was evaluated by measuring the apparent unidirectional rate constant (kf) in the “forward” direction (i.e., in the direction of ATP synthesis from phosphocreatine). kf increased 34% in the visual cortex areas during stimulation without significant changes of steady‐state concentration of high energy phosphate compounds, indicating that CK turnover is elevated during increased neuronal activity.
Determination and characterization of nitric oxide generation in mice by in vivo l‐band EPR spectroscopy
Authors: Fujii Hirotada , Koscielniak Janusz , Berliner Lawrence J. 
Published: March 2007

AbstractThe authors have shown direct, real‐time, in vivo measurement of nitric oxide (NO) in mice by using the water soluble metal chelator complex, N‐methyl‐D‐glucamine dithiocarbamate (MGD), and Fe(II) as monitored by EPR at L‐band. The three‐line EPR spectrum from the product [(MGD)2‐Fe(II)‐NO] was observed noninvasively in lipopolysaccharide (LPS)‐treated mice. The spectrum was markedly suppressed by the administration, before LPS injection, of phenyl N‐tert‐butyl nitrone (PBN), an inhibitor of the expression of induced nitric oxide synthase (iNOS). When 15N‐arginine was administered to LPS‐treated mice, a diagnostic EPR spectrum was observed, consisting of both three‐ and two‐line EPR signals, due to (MGD)2‐Fe(II)‐14NO and (MGD)2‐Fe(II)‐16NO, respectively. The results strongly suggested that the NO detected in these experiments was synthesized by iNOS. In vivo EPR measurements of [(MGD)2‐Fe(II)‐NO] at several regions in the body (from the head to the tail) indicated that the NO was generated mostly in the upper abdomen near the liver. These observations were confirmed by ex vivo EPR measurements on isolated organs where higher NO levels were detected in vivo in the liver and kidney. The spectroscopic results, combined with the pharmacokinetic data, support the model that NO detected in LPS‐treated mice was produced mainly in the liver, and that it did not reflect NO‐adduct complex accumulated in the liver via the blood circulation.
BURST excitation pulses
Authors: Heid Oliver 
Published: March 2007

AbstractA theory of optimum burst excitation is developed in the frame‐work of the Shinnar‐LeRoux spinor formalism. An N pulse RF train of constant or linear phase cannot improve on an average echo strength of M0/N, and a phase modulation of the pulse train is necessary to improve the signal yield to the theoretical maximum value M0√;N. Several methods are presented yielding pulse trains of nearly optimum average amplitude for arbitrary N. It is shown that RF phase spoiling can be analyzed with the same framework. The presented pulse trains may also be useful when ultrawide spectrum hard pulses are required, but only limited RF power is available, e.g., for NMR experiments in extremely inhomogeneous B0 fields.
Simultaneous acquisition of spatial harmonics (SMASH): Fast imaging with radiofrequency coil arrays
Authors: Sodickson Daniel K. , Manning Warren J. 
Published: March 2007

AbstractSiMultaneous Acquisition of Spatial Harmonics (SMASH) is a new fast‐imaging technique that increases MR image acquisition speed by an integer factor over existing fast‐imaging methods, without significant sacrifices in spatial resolution or signal‐to‐noise ratio. Image acquisition time is reduced by exploiting spatial information inherent in the geometry of a surface coil array to substitute for some of the phase encoding usually produced by magnetic field gradients. This allows for partially parallel image acquisitions using many of the existing fast‐imaging sequences. Unlike the data combination algorithms of prior proposals for parallel imaging, SMASH reconstruction involves a small set of MR signal combinations prior to Fourier transformation, which can be advantageous for artifact handling and practical implementation. A twofold savings in image acquisition time is demonstrated here using commercial phased array coils on two different MR‐imaging systems. Larger time savings factors can be expected for appropriate coil designs.
Simulated phase evolution rewinding (SPHERE): A technique for reducing B0 inhomogeneity effects in MR images
Authors: Kadah Yasser M. , Hu Xiaoping 
Published: March 2007

