Triple-quantum-filtered imaging of sodium in presence of B(0) inhomogeneities

Triple quantum filtered sodium MRI techniques have been recently demonstrated in vivo. These techniques have been previously advocated as a means to separate the sodium NMR signal from different physiological compartments based on the differences between their relaxation rates. Among the different triple quantum coherence transfer filters, the three-pulse coherence transfer filter has been demonstrated to be better suited for human imaging than the traditional four-pulse implementation. While the three-pulse structure has distinct advantages in terms of RF efficiency, the lack of a refocusing pulse in the filter introduces an increased dependence on the main magnetic field inhomogeneities, which can sometimes lead to significant signal loss. In this paper, we characterize these dependencies and introduce a method for their compensation through the acquisition of a B(0) map and the use of a modified phase cycling scheme.     

Prospects for the rapid detection of mealiness in apples by nondestructive NMR relaxometry

The potential of nuclear magnetic resonance (NMR) relaxometry for quantitative evaluation of apple mealiness has been investigated. The degree of “mealiness” was defined by several mechanical techniques, including penetration, compression and shear rupture as well as by the BRIX (soluble solids) and juiciness levels. These data were correlated with both magnetic resonance imaging (MRI) and NMR water proton transverse relaxation time measurements on a fruit-by-fruit basis. It was found that increasing mealiness caused a systematic increase in the transverse relaxation rate. The potential for rapid, on-line NMR/MRI detection of apple mealiness is discussed.     

Sequential anisotropic Wiener filtering applied to 3D MRI data

We present three different sequential Wiener filters, namely, isotropic, orientation and anisotropic. The first one is similar to the classical Wiener filter in the sense that it uses an isotropic neighborhood to estimate its parameters. Here we present a sequential version of it. The orientation Wiener filter uses oriented neighborhoods to estimate the structure orientation present at each voxel, giving rise to a modified estimator of the parameters. Finally, the anisotropic Wiener filter combines both approaches adaptively so that the appropriate approach is locally selected. Several synthetic experiments are presented showing the performance of the filters with respect to their parameters. A mean square error analysis is performed using a publicly available magnetic resonance imaging (MRI) brain phantom and a comparison with other filtering approaches is carried out. In addition, results from filtering real MRI data are presented.     

Comparing anisotropic diffusion filters for the enhancement of sodium magnetic resonance images

The anisotropic diffusion (AND) filter, an image processing technique derived from physics, was applied to low-resolution sodium magnetic resonance imaging (MRI) to examine the possibilities of image enhancement by postprocessing. We compared six different variants of AND filters. Besides the qualitative good results on phantom measurements, quantitative analyses on MRI of human kidney yielded major improvements in noise reduction and other quality measures: the noise (i.e., the standard deviation in the image background) could be reduced to 1%-2% of its original value, while linear filters (Gaussian, Fermi, Hamming) achieved a reduction to 42%-64%. Besides that, less than 5% of structures and intensities are lost when using AND filters. Comparing the different variants, the two-dimensional and the three-dimensional AND filter outperformed the histogram-of-gradient and tensor-based AND filter. We envision that by using these AND filters, quantitative analysis of sodium MRI of kidney could be improved.     

Investigations and Modeling of the Effect of Magnetic Nanoparticles on MR Image Contrast

The investigations of nuclear magnetic resonance (NMR) relaxation of protons in aqueous solution and 2% agar–agar gel in the presence of magnetic nanoparticles were performed. To identify the effect of magnetic nanoparticles on the contrast of magnetic resonance images, the dependences of the MR signal intensity on the parameters of the two pulse radio-frequency (RF) sequences (spin-echo, gradient-echo) most commonly used in MRI for different values of the magnetic nanoparticle (MNP) concentration were simulated and analyzed. Recommendations for choosing the optimal values of pulse RF sequence parameters for MR imaging (MRI) in the presence of MNPs are formulated. MRI studies of phantom samples with 2% agar–agar gel containing MNPs have been performed for choosing the fast pulse RF sequence which shows the greatest contrast effect on MR images. A program for modeling magnetic resonance tomograms and determination of optimal values of pulse RF sequence parameters to achieve the best contrast of magnetic resonance images is developed. This program allows to reduce the time of MRI studies, to assess the possibility of using MNPs for contrast of MR images and to simulate the MR image in the presence of magnetic nanoparticles at the planning stage of procedures in MR-guided theranostics.     

