bluuebunny/crossref_metadata_2025_split huggingface dataset:
Constrained length minimum inductance gradient coil design
Authors: Chronik Elaine A. , Rutt Brian K. 
Published: March 2007

AbstractA gradient coil design algorithm capable of controlling the position of the homogeneous region of interest (ROI) with respect to the current‐carrying wires is required for many advanced imaging and spectroscopy applications. A modified minimum inductance target field method that allows the placement of a set of constraints on the final current density is presented. This constrained current minimum inductance method is derived in the context of previous target field methods. Complete details are shown and all equations required for implementation of the algorithm are given. The method has been implemented on computer and applied to the design of both a 1:1 aspect ratio (length:diameter) central ROI and a 2:1 aspect ratio edge ROI gradient coil. The 1:1 design demonstrates that a general analytic method can be used to easily obtain very short gradient coil designs for use with specialized magnet systems. The edge gradient design demonstrates that designs that allow imaging of the neck region with a head‐sized gradient coil can be obtained, as well as other applications requiring edge‐of‐cylinder regions of uniformity.
Comparison of four magnetization preparation schemes to improve blood‐wall contrast in cine short‐axis cardiac imaging
Authors: Semple Scott I. K. , Redpath Thomas W. , McKiddie Fergus I. , Waiter Gordon D. 
Published: March 2007

AbstractImprovements in short‐axis blood‐myocardium contrast in the heart with the use of four magnetization preparation schemes applied before the imaging sequence are demonstrated. Gradient‐echo cine cardiac images are acquired and compared at 0.95 T incorporating T2, T1p, magnetization transfer, and double inversion (black blood) preparations in a series of volunteer studies over the first 550 ms of the cardiac cycle. T2 and T1p preparations exhibit improvements of 100% and above in image contrast. Magnetization transfer preparation exhibits improvements of 50% in image contrast, whereas an initial improvement (50%) followed by a large loss in contrast is observed using the black blood preparation. Improvements in contrast are dependent on tissue relaxation parameters and therefore are suitable for studies involving patients exhibiting poor in‐flow enhancement of blood caused by poor heart function.
Concomitant gradient terms in phase contrast MR: Analysis and correction
Authors: Bernstein Mat A. , Zhou Xiaohong Joe , Polzin Jason A. , King Kevin F. , Ganin Alexander , Pelc Norbert J. , Glover Gary H. 
Published: March 2007

AbstractWhenever a linear gradient is activated, concomitant magnetic fields with non‐linear spatial dependence result. This is a consequence of Maxwell's equations, i.e., within the imaging volume the magnetic field must have zero divergence, and has negligible curl. The concomitant, or Maxwell field has been described in the MRI literature for over 10 years. In this paper, we theoretically and experimentally show the existence of two additional lowest‐order terms in the concomitant field, which we call cross‐terms. The concomitant gradient cross‐terms only arise when the longitudinal gradient Gz is simultaneously active with a transverse gradient (Gx or Gy). The effect of all of the concomitant gradient terms on phase contrast imaging is examined in detail. Several methods for reducing or eliminating phase errors arising from the concomitant magnetic field are described. The feasibility of a joint pulse sequence‐reconstruction method, which requires no increase in minimum TE, is demonstrated. Since the lowest‐order terms of the concomitant field are proportional to G2/B0, the importance of concomitant gradient terms is expected to increase given the current interest in systems with stronger gradients and/or weaker main magnetic fields.
Localized proton NMR spectroscopy in the striatum of patients with idiopathic spasmodic torticollis
Authors: Möller Harald E. , Vollmer‐Haase Juliane , Wiedermann Dirk , Schuierer Gerhard 
Published: March 2007

AbstractSingle voxel proton spectroscopy was used to study brain metabolism in idiopathic spasmodic torticollis. Peak metabolite ratios in long echo time (135 ms) spectra were evaluated in the basal ganglia of 16 patients (29‐65 years). Eight of them were untreated; the other eight were examined 4‐6 weeks after administration of botulinum toxin type A. As compared with 60 control spectra, patients showed a significant, therapy‐resistant decrease in N‐acetyl‐L‐aspartate (NAA)/choline (Cho). NAA/cre‐atine (Cr) and Cho/Cr were close to normal in the post‐treatment group but significantly reduced in untreated patients. This result is consistent with the hypothesis that striatal Cr is reversibly elevated in chronic muscle stimulation. The right and left stria‐tum were affected to the same extent.
Evaluation of 31P metabolite differences in human cerebral gray and white matter
Authors: Mason Graeme F. , Chu Wen‐Jang , Vaughan J. Thomas , Ponder Steven L. , Twieg Donald B. , Adams Dorothy , Hetherington Hoby P. 
Published: March 2007