AbstractA novel method for reducing field inhomogeneity effects in magnetic resonance images is described in this paper. Observing that image degradation arises from B0 inhomogeneity‐induced phase accrual during data acquisition, the present method numerically rewinds the accumulated phase in the k‐space data based on an initial estimate of the image and a corresponding field map. The rewinding process generates a corrected k‐space data set that is subsequently Fourier transformed to produce the final image. In this paper, a theoretical analysis of the method and applications of the technique to magnetic resonance imaging data are presented. The theoretical analysis of the method indicates that it is a general approach applicable to a variety of sequences. Results obtained by applying the method to experimental data acquired with single‐shot echo‐planar imaging, segmented echo‐planar imaging with centric reordering, and spiral sequences demonstrate that it is robust in reducing image degradation induced by B0 inhomogeneity.
STEAM‐Burst: A single‐shot, multi‐slice imaging sequence without rapid gradient switching
Authors: Crémillieux Yannick , Wheeler‐Kingshott Claudia A. , Briguet André , Doran Simon J. 
Published: March 2007

AbstractThe stimulated‐echo acquisition mode‐Burst sequence is a single‐shot, multi‐slice imaging technique that does not involve rapid gradient switching. A Burst excitation pulse train is followed by a 90° hard pulse and, after a mixing time, by a 90° slice‐selective pulse. A read gradient refocuses a set of stimulated echoes, which can be phase encoded to form an image. By repeating the selective pulse N times, each time with the carrier frequency offset differently, it is possible to sample N slices in a single‐shot. A comparison is made of the sequence with other three‐dimensional single‐shot methods. Experiments implementing the technique on a 3 T whole‐body imaging system and a 2 T, 31‐cm bore animal imager are described. Both phantom and brain images are presented. The principal advantages of the new sequence are its speed, the absence of rapid gradient switching and corresponding freedom from artifacts, its insensitivity to static magnetic field inhomogeneities, and its low acoustic noise. The main disadvantages are the low signal‐to‐noise ratio of the images produced and the concomitant limitation in resolution.
Theoretical basis for sodium and potassium MRI of the human heart at 1.5 T
Authors: Parish Todd B. , Fieno David S. , Fitzgerald Steven W. , Judd Robert M. 
Published: March 2007

AbstractKnowledge of the extent and location of viable tissue is important to clinical diagnosis. In principle, sodium (23Na) and potassium (39K) MRI could noninvasively provide information about tissue viability. In practice, imaging of these nuclei is difficult because, compared with water protons (1H), 23Na and 39K have lower MR sensitivities (9.2 and 0.051%, respectively), and lower in vivo concentrations (ca. 1000‐fold). On the other hand, the relatively short T1 relaxation times of 23Na and 39K (ca. 30 and 10 ms, respectively) suggest that optimized imaging pulse sequences may in part alleviate the weak signal of these nuclei. In this study, numerical simulations of high‐speed imaging sequences were developed and used to maximize 23Na and 39K image signal‐to‐noise ratio (SNR) per unit time within the constraints of existing gradient hardware. The simulation demonstrated that decreasing receiver bandwidth at the expense of echo time (TE) results in a substantial increase in 23Na and 39K image SNR/time despite the short T2 and T2* of these nuclei. Referenced to the available 1H signal on existing 1.5 T scanners, the simulation suggested that it should be possible to acquire three‐dimensional 23Na images of the human heart with 7 × 7 × 7 mm resolution and 39K images with 26 × 26 × 26 mm resolution in 30 min. Experimentally in humans at 1.5 T, three‐dimensional 23Na images of the heart were acquired in 15 min with 6 × 6 × 12 mm resolution and signal‐to‐noise ratios of 11 and 7 in the left ventricular cavity and myocardium, respectively, which is very similar to the predicted result. The results demonstrate that by choosing imaging pulse sequence parameters that fully exploit the short relaxation times of 23Na and 39K, potassium MRI is improved but remains impractical, whereas sodium MRI improves to the point where 23Na imaging of the human heart may be clinically feasible on existing 1.5 T scanners.
An orthogonal correlation algorithm for ghost reduction in MRI
Authors: Kruger David G. , Slavin Glenn S. , Muthupillai Raja , Grimm Roger C. , Riederer Stephen J. 
Published: March 2007