Dynamics of paramagnetic agents by off-resonance rotating frame technique

Off-resonance rotating frame technique offers a novel tool to explore the dynamics of paramagnetic agents at high magnetic fields (B0 > 3T). Based on the effect of paramagnetic relaxation enhancement in the off-resonance rotating frame, a new method is described here for determining the dynamics of paramagnetic ion chelates from the residual z-magnetizations of water protons. In this method, the dynamics of the chelates are identified by the difference magnetization profiles, which are the subtraction of the residual z-magnetization as a function of frequency offset obtained at two sets of RF amplitude omega(1) and pulse duration tau. The choices of omega(1) and tau are guided by a 2-D magnetization map that is created numerically by plotting the residual z-magnetization as a function of effective field angle theta and off-resonance pulse duration tau. From the region of magnetization map that is the most sensitive to the alteration of the paramagnetic relaxation enhancement efficiency R(1rho)/R1, the ratio of the off-resonance rotating frame relaxation rate constant R(1rho) verse the laboratory frame relaxation rate constant R(1), three types of difference magnetization profiles can be generated. The magnetization map and the difference magnetization profiles are correlated with the rotational correlation time tauR of Gd-DTPA through numerical simulations, and further validated by the experimental data for a series of macromolecule conjugated Gd-DTPA in aqueous solutions. Effects of hydration water number q, diffusion coefficient D, magnetic field strength B0 and multiple rotational correlation times are explored with the simulations of the magnetization map. This method not only provides a simple and reliable approach to determine the dynamics of paramagnetic labeling of molecular/cellular events at high magnetic fields, but also a new strategy for spectral editing in NMR/MRI based on the dynamics of paramagnetic labeling in vivo.     

磁共振波谱与成像技术

文章简要介绍了核磁共振的基本原理,详细阐述了液体核磁共振在蛋白质结构、功能和动力学等方面的研究进展,论述了增强固体核磁共振分辨率的方法及其应用,讲述了磁共振成像的原理并综述了不同磁共振成像方法的应用研究进展,并对核磁共振的发展作了展望。     

L-赖氨酸长链烷基酯双DTPA酰胺钆(Ⅲ)配合物的合成、表征与弛豫效能

通过二乙三胺五乙酸单环酸酐(DTPA-MA)分别与L-赖氨酸的十八酯、十六酯、十四酯和十二酯的双酰化反应, 制得四种含有双DTPA螯合单元的新型配体. 它们与GdCl3·6H2O配合得到相应的双核钆(Ⅲ)配合物. 表征了配体和配合物的结构, 测试了配合物的纵向弛豫效能(R1). 结果表明: 这四种新钆配合物的R1都高于Gd-DTPA.     

Noninvasive characterization of gonad maturation and determination of the sex of Pacific oysters by MRI

The aim of this study was to test the ability of magnetic resonance imaging (MRI) technique to characterize gonad development and to determine the sex of live Pacific oysters through their shells. A preliminary nuclear magnetic resonance (NMR) relaxometry study was conducted to characterize T1 and T2 NMR relaxation parameters for the main oyster organs. This showed that T1-weighted MRI sequences were most appropriate to optimize contrasts between tissues in images. The results showed that gray levels of gonads in images acquired with gradient-echo sequence were variably affected by T2* weighting effect. However, the ovaries systematically gave a hypersignal in spin-echo T1-weighted images, and stack histograms of female oysters showed a peak well separated from that of male oysters. An automated method is proposed to quantify the development of oysters and their gonad maturation and to identify their sex.     