Abstract31P NMR is commonly used to study brain energetics in health and disease. Due to sensitivity constraints, the NMR measurements are typically made in volumes that do not contain pure gray or white matter. For accurate evaluation of abnormalities in brain metabolite levels, it is necessary to consider the differences in normal levels of 31P metabolites in gray and white matter. In this study, voxels from a three‐dimensional spectroscopic image acquisition were analyzed for their dependence on tissue type to assess differences in metabolite levels between gray and white matter. Specifically, gray matter was found to have significantly higher ratios of phosphocreatine (PCr) to γ‐ATP and PCr to the total 31P metabolite signal, whereas pH and the ratio of PCr to inorganic phosphate (P1) were found to differ insignificantly between gray and white matter. Thus, tissue type can be an important factor to consider for alterations in bioenergetics by 31P NMR spectroscopic studies of the brain.
MR‐guided interstitial cryotherapy of the liver with a novel, nitrogen‐cooled cryoprobe
Authors: Tacke Josef , Adam Gerhard , Speetzen Ralf , Brucksch Kerstin , Sticker Arno , Heschel Ingo , Prescher Andreas , van Vaals Joop J. , Hunter David W. , Rau Günter , Günther Rolf W. 
Published: March 2007

AbstractThe purpose of the study was to test a newly developed, MR‐compatible, liquid nitrogen‐cooled cryoprobe. The probe has an outer diameter of 3.5 mm and was specifically developed for percutaneous, MR‐guided, interstitial cryotherapy. The probe was inserted percutaneously into the livers of 10 rabbits. The cryotherapy procedure was monitored with a surface coil in a 1.5 Tesla magnet using a gradient echo sequence. Follow‐up examinations were performed 3 and 7 days after the freezing procedure using T1‐ and T2‐weighted spin echo sequences. At 7 days the animals were sacrificed and the cryolesions were examined histologically.The cryoprobe enabled artifact‐free MR imaging of the “ice‐ball” formation during freezing of the rabbit liver. After 1 min of freezing, the iceball at the tip of the probe showed an average maximum diameter of 10.8 mm. No bleeding complications were observed during or after the freezing procedure. Histologic examination 7 days after cryotherapy confirmed that the liver lesions were the same size as had been predicted by the images of the acute iceball.This new, percutaneously inserted, MR‐compatible, liquid‐nitrogen cooled cryoprobe allows accurate, artifact‐free MR imaging of interstitial cryotherapy.
Wavelet encoding for improved SNR and retrospective slice thickness adjustment
Authors: Gelman Neil , Wood Michael L. 
Published: March 2007

AbstractA method to encode the slice‐select direction in magnetic resonance imaging through the use of a discrete wavelet transform is presented. The paper reports the first implementation of wavelet encoding using the Haar basis. Theoretically, the signal‐to‐noise ratio (SNR) for two levels of wavelet encoding is expected to be approximately 1.6 times higher than for conventional multislice imaging in which each slice is excited separately. A slightly lower SNR is encountered experimentally because of slice interference. The design of radiofrequency (RF) pulses to excite slices approximating Haar wavelet basis functions is discussed. Wavelet encoding also provides an efficient method for balancing slice thickness and SNR after an image has been acquired. The experiments have been conducted at 1.5 Tesla. Wavelet encoding of slices should be especially suitable for low‐field MRI, in which the SNR is lower and the characteristically lower T, values favor the use of shorter TR, which sometimes forces the slices to be acquired in multiple passes.
Practical design of a high‐strength breast gradient coil
Authors: Maier Cynthia F. , Nikolov Hristo N. , Chu Kenneth C. , Chronik Elaine A. , Rutt Brian K. 
Published: March 2007

