Radiofrequency magnetic field gradient echoes have reduced sensitivity to susceptibility gradients

The amplitudes of gradient-echoes produced using static field gradients are sensitive to diffusion of tissue water during the echo evolution time. Gradient-echoes have been used to produce MR images in which image intensity is proportional to the self-diffusion coefficient of water. However, such measurements are subject to error due to the presence of background magnetic field gradients caused by variations in local magnetic susceptibility. These local gradients add to the applied gradients. The use of radiofrequency (RF) gradients to produce gradient-echoes may avoid this problem. The RF magnetic field is orthogonal to the offset field produced by local magnetic susceptibility gradients. Thus, the effect of the local gradients on RF gradient-echo amplitude is small if the RF field is strong enough to minimize resonance offset effects. The effects of susceptibility gradients can be further reduced by storing magnetization longitudinally during the echo evolution period. A water phantom was used to evaluate the effects of background gradients on the amplitudes of RF gradient-echoes. A surface coil was used to produce an RF gradient of between 1.3 and 1.6 gauss/cm. Gradient-echoes were detected with and without a 0.16 gauss/cm static magnetic field gradient applied along the same direction as the RF gradient. The background static field gradient had no significant effect on the decay of RF gradient-echo amplitude as a function of echo evolution time. In contrast, the effect of the background gradient on echoes produced using a 1.6 gauss/cm static field gradient is calculated to be significant. This analysis suggests that RF gradient-echoes can produce MR images in which signal intensity is a function of the self-diffusion coefficient of water, but is not significantly affected by background gradients.     

Hole-burning diffusion measurements in high magnetic field gradients

We describe methods for the measurement of translational diffusion in very large static magnetic field gradients by NMR. The techniques use a "hole-burning" sequence that, with the use of fringe field gradients of 42 T/m, can image diffusion along one dimension on a submicron scale. Two varieties of this method are demonstrated, including a particularly efficient mode called the "hole-comb," in which multiple diffusion times comprising an entire diffusive evolution can be measured within the span of a single detected slice. The advantages and disadvantages of these methods are discussed, as well as their potential for addressing non-Fickian diffusion, diffusion in restricted media, and spatially inhomogeneous diffusion.     

Application of pulsed high magnetic field to μSR studies

The technical development in the production of pulsed high magnetic field to be used for μSR studies is described. A pulsed field up to 16 T with half-sine wave shape and 1 msec duration is repeated every 2 sec and the muon beam pulses are synchronized at the peak of the field. Paramagnetic relaxation measurement on MnO has been performed at above 10 T as a test of this equipment and relaxation in PrCo₂Si₂ has been studied in the vicinity of the metamagnetic transition at 12 T at He temperature.     

脉冲强磁场下的电极化测量系统

多铁性材料是当前物质科学研究的热点,具有重要的科学研究意义和应用前景.低温和强磁场实验环境为研究多铁性材料提供了一种有效途径.脉冲强磁场下的电极化测量系统能实现最高磁场强度60 T、最低温度0.5 K的铁电特性测量.该系统采用热释电方法,具有磁场强度高、控温范围广、转角测量等特点,可用于强磁场下的磁电特性研究.本文介绍了该系统的测量装置和实验原理,并展示了其在多铁性材料研究中的一系列应用,揭示了脉冲强磁场电极化测量系统在磁电特性探索中的重要作用.     

t(1) noise and sensitivity in pulsed field gradient experiments

In this paper, a calculation routine based on product operator formalism and coherence pathway is presented, which describes t(1) noise and sensitivity in pulsed field gradient experiments. Several examples including the absolute value mode pulsed field gradient COSY, MQF-COSY, MQC, HMQC, and NOESY sequences are investigated to study the t(1) noise stemming from the phase errors of radiofrequency (RF) pulses and the sensitivity affected by RF pulse rotation angles. Our theoretical results indicate that the t(1) noises in P-type COSY, multiple-quantum-filtered (MQF)-COSY, and multiple-quantum coherence (MQC) are lower than those in the N-type corresponding experiments, while in HMQC and NOESY there is no difference in t(1) noise effects between P-type and N-type spectra. Meanwhile, from the calculations, we obtained the optimized RF pulse rotation angles in those sequences. In MQF-COSY, an increase in sensitivity of about 4(cos(2)θ sin(q-1)2θ)(2) - 1 (θ = arc cot can be achieved by using the optimized angles. In MQC, the increase is 2 cos(2)θ sin(q-1)2θ - 1 (θ = arc cot. MQF-COSY experiments are also carried out to support our corresponding theoretical results.     