Quantitative multi-modal functional MRI with blood oxygenation level dependent exponential decays adjusted for flow attenuated inversion recovery (BOLDED AFFAIR)

A magnetic resonance imaging (MRI) method is described that allows interleaved measurements of transverse (R(2)(*) and R(2)) and longitudinal (R(1)) relaxation rates of tissue water in conjunction with spin labeling. The image-contrasts are intrinsically blood oxygenation level dependent (BOLD) and cerebral blood flow (CBF) weighted, but each contrast is made quantitative by two echo time (TE) and inversion recovery time (TIR) acquisitions with gradient echo (GE) and spin echo (SE) weighted echo-planar imaging (EPI). The EPI data were acquired at 7 Tesla with nominal spatial resolution of 430 x 430 x 1000 microm(3) in rat brain in vivo. The method is termed as blood oxygenation level dependent exponential decays adjusted for flow attenuated inversion recovery (BOLDED AFFAIR) and allows acquisition of R(2)(*), R(2), and CBF maps in an interleaved manner within approximately 12 minute. The basic theory of the method, associated experimental/systematic errors, and temporal restrictions are discussed. The method is validated by comparison of multi-modal maps obtained by BOLDED AFFAIR (i.e., two TE and TIR values with GE and SE sequences) and conventional approach (i.e., multiple TE and TIR values with GE and SE sequences) during varied levels of whole brain activity. Preliminary functional data from a rat forepaw stimulation model demonstrate the feasibility of this method for functional MRI (fMRI) studies. It is expected that with appropriate precautions this method in conjunction with contrast agent-based MRI has great potential for quantitative fMRI studies of mammalian cortex.     

Difference-NMR techniques for selection of components on the basis of relaxation times

This work describes a numerical methodology to obtain more efficient relaxation filters to selectively retain or remove components based on relaxation times. The procedure uses linear combinations of spectra with various recycle or filter delays to obtain components that are both quantitative and pure. Modulation profiles are calculated assuming exponential relaxation behavior. The method is general and can be applied to a wide range of solution or solid-state NMR experiments including direct-polarization (DP), or filtered cross-polarization (CP) spectra. 13C NMR experiments on isotactic poly(1-butene) and dimethyl sulfone showed the utility of the technique for selectively suppressing peaks.     

基于AD607芯片的核磁共振模拟接收机

核磁共振波谱和核磁共振成像技术（MRI）已经成为一种广泛应用于物理、化学、材料、生物医学等领域的重要研究工具和医疗诊断手段,但是仪器复杂、价格昂贵. 本文提出了一种改进的基于AD607芯片的模拟接收机设计. 相对于传统的模拟接收机,此设计可以减少接收机使用的外围器件的数量,从而简化设备. 而相对于全数字接收机,本文提出的设计结构简单,并且可以工作在较高的频率. 文章最后给出了核磁共振成像实验的结果.     

Coexistence of two different Cr ions by self-doping in half-metallic CrO2 nanorods

We investigated the physical properties of Cr ions in half-metallic CrO(2) using 53Cr nuclear magnetic resonance (NMR). The 53Cr NMR spectra showed two peaks with a similar intensity instead of the single peak anticipated from a uniform single valence Cr(+4). Both Cr peaks exhibited a strong anisotropy in the hyperfine field and similar values in the enhancement factor and T2 relaxation time. These results suggest the coexistence of two different Cr ions (provisionally, Cr(+4 +/- 1/3)) by a self-doping effect, which is a likely origin of the metallic ferromagnetism in CrO(2).     

