共查询到20条相似文献,搜索用时 0 毫秒
1.
Hartwig V Vanello N Giovannetti G De Marchi D Lombardi M Landini L Santarelli MF 《Magnetic resonance imaging》2011,29(5):717-722
Knowledge of the spatial distribution of transmission field B1+ and reception sensitivity maps is important in high-field (≥3 T) human magnetic resonance (MR) imaging for several reasons: these include post-acquisition correction of intensity inhomogeneities, which may affect the quality of images; modeling and design of radiofrequency (RF) coils and pulses; validating theoretical models for electromagnetic field calculations; testing the compatibility with MR environment of biomedical implants. Moreover, inhomogeneities in the RF field are an essential source of error for quantitative MR spectroscopy. Recent studies have also shown that B1+ and reception sensitivity maps can be used for direct calculation of tissue electrical parameters and for estimating the local specific absorption rate (SAR) in vivo.Several B1+ mapping techniques have been introduced in the past few years based on actual flip angle (FA) mapping, but, to date, none has emerged as a standard. For reception sensitivity calculation, the signal intensity equation can be used where the nominal FA distribution must be replaced with the actual FA distribution calculated by one of the B1+ mapping techniques.This study introduces a quantitative comparison between two known methods for B1+/actual FA and reception sensitivity mapping: the double-angle method (DAM) and the fitting (FIT) method. Experimental data obtained using DAM and FIT methods are also compared with numerical simulation results. 相似文献
2.
To simultaneously acquire the B1+ magnitude and B1+ phase, a modified multi-echo actual flip-angle imaging (AFI) sequence is proposed. A multi-echo gradient echo sequence was integrated into every even TR of AFI to measure both magnitude and phase of B1+. In addition, to increase the signal-to-noise ratio of the B1+ phase, a double-angle multi-echo AFI sequence, in which the flip-angle of the RF pulses is α at the odd TR and 2α at the even TR is proposed. Images were simulated to evaluate the performance of this method under various imaging and physical parameters. The performance was compared to the spin echo based B1+ mapping method in phantom and in vivo studies. 相似文献
3.
Carl D. Gregory 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1997,129(2):173-183
Shimming is important. Noniterative methods are desirable. Such methods exist for shimming a spectrometer with pulsed field gradients, generally based on field maps made by spin-warp Fourier imaging. For spectrometers with no pulsed gradients (or for cases whereT2is too short to permit echo imaging), an alternative method is presented: projection-reconstruction spectroscopic imaging, which can be accomplished using only the shim coils of a conventional spectrometer. Images so acquired can be used to map the field, even in the presence of multiple spectral components. Noniterative optimization of the axial shims of a GN-300 spectrometer is demonstrated using 1D + 1D spectroscopic images. Prospects for extending the technique to include the radial shims using 3D + 1D spectroscopic images are discussed. 相似文献
4.
Sandra M. Meyers Cornelia Laule Irene M. Vavasour Shannon H. Kolind Burkhard Mädler Roger Tam Anthony L. Traboulsee Jimmy Lee David K.B. Li Alex L. MacKay 《Magnetic resonance imaging》2009
This study compared region of interest (ROI) and voxel-based analysis (VBA) methods to determine the optimal method of myelin water fraction (MWF) analysis. Twenty healthy controls were scanned twice using a multi-echo T2 relaxation sequence and ROIs were drawn in white and grey matter. MWF was defined as the fractional signal from 15 to 40 ms in the T2 distribution. For ROI analysis, the mean intensity of voxels within an ROI was fit using non-negative least squares. For VBA, MWF was obtained for each voxel and the mean and median values within an ROI were calculated. There was a slightly higher correlation between Scan 1 and 2 for the VBA method (R2=0.98) relative to the ROI method (R2=0.95), and the VBA mean square difference between scans was 300% lower, indicating VBA was the most consistent between scans. For the VBA method, mean MWF was found to be more reproducible than median MWF. As the VBA method is more reproducible and gives more options for visualization and analysis of MWF, it is recommended over the ROI method of MWF analysis. 相似文献
5.
