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31.
32.
Rolf Pohmann 《NMR in biomedicine》2013,26(1):74-82
The accuracy and precision of the quantification of metabolite concentrations in in vivo 1H NMR spectroscopy are affected by linewidth and signal‐to‐noise ratio (SNR). To study the effect of both factors in in vivo 1H NMR spectra acquired at ultrahigh field, a reference spectrum was generated by summing nine in vivo 1H NMR spectra obtained in rat brain with a STEAM sequence at 16.4 T. By progressive deterioration of linewidth and SNR, 6400 single spectra were generated. In an accuracy study, the variation in the mean concentrations of five metabolites was mainly dependent on SNR, whereas 11 metabolites were predominantly susceptible to the linewidth. However, the standard deviations of the concentrations obtained were dependent almost exclusively on the SNR. An insignificant correlation was found between most of the heavily overlapping metabolite peaks, indicating independent and reliable quantification. Two different approaches for the consideration of macromolecular signals were evaluated. The use of prior knowledge derived by parameterization of a metabolite‐nulled spectrum demonstrated improved fitting quality, with reduced Cramér–Rao lower bounds, compared to the calculation of a regularized spline baseline. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
33.
Improving hyperpolarized 129Xe ADC mapping in pediatric and adult lungs with uncertainty propagation
34.
In this work, a multiecho parallel echo‐planar imaging (EPI) acquisition strategy is introduced as a way to improve the acquisition efficiency in parallel diffusion tensor imaging (DTI). With the use of an appropriate echo combination strategy, the sequence can provide signal‐to‐noise ratio (SNR) enhancement while maintaining the advantages of parallel EPI. Simulations and in vivo experiments demonstrate that a weighted summation of multiecho images provides a significant gain in SNR over the first echo image. It is experimentally demonstrated that this SNR gain can be utilized to reduce the number of measurements often required to ensure adequate SNR for accurate DTI measures. Furthermore, the multiple echoes can be used to derive a T2 map, providing additional information that might be useful in some applications. Magn Reson Med 60:1512–1517, 2008. © 2008 Wiley‐Liss, Inc. 相似文献
35.
Concentric coil arrays for parallel MRI. 总被引:1,自引:0,他引:1
Michael A Ohliger Robert L Greenman Randy Giaquinto Charles A McKenzie Graham Wiggins Daniel K Sodickson 《Magnetic resonance in medicine》2005,54(5):1248-1260
A new type of coil array is proposed that consists of concentrically placed coil elements, each of which is characterized by symmetrically arranged lobes that have alternating current directions. Symmetries in the coil elements' conductor paths allow for the minimization of mutual inductance and noise correlations. In addition, the concentric arrangement of the coil elements provides spatial encoding capabilities in multiple directions, which is valuable when arrays are used with parallel MRI. Simulations are presented that describe the signal-to-noise ratio (SNR) properties of individual concentric array elements, and a four-element prototype concentric array is constructed. This prototype array is compared experimentally with three alternative four-element array designs. The overall SNR of the concentric array is comparable to the SNR of the competing arrays. Reconstruction of twofold undersampled data using the concentric array yields an average g-factor of less than 1.3 in all directions parallel to the plane of the array. There is some degradation in performance when threefold undersampled data are reconstructed, but the array still shows substantial directional invariance compared to alternative designs. Both fully-sampled and undersampled cardiac images acquired using the concentric array are shown. These results suggest that concentric structures can be useful tools for designing specialized coil arrays for parallel MRI. 相似文献
36.
A Liffers H H Quick C U Herborn H Ermert M E Ladd 《Magnetic resonance in medicine》2003,50(2):439-443
The geometry of an RF phased-array receiving coil for high-resolution MRI of the carotid artery, particularly the bifurcation, was optimized with respect to signal-to-noise ratio (SNR). A simulation tool was developed to determine homogeneity, sensitivity, and SNR for a given imaging situation. The algorithm takes into account the coil geometry, the parameters of the measured object, and the imaging parameters of the pulse sequence. The coil with the optimum geometry was implemented as a receive-only coil for 1.5 T and comparative SNR measurements with different coils were performed. The experimental SNR measurements verified the simulations. The optimized carotid artery phased array offered the best SNR over the desired field of view. In vivo high-resolution MRI of the carotid arteries of healthy volunteers and patients with known stenosis was conducted with the optimized phased array coil. The capability of the phased array coil for resolving components within the carotid artery walls is demonstrated. Magn Reson Med 50:439-443, 2003. 相似文献
37.