AbstractA high‐strength three‐axis local gradient coil set was constructed for MRI of the breast. Gradient fields with good uniformity (<10% deviation from the desired gradient) over most of the volume required for breast imaging were generated with efficiencies of up to 3.3 mT/m/A. The coils will allow diffusion breast imaging in clinically acceptable examination times. The electrical design, water cooling system, and fabrication techniques are described. Preliminary tests of the coil included images of a grid phantom and diffusion measurements in a short‐T2 agarose gel phantom.
Functional MRI of brain activation induced by scanner acoustic noise
Authors: Bandettini Peter A. , Jesmanowicz A. , van Kylen Joel , Birn Rasmus M. , Hyde James S. 
Published: March 2007

AbstractA method is introduced by which brain activation caused by the acoustic noise associated with echo planar imaging (EPI) is mapped. Two types of time series were compared. The first time series, considered the “task,” involved applying only EPI gradients for 20 s without the application of RF pulses, then, without pause, starting image collection. The second, considered the “control,” involved typical sequential image acquisition without the prior gradient pulses. Subtraction of the first 5 s of the two time series revealed signal enhancement mainly in the primary auditory cortex. The technique was validated using a motor cortex task that mimicked the hypothesized scanner noise induced activation.
Quantitative 1H MRS of the human brain in vivo based on the simulation phantom calibration strategy
Authors: Due Corinne O. , Weber Oliver M. , Trabesinger Andreas H. , Meier Dieter , Boesiger Peter 
Published: March 2007

AbstractNormal metabolite concentrations were determined in five different brain regions of healthy adult volunteers using proton magnetic resonance spectroscopy (1H MRS) in vivo. The absolute in vivo concentrations of N‐acetylaspartate (NAA), creatine and phosphocreatine (Cre), and choline containing compounds (Cho) were quantified from measurements obtained with a head‐shaped simulation phantom. Scanner performance and calibration accuracy were assessed by phantom experiments. Localized spectra were acquired on clinical 1.5 T systems using the PRESS localization sequence with frequency selective water suppression. Comparison of the results obtained from phantom experiments and human brain in vivo strongly suggests that reproducibility in vivo mainly depends on the topologic metabolite heterogeneity of brain tissue in combination with relative volume dislocalization.
A critique of the use of the Kolmogorov‐Smirnov (KS) statistic for the analysis of BOLD fMRI data
Authors: Aguirre Geoffrey K. , Zarahn Eric , D′Esposito Mark 
Published: March 2007

AbstractThe use of the Kolmogorov‐Smirnov (KS) statistic for testing hypotheses regarding activation in blood oxygenation level‐dependent functional MRI data is critiqued on both theoretical and empirical grounds. Theoretically, it is argued that the KS test is formally unable to support inferences of interest to most neuro‐imaging studies and has reduced sensitivity compared with parametric alternatives. Empirically, false‐positive rates yielded by the KS test in human data collected under the null hypothesis were significantly in excess of tabular values. These excessive false‐positive rates could be explained by the presence of temporal autocorrelation. We also present evidence that the distribution of blood oxygenation level‐dependent functional MRI data is only slightly nonnormal, questioning the initial impetus for the use of the KS test in this context. Finally, it is noted that parametric alternatives exist that do provide adequate control of the false‐positive rate and can support inferences of interest.
Improved coverage in dynamic contrast‐enhanced cardiac MRI using interleaved gradient‐echo EPI
Authors: Ding Shujun , Wolff Steven D. , Epstein Frederick H. 
Published: March 2007

AbstractAn interleaved gradient‐echo echo‐planar imaging (IGEPI) sequence was modified for and applied to dynamic contrast‐enhanced imaging of the heart. Using IGEPI, images with 3.0 × 3.9 mm nominal in‐plane resolution are acquired in 100 ms, enabling eight slices per heartbeat for a heart rate of 60 beats/min. The acquisition speed and use of saturation pre‐pulses allows acquisition of short‐ and long‐axis images during the same contrast bolus. IGEPI maintains the acquisition characteristics required for performing a quantitative first‐pass perfusion analysis as well as providing improved coverage compared with conventional fast gradient echo.
Spatial and temporal differentiation of fMRI BOLD response in primary visual cortex of human brain during sustained visual simulation
Authors: Chen Wei , Zhu Xiao‐Hong , Kato Toshinori , Andersen Peter , Uǧurbil Kâmil 
Published: March 2007