T1ρ imaging is useful in a number of clinical applications. T1ρ preparation methods, however, are sensitive to non-uniformities of the B0 magnetic field and the B1 RF field. These common system imperfections can result in image artifacts and quantification errors in T1ρ imaging. We report on a phase-cycling method which can eliminate B1 RF inhomogeneity effects in T1ρ imaging. This method does not only correct for image artifacts but also for T2ρ contamination caused by B1 RF inhomogeneity. The presence of B0 magnetic field inhomogeneity can compromise the effectiveness of this method for B1 RF inhomogeneity correction. We demonstrate that, by combining the spin-locking scheme reported by Dixon et al. (Myocardial suppression in vivo by spin locking with composite pulses. Magn Reson Med 1996; 36:90-94) with phase cycling, we can simultaneously correct B0 magnetic field inhomogeneity effects and B1 RF inhomogeneity effects in T1ρ imaging. Phantom and in vivo data sets are used to demonstrate the proposed methods and to compare them with other existing T1ρ preparation methods. 相似文献
6.
Sensitive and automated detection of iron-oxide-labeled cells using phase image cross-correlation analysis 总被引:2,自引:1,他引:1
Superparamagnetic iron oxide (SPIO) nanoparticles are increasingly being used to noninvasively track cells, target specific molecules and monitor gene expression in vivo. Contrast changes that are subtle relative to intrinsic sources of contrast present a significant detection challenge. Here, we describe a postprocessing algorithm, called Phase map cross-correlation Detection and Quantification (PDQ), with the purpose of automating identification and quantification of localized accumulations of SPIO agents. The method is designed to sacrifice little flexibility - it works on previously acquired data and allows the use of conventional high-SNR pulse sequences with no extra scan time. We first investigated the theoretical detection limits of PDQ using a simulated dipole field. This method was then applied to three-dimensional (3D) MRI data sets of agarose gel containing isolated dipoles and ex vivo transplanted allogenic rat hearts infiltrated by numerous iron-oxide-labeled macrophages as a result of organ rejection. A simulated dipole field showed this method to be robust in very low signal-to-noise ratio images. Analysis of agarose gel and allogenic rat heart shows that this method can automatically identify and count dipoles while visualizing their biodistribution in 3D renderings. In the heart, this information was used to calculate a quantitative index that may indicate its degree of cellular infiltration. 相似文献
7.
Previous studies have shown that iterative in-line X-ray phase retrieval algorithms may have higher precision than direct retrieval algorithms. This communication compares three iterative phase retrieval algorithms in terms of accuracy and efficiency using computer simulations. We found the Fourier transformation based algorithm (FT) is of the fastest convergence, while the Poisson-solver based algorithm (PS) has higher precision. The traditional Gerchberg-Saxton algorithm (GS) is very slow and sometimes does not converge in our tests. Then a mixed FT-PS algorithm is presented to achieve both high efficiency and high accuracy. The mixed algorithm is tested using simulated images with different noise level and experimentally obtained images of a piece of chicken breast muscle. 相似文献
8.
Bhoj Raj Gautam Igor Dubenko Arjun Kumar Pathak Shane Stadler Naushad Ali 《Journal of magnetism and magnetic materials》2009,321(1):29-33
The influence of the substitution of manganese by boron on the crystal structure and magnetic properties of Ni2Mn1−xBxGa Heusler alloys with 0?x?0.5 has been investigated using X-ray diffraction, thermal expansion, resistivity, and magnetization measurements. The samples with concentrations x<0.25 were found to be of single phase and belonged to the cubic L21 crystal structure at room temperature. Crystal cell parameters of the alloys decreased from 5.830 to 5.825 Å with increasing boron concentration (x) from 0 to 0.25. The alloys were ferromagnetically ordered at 5 K and the saturation magnetization decreased with increasing boron concentration. The ferromagnetic ordering and structural transition temperatures for 0?x?0.3 have been observed and the phase (x−T) diagram of the Ni2Mn1−xBxGa system was constructed. The phase (x−T) diagram indicates that the ground state of Ni2Mn1−xBxGa alloys belongs to ferromagnetic martensitic, premartensitic, and austenitic phases in x?0.12, 0.12<x?0.18, and 0.18<x?0.3, respectively. The relative influence of cell parameters and electron concentrations on the phase diagram is discussed. 相似文献
9.