对称脉冲强磁场发生器的研制

 对称脉冲强磁场发生器是一台能够产生周期为10 μs左右的强磁场装置,由10 μF的脉冲电容作为储能系统,平板传输线、电缆和气体开关作为传输系统,单匝线圈作为负载。为了使两个对称布置的负载能同时工作,设计了一个能同时输出8路脉冲信号的同步触发系统,其时间分散性约为20 ns。在常温常压下,当储能电容和同步触发系统分别加压到28 kV和49 kV并且气体开关工作气压为80 kPa时,线圈中心最大磁感应强度约为18 T。     

脉冲强磁场磁特性科学测量系统及其应用

作为一种极端条件下的实验技术,脉冲强磁场下的磁化测量在磁性材料等研究领域发挥着重要作用。武汉国家脉冲强磁场科学中心建成了两套先进的脉冲强磁场磁化测量系统,并已对外开放运行。文章介绍了该测量系统的组成、测量原理和系统特点,以及该系统在量子磁性材料、功能磁性材料、强关联材料等研究领域中的应用,并对未来的发展提出了展望。     

65 T脉冲强磁体设计与实验研究

研制并实验测试一台65 T脉冲强磁场磁体。磁体线圈采用14层纯铜导线与Zylon纤维交替缠绕而成,内绕组采用分布式加固技术,实现应力分布的优化,外层由不锈钢套筒与碳纤维加固。内外绕组分别采用两种不同横截面导线平衡能量分布,实现内外绕组温度分布相对均匀,避免局部过热。磁体孔径为12 mm,高为120 mm,通过1 MJ/3.2 mF/25 kV高储能密度电容器供电,在20.6 kV电压时,获得了磁感应强度为65 T、持续时间为20 ms的脉冲磁场。     

65T脉冲强磁体设计与实验研究

研制并实验测试一台65T脉冲强磁场磁体。磁体线圈采用14层纯铜导线与Zylon纤维交替缠绕而成,内绕组采用分布式加固技术,实现应力分布的优化,外层由不锈钢套筒与碳纤维加固。内外绕组分别采用两种不同横截面导线平衡能量分布,实现内外绕组温度分布相对均匀,避免局部过热。磁体孔径为12mm,高为120mm,通过1MJ/3.2mF/25kV高储能密度电容器供电,在20.6kV电压时,获得了磁感应强度为65T、持续时间为20ms的脉冲磁场。     

An approach to modeling the dependence of magnetization on magnetic field in the high field regime

The “law of approach to saturation” is a well-known mathematical model for describing the behavior of ferromagnets at high magnetic field strengths which has the additional advantage that it can be linked to anisotropy through one of the terms in the mathematical expression of the law. In this paper, two recent and more comprehensive models of the magnetization process are compared with the law of approach and with each other in terms of their capability to describe the dependence of the magnetization curve in the high field regime. The comparison leads to relations between these two models and the interpretation of certain aspects of the models in terms of anisotropy. It is shown that the effects of anisotropy can be incorporated directly into these models without any additional assumptions.     

An NMR technique for measurement of magnetic field gradient waveforms

Almost all NMR imaging and localized spectroscopic methods fundamentally rely on the use of magnetic field gradients. It follows that precise information on gradient waveform shape and rise-times is often most useful in experimental MRI. We present a very simple and robust method for measuring the time evolution of a magnetic field gradient. The method is based on the analysis of the NMR signal in the time domain, and requires no specialized field measurement probes for its implementation. The technique makes use of the principal that for small flip angles the excitation profile is a good approximation to the Fourier transform of the radio frequency pulse shape. Creation of the NMR signal can be considered as an inverse Fourier transform and thus variation of the gradient strength during the excitation pulse influences the shape of the NMR signal. Although originally designed for measurement of the rise time only, we have now extended the technique to measure the exact time course of the gradient. The theory is confirmed by experimental results for gradient waveform field measurements in a high-field vertical bore system.     