AbstractThe blood oxygenation level dependent (BOLD) response during sustained visual stimulation has been studied by several groups using fMRI with controversial conclusions. This issue was investigated for the human brain at high (4 Tesla) magnetic field strength using a flashing goggle at 8 Hz. The results demonstrate that the overall BOLD response in the primary visual cortex has an initial overshoot after the onset of visual stimulation and an undershoot after the termination of visual stimulation. A significant and positive BOLD response, however, remains constant between the initial and terminal transient responses. The temporal BOLD responses in the primary visual cortex were spatially dependent. The regions identified as draining veins in images displayed proportionately larger initial and terminal transient responses, whereas regions devoid of such vessels and associated mainly with parenchyma exhibited a more time‐independent BOLD response. These results reveal that the BOLD effect and, presumably, the uncoupling between cerebral blood flow and cerebral metabolic rate of oxygen consumption, are maintained in the primary visual cortex during sustained visual stimulation, and the temporal characteristics of the BOLD effect are spatially dependent.
Multiple‐echo proton spectroscopic imaging using time domain parametric spectral analysis
Authors: Kiefer Andreas P. , Govindaraju Varanavasi , Matson Gerald B. , Weiner Michael W. , Maudsley Andrew A. 
Published: March 2007

AbstractA multiple‐echo MR spectroscopic imaging (MRSI) method is presented that enables improved metabolite imaging in the presence of local field inhomogeneities and measurement of transverse relaxation parameters. Short echo spacing is used to maximize signal energy from inhomogeneously line‐broadened resonances, and time domain parametric spectral analysis of the entire echo train is used to obtain sufficient spectral resolution from the shortened sampling periods. Optimal sequence parameters for 1H MRSI are determined by computer simulation, and performance is compared with conventional single‐echo acquisition using phantom studies at a field strength of 4.7 T. A preliminary example for use at 1.5 T is also presented using phantom and human brain MRSI studies. This technique is shown to offer improved performance relative to single‐echo MRSI for imaging of metabolites with shortened T2* values due to the presence of local field inhomogeneities. Additional advantages are the intrinsic measurement of metabolite T2 values and determination of metabolite integrals without T2 weighting, thereby facilitating quantitative metabolite imaging.
Simultaneous intracellular and extracelular pH measurement in the heart by 19F NMR of 6‐fluoropyridoxol
Authors: Hunjan Sandeep , Mason Ralph P. , Mehta Vimal D. , Kulkarni Padmakar V. , Aravind Subramanian , Arora Veeva , Antich Peter P. 
Published: March 2007

Abstract6‐Fluoropyridoxol (6‐FPOL) was evaluated as a simultaneous indicator of intracellular and extracellular pH and, hence, pH gradient in perfused rat hearts. After infusion, 19F NMR spectra rapidly showed two well‐resolved peaks assigned to the intracellular and extracellular compartments, and pH was calculated on the basis of chemical shift with respect to a sodium trifluoroacetate standard. To demonstrate use of this molecule, dynamic changes in myocardial pH were assessed with a time resolution of 2 min during respiratory and metabolic alkalosis or acidosis and ischemia. For a typical heart, intracellular pH (pHi) = 7.14 ± 0.01 and extracellular pH (pHe) = 7.52 ± 0.02. In response to metabolic alkalosis, pHi remained relatively constant and the pH gradient increased. In contrast, respiratory challenge caused a significant increase in pHi. Independent measurements using pH electrodes and 31P NMR confirmed validity of the 19F NMR results.
Quantitative measurements of cerebral blood flow in rats using the FAIR technique: Correlation with previous lodoantipyrine autoradiographic studies
Authors: Tsekos Nikolaos V. , Zhang Fangyi , Merkle Hellmut , Nagayama Masao , Ladecola Costantino , Kim Seong‐Gi 
Published: March 2007

AbstractFlow‐sensitive alternating inversion recovery (FAIR) is a recently introduced MRI technique for assessment of perfusion that uses blood water as an endogenous contrast agent. To characterize the FAIR signal dependency on spin tagging time (inversion time (Tl)) and to validate FAIR for cerebral blood flow (CBF) quantification, studies were conducted on the rat brain at 9.4 T using a conventional gradient‐recalled echo sequence. The 71, of cerebral cortex and blood was found to be 1.9 and 2.2 s, respectively, and was used for CBF calculations. At short Tls (<0.8 s), the FAIR signal originates largely from vascular components with fast flows, resulting in an overestimation of CBF. For Tl > 1.5 s, the CBF calculated from FAIR is independent of the spin tagging time, suggesting that the observed FAIR signal originates predominantly from tissue/capillary components. CBF values measured by FAIR with Tl of 2.0 s were found to be in good agreement with those measured by the iodoantipyrine technique with autoradiogra‐phy in rats under the same conditions of anesthesia and arterial pCO2. The measured pCO2 index on the parietal cortex using the FAIR technique was 6.07 ml/100 g/min per mmHg, which compares well with the pCO2 index measured by other techniques. The FAIR technique was also able to detect the regional reduction in CBF produced by middle cerebral artery occlusion in rats.
Off‐resonance proton T1p dispersion imaging of 17O‐enriched tissue phantoms
Authors: Charagundla Sridhar R. , Stolpen Alan H. , Leigh John S. , Reddy Ravinder 
Published: March 2007