Two-dimensional (2D)-SE, 2D-GE and tri-dimensional (3D)-GE two-point T(1)-weighted MRI methods were evaluated in this study in order to maximize the accuracy of temperature mapping of bread dough during thermal processing. Uncertainties were propagated throughout each protocol of measurement, and comparisons demonstrated that all the methods with comparable acquisition times minimized the temperature uncertainty to similar extent. The experimental uncertainties obtained with low-field MRI were also compared to the theoretical estimations. Some discrepancies were reported between experimental and theoretical values of uncertainties of temperature; however, experimental and theoretical trends with varying parameters agreed to a large extent for both SE and GE methods. The 2D-SE method was chosen for further applications on prefermented dough because of its lower sensitivity to susceptibility differences in porous media. It was applied for temperature mapping in prefermented dough during chilling prior to freezing and compared locally to optical fiber measurements. 相似文献
10.
Petr Brázda Pingo Mutombo Martin Ondráček Cinthia Antunes Corrêa Jaromír Kopeček 《哲学杂志》2018,98(13):1131-1150
The bulk and surface structures of calcium and strontium disilicides are investigated by computational methods using density functional theory. The investigated structures are R6, R3 and P1-CaSi2 and P1-SrSi2. The investigated properties are the cleavage energy at the silicene sheet, buckling of the bulk and surface silicene layers, charge transfer from calcium to silicon, band structure of bulk and surface-terminated structures and adsorption energies on H atoms and H2 molecules on the silicene-terminated surface of the R3 phase. The cleavage energy at the silicene surface is low in all cases. Structures P1-CaSi2 and R3-CaSi2 contain silicene sheets with different coordination to Ca, while R6-CaSi2 contains both types of the sheets. It is shown that the properties of the two types of silicene-like sheets in R6-CaSi2 are similar to those of the corresponding sheets in P1-CaSi2 and R3-CaSi2, and the thermodynamically stable R6 phase is a good candidate for experimental investigation of silicene-terminated surface in calcium disilicide. 相似文献
11.
Liqiong Xing 《Applied Surface Science》2010,256(11):3586-602
A CrOx-Y2O3 sample was prepared by a deposition-precipitation method and phase transformation of the sample under N2 and air atmospheres was characterized by in situ Raman spectroscopy and X-ray diffraction (XRD) techniques. It was found that when the CrOx-Y2O3 sample was heated, CrO3 transformed to YCrO4 and then to YCrO3 and Cr2O3. Also, the transformation started from the surface region of the sample and then extended to the bulk, due to the fact that the phase transformation was detected by Raman spectroscopy at lower temperature compared to that by XRD. In addition, both atmosphere and temperature had influence on the phase transformation in the surface region, while the phase transformation in the bulk was merely dependent on the temperature. It was also found that low oxidation state Cr(III) species on the surface could be re-oxidized to high oxidation state Cr(V) or Cr(VI) species when the thermal treated sample was exposed to ambient air. 相似文献
12.
Jing Zhang Zheng Chen MingYi Zhang QingBo Lai YanLi Lu YongXin Wang 《中国科学G辑(英文版)》2009,52(8):1154-1160
Microscopic phase field simulation is performed to study antisite defect type and temporal evolution characteristic of D022-Ni3V structure in Ni75Al
x
V25−x
ternary system. The result demonstrates that two types of antisite defect VNi and NiV coexist in D022 structure; however, the amount of NiV is far greater than VNi; when precipitates transform from D022 singe phase to two phases mixture of D022 and L12 with enhanced Al:V ratio, the amount of VNi has no evident response to the secondary L12 phase, while NiV exhibits a definitely contrary variation tendency: NiV rises without L12 structure precipitating from matrix but declines with it; temporal evolution characteristic and temperature dependent antisite
defect VNi, NiV are also studied in this paper: The concentrations of the both defects decline from high antistructure state to equilibrium
level with elapsed time but rise with elevated temperature; the ternary alloying element aluminium atom occupies both α and
β sublattices of D022 structure with a strong site preference of substituting α site.
Supported by the National Natural Science Foundation of China (Grant Nos. 50671084 and 50875217), the Doctorate Foundation
of Northwestern Polytechnical University of China (Grant No. CX200806), the China Postdoctoral Science Foundation Funded Project
(Grant No. 20070420218), and the Natural Science Foundation of Shaanxi Province of China 相似文献
13.
Coalescence of BnNn fullerenes: A new pathway to produce boron nitride nanotubes with small diameter
Using density functional theory calculations, we predict that single-walled hemispherical-caped boron nitride (BN) nanotubes with small diameters can be produced via the coalescence of stable nanoclusters. Specifically, the assembly of BnNn (n=12,24) clusters exhibiting particularly high stability and leading to armchair (3,3) and (4,4) BN nanotubes, respectively, are considered. The formed finite-length BN nanotubes have semiconducting properties with wide band gaps attractive to nano-device applications. 相似文献
14.
