摘要:

AbstractThe addition of a navigator echo acquisition to a1H spectroscopy sequence allows retrospective motion correction using acceptance criteria based on measured displacement of the diaphragm. This technique has been applied to the acquisition of short echo‐time1H spectroscopy of the liver in seven normal subjects. Navigation correction significantly improved overall spectral quality as measured by the phase variance, linewidth and suppression ratio of the water resonance. The hepatic choline resonance was consistently observed in all navigated spectra. Retrospective navigation makes close to optimal use of data acquisition time and, unlike external respiratory transducers, allows direct measurement of diaphragm displacement. The reconstruction of multiple voxel spectra from a single data set was also demonstrate

ISSN:0740-3194    

DOI:10.1002/mrm.1910390102

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1998

数据来源: WILEY

摘要:

AbstractThe goal of this work was to develop and evaluate a numerically optimized inversion pulse to be used with a homonuclear editing sequence to measure human cerebral GABA 117vivoat 4.1 T in the occipital lobe. The optimized pulse was constructed using pallindromic symmetry with 30 pulses and 29 delays. The optimized pulse provided greater selectivity than the equivalent bandwidth matched DANTE pulse and sine shaped DANTE. The improved selectivity reduced the co‐editing of the macromolecule resonance, permitting the GABA edited doublet to be resolvedin vivo.Using cerebral creatine as a reference, 7.1 mM, the measured GABA level was 1.15 ± 0.13 mM in the occipital lo

ISSN:0740-3194    

DOI:10.1002/mrm.1910390103

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1998

数据来源: WILEY

摘要:

AbstractThe observation of a1H double quantum filtered (DQF) NMR signal of water in bovine sciatic nerve, bovine articular cartilage, rat tail tendon, and rat brain is reported. The origin of this signal in rat tail tendon was found to be a result of residual dipolar interaction between water protons and macromolecular protons. The dependence of the width of the1H DQF spectra on the orientation indicated that in rat tail tendon the effective director of the residual dipolar interaction is parallel to the collagen fibers.1H DQF NMR may be applied in imaging where the contrast obtained is related to the degree of order in the tissue.

ISSN:0740-3194    

DOI:10.1002/mrm.1910390104

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1998

数据来源: WILEY

摘要:

AbstractThe technical limitations of electroencephalography (EEG) and flashed visually‐evoked potentials (VEP) recordings in the static magnetic field of the MR system were systematically studied. A main artifact occurring in the magnetic field was found to be correlated with the heart cycle and had amplitudes in the range of EEG and VEP signals. For VEP recordings, a substantial reduction of this effect was achieved by subtraction of the averaged artifact from the averaged composed signal (VEP and artifact) resulting in the VEP signal alone. However, for continuous EEG recordings, there is no such solution, since the observed effect is not sufficiently constant in amplitude, and the standard deviation of the amplitude of the effect is often larger than the EEG amplitud

ISSN:0740-3194    

DOI:10.1002/mrm.1910390105

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1998

数据来源: WILEY

摘要:

AbstractThis paper presents a fast spin echo (FSE) imaging method that employs circular sampling of Jr‐space. The technique has been implemented on a 2 Tesla imaging system and validated on both phantoms and living animals. Experimental studies have shown that circular sampling can produce artifact‐free FSE images without the need of phase correction. Although not fully explored, preliminary results also show that circular sampling may have advantages over the conventional rectilinear FSE in signal‐to‐noise ratio and imaging efficiency. A major disadvantage is the increased sensitivity to off‐resonance effects. The authors expect that the FSE technique with circular sampling will find its applications in magnetic resonance microscopy, neuro‐functional imaging, and real‐time dy

ISSN:0740-3194    

DOI:10.1002/mrm.1910390106

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1998

数据来源: WILEY

摘要:

AbstractDifferentiation of absolute metabolite concentrations between gray and white matter in the occipital region of normal human brain was performed byin vivolocalized single‐voxel1H magnetic resonance spectroscopy at 1.5 Testa with long echo time (136 ms). With the combination of image segmentation between white and gray matter and cerebrospinal fluid, signal compensation of T, and T2effects, tissue water signal as the internal concentration reference, as well as compensation by different water contents in gray and white matters, it was determined that the levels of N‐acetylaspartate (NAA), creatine and/or phosphocreatine (Cr), and choline‐containing compounds (Cho) in gray matter were significantly higher than in white matter. The averaged NAA, Cr, and Cho concentrations in gray matter were 11.0, 9.7, and 1.9 mM/liter, respectively, in comparison with 7.5, 5.2, and 1.6 mM/liter in white matter. These results suggest that precise composition of white and gray matter and cerebrospinal fluid is necessary to avoid partial voluming effect in a single voxel and to accurately quantify the metabolite concentra

ISSN:0740-3194    

DOI:10.1002/mrm.1910390107

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1998

数据来源: WILEY

摘要:

AbstractMultivoxel 3D localized proton spectroscopy using a hybrid of 1D 8th‐order transverse Hadamard spectroscopic imaging (HSI) with 2D chemical shift imaging (CSI) is demonstrated in human brain. The spatially selective HSI pulse incorporates naturally into the PRESS sequence (TE= 135 ms), which then both excites an 8 × 8 × 6 cm parallelepiped volume of interest (VOI) and subdivides it into eight slices. The planes of these slices are further partitioned into 16 × 16 voxel arrays using 2D CSI to yield 8 × 8 × 8 voxels within the VOI. Simultaneous 3D coverage yields good voxel signal‐to‐noise (8, 12, and 22 for choline, creatine, andN‐acetylaspartate, respectively) from these 0.75‐ml voxels, in ˜45 min. The high spatial isolation allows localization to within less than 1 cm from the skull without fa

ISSN:0740-3194    

DOI:10.1002/mrm.1910390108

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1998

数据来源: WILEY

摘要:

AbstractThis paper presents an approach to characterizing evoked hemodynamic responses in fMRI based on nonlinear system identification, in particular the use of Volterra series. The approach employed enables one to estimateVolterra kernelsthat describe the relationship between stimulus presentation and the hemodynamic responses that ensue. Volterra series are essentially high‐order extensions of linear convolution or “smoothing.” These kernels, therefore, represent a nonlinear characterization of the hemodynamic response function that can model the responses to stimuli in different contexts (in this work, different rates of word presentation) and interactions among stimuli. The nonlinear components of the responses were shown to be statistically significant, and the kernel estimates were validated using an independent event‐related fMRI experiment. One important manifestation of these nonlinear effects is a modulation of stimulus‐specific responses by preceding stimuli that are proximate in time. This means that responses at high‐stimulus presentation rates saturate and, in some instances, show an inverted U behavior. This behavior appears to be specific to BOLD effects (as distinct from evoked changes in cerebral blood flow) and may represent a hemodynamic “refractoriness.” The aim of this paper is to describe the theory and techniques upon which these conclusions were based and to discuss the implications for experimental desi

ISSN:0740-3194    

DOI:10.1002/mrm.1910390109

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1998

数据来源: WILEY

摘要:

AbstractThe regional distribution of brain metabolites was studied in several cortical white and gray matter areas, cerebellum, and thalamus of young adults with use of quantitative single‐voxel proton MRS at 2.0 T. Whereas the neuronal compoundN‐acetylaspartate is distributed homogeneously throughout the brain,N‐acetylaspartylglutamate increases caudally and exhibits higher concentrations in white matter than in gray matter. Creatine,myo‐inositol, glutamate, and glutamine are less concentrated in cortical white matter than in gray matter. The highest creatine levels are found in cerebellum, parallel to the distribution of creatine kinase and energy‐requiring processes in the brain. Alsomyo‐inositol has highest concentrations in the cerebellum. Choline‐containing compounds exhibit a marked regional variability with again highest concentrations in cerebellum and lowest levels and a strong caudally decreasing gradient in gray matter. The present findings neither support a metabolic gender difference (except for a 1.3‐fold highermyo‐inositol level in parietal white matter of female subjects) nor a metabolic hemi

ISSN:0740-3194    

DOI:10.1002/mrm.1910390110

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1998

数据来源: WILEY

摘要:

AbstractTo evaluate the sensitivity of fast, gradient‐echo MR scan techniques in their ability to detect blood oxygenation level dependent (BOLD) signal changes in task activation studies, three dedicated fast scan techniques, each with whole‐brain coverage, were compared during a 3‐min finger tapping paradigm on nine normal volunteers on a clinical 1.5 T scanner. Multislice (2D) single‐shot spiral, 3D spiral, and multislice (2D) single‐shot EPI scan techniques were done with similar temporal and spatial resolutions on each of the volunteers in random order. After image registration and statistical analysis, the sensitivity to detect activation was evaluated for the techniques by calculatingtscores and number of activated voxels in predetermined regions of interest, including the contralateral primary sensorimotor cortex, the premotor region, the parietal region, the supplementary motor area, and the ipsilateral cerebellum. Baseline images acquired with the three techniques were qualitatively comparable and had a similar effective spatial resolution of around 5 × 5 × 5 mm3, as determined from autocorrelation analysis. The anatomical coverage was somewhat reduced (4 less slices per volume) with EPI at the identical temporal resolution of 1.76 s for all techniques. The use of multislice 2D spiral scan for motor cortex fMRI experiments provided for a superior overall temporal stability, and an increased sensitivity compared with multislice 2D EPI, and 3D spiral scan. The difference in sensitivity between multislice 2D spiral and EPI scans was small, in particular in the case of a ramp‐sampled version of EPI. The difference in performance is attributed mainly to the difference in scan‐to

ISSN:0740-3194    

DOI:10.1002/mrm.1910390111

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1998

数据来源: WILEY