Linear signal combination T2 spectroscopy

A technique is presented for performing T2 spectroscopy in magnetic resonance imaging (MRI). It is based on a weighted linear combination of T2 decay data. The data is combined in a manner that acts like a filter on the T2 spectrum. The choice of weighting coefficients determines the filter specifications (e.g. passband/stopband locations, stopband suppression factors). To perform spectroscopy, a series of filters are designed with narrow passbands centered about consecutive regions of the T2 spectrum. This provides an estimate of every region of the spectrum. Taken together, an initial estimate of the full T2 spectrum is thus obtained. However, the filtering process causes a distortion of the estimate relative to the true spectrum. To reduce this distortion, deconvolution is performed. The characteristics of the technique are first evaluated through simulation. The technique is then applied to experimental MRI data to demonstrate practical feasibility. T2 spectroscopy falls into a class of problems requiring inverse transformation with a set of exponential basis functions (i.e. the Laplace Transform). It is demonstrated how the present technique may be applied to problems involving non-exponential basis functions as well.     

Fast and high-resolution MRI on the basis of interleaved-spiral technique at 4.7 T and its application for imaging of ischemic rat brain

The interleaved-spiral magnetic resonance imaging (MRI) technique was implemented and optimized on a Bruker Biospec 47/30 scanner. The method gives rise to high-resolution images with a time saving factor of up to 8, as compared to the conventional approach. A multifunctional pulse sequence for the fast interleaved-spiral MRI was composed. These functions include spin intensity imaging, transverse relaxation timeT ₂ and apparent diffusion-weighted imaging. The method was used to obtain the dynamic responses of a rat brain during ischemia.     

An Adjustable TD-NMR Method for Rapid and Quantitative Analysis of Body Composition in Awake Mice

This paper describes an adjustable method for body composition analysis of awake mice based on time domain nuclear magnetic resonance (TD-NMR) technology. A T1-weighted CPMG pulse sequence was introduced to acquire the NMR signals that contain both longitudinal (T1) and transversal (T2) relaxation information of mice. It took less than 90 s to dramatically lighten the burden on animals. A model training according to a well-designed factorial experiment was conducted by measuring a series of artificial samples, and the relations between NMR signals and body components (lean, fat and free fluid) were established. This NMR model gave a good fitting (R2 > 0.99) between real values and predictive values and measuring stability were also demonstrated. A calibration method was highlighted to figure out how to keep NMR model effective on another NMR scanner or avoid the error from fluctuation of machine without a time-consuming re-modeling process.     

Standardization of results of NMR relaxation experiments in rubber investigation

Since low-field nuclear magnetic resonance (LF NMR) becomes a widely used technique of polymer structure analysis, the unification of the variety of measurements made on different LF NMR devices is particularly necessary. The basic approach to make them comparable for different kinds of hardware is given. It is based on the experimentally proven fact that artefact contributions to the values of spin-spin relaxation time have a multiplicative character. Recommendations of how to use reference samples are presented as well.     

Selective imaging of biofilms in porous media by NMR relaxation.

Nuclear magnetic resonance imaging (NMRI) techniques were employed to identify and selectively image biological films (biofilm) growing in aqueous systems. Biofilms are shown to affect both the longitudinal (T1) and transverse (T2) NMR relaxation time values of proximal water hydrogens. Results are shown for biofilm growth experiments performed in a transparent parallel-plate reactor. A comparison of biofilm distributions by both NMR and optical imaging yielded general agreement for both an open-flow system and an idealized porous system (the reactor without and with packed glass beads, respectively). The selective imaging of biofilm by relaxation NMRI is dependent upon the resolution of relaxation times for the fluid phases, dynamic range, and signal-to-noise ratio. For open-flow systems, the use of a rapid and quantitative T2-sorted NMRI technique was preferred. For porous systems where T2 values are generally more similar, a T1-weighted technique was preferred.     