AbstractGhosting in MRI due to modulation of k‐space data can be caused by motion of the subject or characteristics of the sequence. A general solution for 2DFT MRI that reduces ghosting without causal modeling is presented. Separate image data sets are acquired in which the phase and frequency directions are swapped. In these two data sets, the image signal is correlated, whereas the ghost signals are not. By taking a correlation of these two data sets, an image with greatly reduced ghosting is obtained. The reduction is shown to depend both on the correct signal intensity of the image, as well as the ghost intensity in the ghosted region. The reduction approaches 100% in regions of low image signal, and is more moderate in regions of higher image signal. The process was applied to conventional spin‐echo, fast‐spin‐echo, and gradient echo imaging of volunteers and a phantom. Results of a reader study of the volunteer images reflected a significant overall reduction of ghosting artifacts in all volunteer experiments.
Multi‐turn split‐conductor transmission‐line resonators
Authors: Serfaty Stéphane , Haziza Nathalie , Darrasse Luc , Kan Siew 
Published: March 2007

AbstractMulti‐turn split‐conductor transmission‐line resonators (MSTR) can be designed to operate over a wide range of NMR operating frequencies without lumped tuning capacitors. This architecture constitutes an alternative solution to recent designs proposed for high‐Q, thin‐film, high‐temperature superconducting NMR probes. An advantage is that the resonant frequency can be calculated in a relatively simple manner in terms of coil turns or total coil length, coil width, substrate thickness, and dielectric constant. Analytical calculation of the resonant frequency is provided. Also, a series of MSTRs was constructed on a double copper‐clad substrate, and their resonant frequencies are noted. The results obtained were in good agreement with the predicted values.
Lateral geniculate activations can be detected using intersubject averaging and fMRI
Authors: Büchel Christian , Turner Robert , Friston Karl 
Published: March 2007

AbstractApplications of fMRI in functional brain imaging are mainly confined to single subject designs, prohibiting the assessment of subject or group by condition interactions (i.e., differential activations) or areas of conjoint activation. In this paper a framework for fMRI group designs, using statistical parametric mapping, is introduced. It is generally believed that intersubject averaging, which requires spatial normalization and smoothing, will decrease the effective spatial resolution of fMRI or its sensitivity. A subcortical activation of the lateral geniculate nucleus (LGN) was therefore chosen to demonstrate the feasibility and power of intersubject averaging in the context of fMRI. Seven volunteers were studied, while looking at a blank screen or radially moving dots. LGN activation was demonstrated in all single subject analyses and in the group analysis.
Brain MRI with laser‐polarized 129xe
Authors: Swanson Scott D. , Rosen Matthew S. , Agranoff Bernard W. , Coulter Kevin P. , Welsh Robert C. , Chupp Timothy E. 
Published: March 2007

AbstractThe feasibility of brain MRI with laser‐polarized 129Xe in a small animal model is demonstrated. Naturally abundant 129Xe is polarized and introduced into the lungs of Sprague‐Dawley rats. Polarized xenon gas dissolves in the blood and is transported to the brain where it accumulates in brain tissue. Spectroscopic studies reveal a single, dominant, tissue‐phase NMR resonance in the head at 194.5 ppm relative to the gas phase resonance. Images of 129Xe in the rat head were obtained with 98‐μl voxels by 2D chemical shift imaging and show that xenon is localized to the brain. This work establishes that nuclear polarization produced in the gas phases survives transport to the brain where it may be imaged. Increases in polarization and delivered volume of 129Xe will allow clinical measurements of regional cerebral blood flow.
Low visibility of lactate in excised rat muscle using double quantum proton spectroscopy
Authors: Jouvensal Laurence , Carlier Pierre G. , Bloch Gilles 
Published: March 2007