AbstractProton T1p dispersion imaging is a recently described method for indirect detection of 17O. However, clinical implementation of this technique is hindered by the requirement for a high‐amplitude spin‐locking field (γB1 > 1 kHz) that exceeds current limitations in specific absorption rate (SAR). Here, a strategy is offered for circumventing high SAR in T1p dispersion imaging of 17O through the use of low‐amplitude off‐resonance spin‐locking pulses (γB1 < 300 Hz). Proton spin‐lattice relaxation times in the off‐resonance rotating frame were measured in H217O‐enriched tissue phantoms. On‐ and off‐resonance T1p dispersion imaging was implemented at 2 T using a spin‐locking preparatory pulse cluster appended to a standard spin‐echo sequence. On‐ and off‐resonance dispersion images exhibited similar 17O‐based image contrast. Magnetization transfer effects did not depend on 17O concentration and had no effect on image contrast. In conclusion, off‐resonance proton T1p dispersion imaging shows promise as a safe, sensitive technique for generating 17O‐based T1p contrast without exceeding SAR limitations.
Concomitant magnetic‐field‐induced artifacts in axial echo planar imaging
Authors: Zhou Xiaohong Joe , Du Yiping P. , Bernstein Matt A. , Reynolds H. Glenn , Maier Joseph K. , Polzin Jason A. 
Published: March 2007

AbstractWhen a linear magnetic field gradient is used, spatially higher‐order magnetic fields are produced to satisfy the Maxwell equations. It has been observed that the higher‐order magnetic field produced by the readout gradient causes axial echo planar images acquired with a horizontal solenoid magnet to shift along the phase‐encoding direction and lose image intensities. Both the shift and intensity reduction become increasingly severe as the slice offset from the isocenter increases. These phenomena are quantitatively analyzed, and good correlation between experiments and theory has been established. The analysis also predicts a previously unre‐ported Nyquist ghost on images with very large slice offsets. This ghost has been verified with computer simulations. Based on the analysis, several methods have been developed to eliminate the image shift, the intensity reduction, and the ghost. Selected methods have been implemented on a commercial scanner and proved effective in removing these image artifacts.
Proton T2 relaxation of cerebral metabolites during transient global ischemia in rat brain
Authors: Fujimori Hiroyuki , Michaelis Thomas , Wick Markus , Frahm Jens 
Published: March 2007

AbstractPutative changes of metabolite T2 relaxation times were investigated before and after a 20‐min period of global ischemia in rat brain in vivo (n= 10) using localized proton MRS at different echo times (2.35 T). Neither absolute T2 relaxation times (TE = 20‐270 ms) nor time courses of T2‐weighted metabolite signals (TE = 135 ms) revealed statistically significant changes during the occlusion or early reperfusion relative to pre‐ischemic baseline. These findings are in line with reports of relaxation changes at much later stages and further demonstrate that altered T2 relaxation is not a confounding factor in diffusion‐weighted long‐TE proton MRS during early ischemic events.
Improved sampling of myocardial motion with variable separation tagging
Authors: McVeigh Elliot R. , Bolster Bradley D. 
Published: March 2007

AbstractA novel tagging method is introduced that increases the spatial resolution of estimates of myocardial radial thickening. The separation of adjacent parallel tag planes is customized to match the expected motion of specific regions of the heart wall. In regions in which the tags increase their separation over systole (radial thickening), the tag planes are placed close together at end diastole. In regions in which the tags decrease their separation over systole (circumferential shortening), the tags are placed farther apart so they remain detectable at end systole. With variable separation tagging (VTAG), parallel plane tagging can be used to obtain higher‐resolution estimates of radial thickening and circumferential shortening simultaneously.