15.
A high-resolution (0.002 cm−1) infrared absorption spectrum of methylene fluoride-d2 (CD2F2) of the lowest fundamental mode ν4 in the region from 460 to 610 cm−1 has been measured on a Bruker IFS 120-HR Fourier transform infrared spectrometer. More than 3500 transitions have been assigned in this B-type band centered at 521.9 cm−1. The data have been combined with upper state pure rotational measurements in a weighted least-squares fit to obtain molecular constants for the upper state resulting in an overall standard deviation of 0.00018 cm−1. Accurate value for the band origin (521.9578036 cm−1) has been obtained and inclusion of transitions with very high J (?60) and Ka (?34) values has resulted in improved precision for sextic centrifugal distortion constants, in particular DK, HKJ, and HK. 相似文献
16.
D. Jaiswal-nagar D. Pal M. R. Eskildsen P. C. Canfield H. Takeya S. Ramakrishnan A. K. Grover 《Pramana》2006,66(1):113-129
We present a study of magnetization measurements performed on the single crystals of YNi2B2C and LuNi2B2C. For both the compounds, we find flux jumps in magnetisation values in the respective field regions, where the structural
transitions in the flux line lattice symmetry have been reported in these systems via the small angle neutron scattering experiments.
The magnetisation hysteresis loops and the AC susceptibility measurements show pronounced peak effect as well as second magnetisation
peak anomaly for both YNi2B2C and LuNi2B2C. Based on these results, a vortex phase diagram has been constructed for YNi2B2C forH∥c depicting different glassy phases of the vortex matter. 相似文献
17.
Experimental and theoretical study of silicon-doped Sb2Te3 thin films: Structure and phase stability
Xuelai Li Feng RaoZhitang Song Kun Ren Weili LiuZhimei Sun 《Applied Surface Science》2011,257(10):4566-4568
The influence of Si in Sb2Te3 on structure and phase stability was studied by experiments and ab initio calculations. With the increase of Si content in Sb2Te3 samples, the crystallization temperature increases and the crystalline grain size decreases. The incorporation of Si atoms into Sb2Te3 lattice is energetically unfavorable and hence Si atoms most probably accumulated in the boundaries of Sb2Te3 grains. 相似文献
18.
The electrical conductivity of ZrO2 doped with Co3O4 has been measured at various temperatures for different molar ratios. The conductivity increases due to the migration of vacancies created by doping. The conductivity is also found to increase with rise in temperature up to 120°C, and after attaining a maximum the conductivity decreases due to a collapse of the lattice framework. A second rise in conductivity around 460°C in all the compositions confirms the phase transition in ZrO2 from monoclinic to tetragonal symmetry. X-ray powder diffraction and DTA studies were carried out for confirming the doping effects and the transition in ZrO2. 相似文献
19.
G. Polla D. Vega A. G. Leyva P. K. De Perazzo H. Lanza M. A. R. De Benyacar 《Phase Transitions》2013,86(1):15-24
α-Ba2Cu(HCOO)6 grown at room temperature crystallizes in space group P21. On heating a reversible, hysteretic, equitranslational, first-order phase transition at about 60-90°C takes place. Here we discuss the influence of thermal treatments on the occurrence, coexistence and stability ranges of the two observed phases, as studied by several techniques (optical microscopy, differential scanning calorimetry and X-ray diffraction). On cooling a single set of domain walls parallel to (001) have been observed. Taking into account experimental results and the crystal pseudosymmetry of the α phase we propose that the high-temperature β phase crystallizes in space group Pbnm. 相似文献
20.
The results of our NMR, EPR and magnetic susceptibility measurements in the paramagnetic state of TlMnCl3 are reported here. The NMR paramagnetic shift of thallium is found to be small but positive. Mn2+ EPR line is exchange narrowed. The susceptibility measurements indicate an antiferromagnetic transition. The heat of crystallographic
phase transition ΔH, in TlMnCl3 has been measured using differential scanning calorimetry. The crystallographic phase transition appears to be first order
and ΔH is unusually low viz. 10 cal mole−1. In the case of KMnF3 ΔH, which is reported here for the first time, is determined to be 2 cal mole−1. 相似文献