宽带磁共振T/R开关的设计与实现

磁共振系统中的发射/接收（transmit/receive,T/R）开关用于收发通路的切换,要求在隔离大功率信号的同时具有较小的插入损耗;另外,在宽带磁共振谱仪中,一般的窄带T/R开关也不再适用.本文基于PIN（positive-intrinsic-negative）二极管设计了工作频率为10~100 MHz的宽带磁共振T/R开关.通过减小电平跳变、优化开关过冲和解决二极管被动导通等问题,获得了插入损耗小于1 dB、大功率信号隔离度大于75 dB、开关时间1 μs的整体性能.用本文设计的宽带T/R开关替代一般探头中的窄带T/R开关,在0.5 T核磁共振（NMR）波谱仪上测得了氢核和氟核两个不同频率的磁共振信号,验证了宽带T/R开关的宽带性能.     

Equilibrium signal intensity mapping, an MRI method for fast mapping of longitudinal relaxation rates and for image enhancement

INTRODUCTION: Inhomogeneity of magnetic fields, both B(0) and B(1), has been a major challenge in magnetic resonance imaging (MRI). Field inhomogeneity leads to image artifacts and unreliability of signal intensity (SI) measurements. This work proposes and shows the feasibility of generating equilibrium signal intensity (SI(Eq)) maps that can be utilized either to speed up relaxation-rate measurement or to enhance image quality and relaxation-rate-based weighting in various applications. METHODS: A 1.5-T MRI scanner was used. In canines (n=4), myocardial infarction was induced, and 48 h after the administration of 0.05 mmol kg(-1) Gd(ABE-DTTA), a contrast agent with slow tissue kinetics, in vivo R(1) mapping was carried out using an inversion recovery (IR)-prepared, fast gradient-echo sequence with varying inversion times (TIs). To test the SI(Eq) mapping method without the confounding effects of motion and blood flow, we carried out ex vivo R(1) mapping after the administration of 0.2 mmol kg(-1) Gd(DTPA) using an IR-prepared, fast spin-echo sequence in another group of dogs (n=2). R(1,full) maps and SI(Eq) maps were generated from the data from both sequences by three-parameter nonlinear curve fitting of the SI versus TI dependence. R(1,full) maps served as the reference standard. Raw IR images were then divided by the SI(Eq) maps, yielding corrected SI maps (COSIMs). Additionally, R(1) values were calculated from each single-TI image separately, using the SI(Eq) value and a one-parameter curve-fitting procedure (R(1,single)). Voxelwise correlation analysis was carried out for the COSIMs and the R(1,single) maps, both versus the standard R(1,full) maps. Deviations of R(1,single) from R(1,full) were statistically evaluated. RESULTS: In vivo, COSIM versus R(1,full) showed significantly (P<.05) better correlation [correlation coefficient (CC)=0.95] than SI versus R(1,full) with a TI=700-800 ms, which is 200-300 ms longer than the tau(null) (500 ms) of viable myocardium. With such TIs, SI versus R(1,full) yielded CCs of 0.86-0.88. R(1,single) versus R(1,full) yielded a peak CC of 0.96 at TI=700-900 ms. Mean deviations of R(1,single) from R(1,full) were below 5% for TIs between 500 and 1000 ms. Ex vivo, where tau(null) was 300 ms, the advantage of correction with SI(Eq) was not in the improvement of linear correlation but more in the reduction of scatter. Peak CCs for SI versus R(1,full) and COSIM versus R(1,full) at TI=500 ms were 0.96 for both. The ex vivo CC for R(1,single) versus R(1,full) at TI=500 ms was 0.98. Mean deviations of R(1,single) from R(1,full) were below 5% for TIs between 400 and 700 ms. CONCLUSIONS: Once the corresponding SI(Eq) map is obtained from a control stack, R(1) can be obtained accurately, using only a single IR image and without the need for a stack of TI-varied images. This approach could be applied in various dynamic MRI studies where short measurement time, once the dynamics has started, is of essence. When using this method with IR-prepared T(1)-weighted images, it is essential that the single TI be chosen such that the longitudinal relaxation in all voxels of interest would have passed tau(null). SI(Eq) maps are also useful in eliminating confounders from MR images to allow obtaining SI values that reflect more faithfully the relaxation parameter (R(1)) sought.