AbstractLactate NMR visibility was investigated in excised rat muscle at 3 T by comparing the concentration determined in situ by double quantum (DQ) proton spectroscopy (150 ms effective echo time) to the concentration measured in vitro from perchloric acid extracts of the same muscle samples. After 1‐2 h of ischemia, lactate NMR visibility was 32 ± 3% (±SE, n = 9), and was only 21 ± 1% (n = 6) after 10–12 h. Muscle lactate T2 was 140 ± 11 ms and 184 ± 6 ms, respectively. All potential mechanisms of DQ lactate signal attenuation (B0, and B1 inhomogeneity, DQ transverse relaxation, diffusion) were examined, and accounted for when necessary. A significant increase in lactate NMR visibility was demonstrated using a shorter effective echo time (79 ms) DQ editing sequence. These results are interpreted as reflecting muscle lactate compartmentation between a long T2 pool predominantly detected by DQ spectroscopy, and a short T2 pool virtually invisible with longer echo time NMR techniques.
Dual‐frequency, dual‐quadrature, birdcage RF coil design with identical b1pattern for sodium and proton imaging of the human brain at 1.5 T
Authors: Shen Gary X. , Boada Fernando E. , Thulborn Keith R. 
Published: March 2007

AbstractRapid quantification of tissue metabolitesin vivoby MRS or MRI can be achieved using dual‐frequency RF coils with identical B1field distributions at the observation frequencies of the metabolites and tissue water protons. Tissue sodium is used as an example for optimizing the dual‐frequency, dual‐quadrature RF coils for such measurements in humans. In the setting of sodium imaging, the challenge of dual‐quadrature birdcage configurations is to decouple the sodium and proton channels because the fourth harmonic of the sodium frequency is very close to the proton frequency. A generalizable method for effectively decoupling these two RF frequencies is presented in this paper. The method is demonstrated with the design of an EPI compatible, dual‐quadrature, double‐tuned,23Na/1H birdcage coil. The performance of the RF probe is reported at 1.5 Tesla in terms of signal‐to‐noise ratio, B1homogeneity and image quality.
Theory of the quadrature elliptic birdcage coil
Authors: Leifer Mark C. 
Published: March 2007

AbstractThis paper presents the theory of the quadrature birdcage coil wound on an elliptic cylindrical former. A conformal transformation of the ellipse to a circular geometry is used to derive the optimal sampling of the continuous surface current distribution to produce uniform magnetic fields within an elliptic cylinder. The analysis is rigorous for ellipses of any aspect ratio and shows how to produce quadrature operation of the elliptic birdcage with a conventional hybrid combiner. Insight gained from the transformation is also used to analyze field homogeneity, find the optimal RF shield shape, and specify component values to produce the correct current distribution in practice. Measurements and images from a 16‐leg elliptic birdcage coil at both low and high frequencies show good quadrature performance, homogeneity, and sensitivity.
Axonal stimulation under MRI magnetic field z gradients: A modeling study
Authors: Carbunaru Rafael , Durand Dominique M. 
Published: March 2007

AbstractThe stimulation of axons under MRI magnetic fields is analyzed solving the cable equation along the axons in the presence of magnetic field z gradients. Axons are represented using a one‐dimensional compartmental cable model including the kinetics of mammalian myelinated fibers. Computer simulations of the model were performed for sinusoidal and trapezoidal fields. Several axon and field parameters were tested to determine the threshold values for axonal stimulation of the magnetic field, induced electric field and time derivative of the magnetic field. The results indicate that 1) threshold for stimulation is lowest for the largest diameter axons terminating in the region where the magnetic field is maximum, 2) trapezoidal waveforms can be optimized to allow better sub‐threshold resolution than sinusoidal wave‐forms, and 3) the induced electric field is better than the magnetic field and time derivative of the magnetic field as indicators of stimulation threshold.