Enhancing measured diffusion anisotropy in gray matter by eliminating CSF contamination with FLAIR. 总被引:1,自引:0,他引:1
Xiangyang Ma Yasser M Kadah Stephen M LaConte Xiaoping Hu 《Magnetic resonance in medicine》2004,51(2):423-427
In this work, the effect of fluid-attenuated inversion recovery (FLAIR) on measured diffusion anisotropy was investigated in gray matter. DTI data were obtained with and without FLAIR in six normal volunteers. The application of FLAIR was experimentally demonstrated to lead to a consistent increase in fractional anisotropy (FA) in gray-matter regions, which was attributed to suppressed partial volume effects from CSF. In addition to these experimental results, Monte Carlo simulations were performed to ascertain the effect of noise on the measured FA under the experimental conditions of this study. The experimentally observed effect of noise was corroborated by the simulation, indicating that the increase in the measured FA was not due to a noise-related bias but to an actual increase in diffusion anisotropy. This enhanced measurement of diffusion anisotropy can be potentially used to differentiate directionally dependent structure and tracking fibers in gray matter. 相似文献
38.
The effect of gradient system performance on segmented k-space gradient echo imaging is presented. Three cases were investigated. First, an ideal system that has infinite slew rates and unlimited maximum gradient strengths was considered. Second, a “high speed” imaging system (2.3 (G/cm), 23 (G/cm)/ms) was considered. These two cases were compared with a “conventional” imaging system (1(G/cm), 1.67 (G/cm)/ms). It was found that substantial increases in SNR can be achieved (≈? 45%) by using high speed versus a conventional gradient system, for a TR of 6 ms. For trapezoidal gradient waveforms, there exists an optimum maximum gradient strength for a given slew rate, and any increase in gradient strength above this optimum will not be utilized by an optimized sequence. These studies have shown that increasing TR without decreasing the bandwidth is not a good way to increase SNR for constant scan time. 相似文献
39.
Potential and feasibility of parallel MRI at high field 总被引:4,自引:0,他引:4
Wiesinger F Van de Moortele PF Adriany G De Zanche N Ugurbil K Pruessmann KP 《NMR in biomedicine》2006,19(3):368-378
This survey focuses on the fusion of two major lines of recent progress in MRI methodology: parallel imaging with receiver coil arrays and the transition to high and ultra-high field strength for human applications. As discussed in this paper, combining the two developments has vast potential due to multiple specific synergies. First, parallel acquisition and high field are highly complementary in terms of their individual advantages and downsides. As a consequence, the joint approach generally offers enhanced flexibility in the design of scanning strategies. Second, increasing resonance frequency changes the electrodynamics of the MR signal in such a way that parallel imaging becomes more effective in large objects. The underlying conceptual and theoretical considerations are reviewed in detail. In further sections, technical challenges and practical aspects are discussed. The feasibility of parallel MRI at ultra-high field is illustrated by current results of parallel human MRI at 7 T. 相似文献
40.
A phased-array coil was developed to facilitate imaging the vasculature of the lower extremities. The array consists of four surface coils placed in a Plexiglas “I-beam” frame that are configured to allow bilateral studies with up to a 40-cm field of view (FOV). Data from phantoms indicate an increase in signal-to-noise ratio (SNR) in the regions of interest by an average factor of 2.8 ± 0.9 over that of the body coil. Preliminary in vivo data have also been obtained from n = 8 subjects and demonstrate significant improvements in image quality. The coil design described here should lead to reduced scan times through the ability to image both legs simultaneously with less need for patient repositioning. 相似文献