Reduced apparent longitudinal relaxation times in slice-selective experiments in strong magnetic field gradients

In contrast to transverse nuclear magnetizations, longitudinal spin magnetizations are usually considered as insensitive to magnetic field gradients. While this assumption is valid for homogeneously excited samples, the apparent longitudinal spin relaxation behavior of thin magnetization slices in high magnetic fields is strongly modified by diffusion. In this contribution, we present the results of theoretical and experimental studies on this effect. Furthermore, possible applications and the impact on different types of NMR techniques using strong magnetic field gradients are discussed.     

A simple method to determine magnetic field gradients

The position of a straight domain wall in a magnetic field gradient in a narrow bar of magnetically uniaxial material is connected with the magnitude of the field gradient. The corresponding relationship is derived. It allows one to determine the field gradient. from a visual measurement of the coordinate of a domain wall and the known parameters of the platelet.     

Diffusion in porous building materials with high internal magnetic field gradients

An algorithm to calculate NMR signals of a multi-echo pulse sequence with arbitrary position dependent B0 and B1 fields taking into account relaxation and spin-diffusion is presented. The multi-echo pulse sequence consists of an initial RF pulse ("90 degrees " RF pulse) and a series of L refocusing RF pulses with arbitrary phases and flip-angles. The calculation is exact and takes into account all the magnetization pathways that contribute to the signal on a predefined spatial grid. The theoretical prediction is verified experimentally using a high field NMR microscopy system. The algorithm was implemented in a simulation program in order to optimize the design of an inside-out MR intra-vascular catheter that is used for characterization of vessel wall tissue. Measured data obtained with the catheter are in good agreement with the theoretical prediction of the simulation.     

The virtual NMR spectrometer: a computer program for efficient simulation of NMR experiments involving pulsed field gradients

This paper presents a software program, the Virtual NMR Spectrometer, for computer simulation of multichannel, multidimensional NMR experiments on user-defined spin systems. The program is capable of reproducing most features of the modern NMR experiment, including homo- and heteronuclear pulse sequences, phase cycling, pulsed field gradients, and shaped pulses. Two different approaches are implemented to simulate the effect of pulsed field gradients on coherence selection, an explicit calculation of all coherence transfer pathways, and an effective approximate method using integration over multiple positions in the sample. The applications of the Virtual NMR Spectrometer are illustrated using homonuclear COSY and DQF COSY experiments with gradient selection, heteronuclear HSQC, and TROSY. The program uses an intuitive graphical user interface, which resembles the appearance and operation of a real spectrometer. A translator is used to allow the user to design pulse sequences with the same programming language used in the actual experiment on a real spectrometer. The Virtual NMR Spectrometer is designed as a useful tool for developing new NMR experiments and for tuning and adjusting the experimental setup for existing ones prior to running costly NMR experiments, in order to reduce the setup time on a real spectrometer. It will also be a useful aid for learning the general principles of magnetic resonance and contemporary innovations in NMR pulse sequence design.     

An experimental approach to the analysis of electric field gradients in ionic crystals

The spectrum of a partially oriented sample of ethanol has been analysed by making use of the simpler spectra obtained from the species CD₃CH₂OH and CH₃CD₂OH, together with ¹H?{²H} double resonance. With p-ethoxy-benzylidene-p-n-butylaniline (EBBA) as the nematic solvent the dipolar couplings of CH₃ and CH₂ protons with the OH proton can be observed, and their magnitudes are compared with values calculated assuming different models for C-O-H internal rotation. Information on the quadrupole coupling constant tensor elements for CD₃ and CD₂ deuterium nuclei is obtained.     

NMR in pulsed magnetic field

Nuclear magnetic resonance (NMR) experiments in pulsed magnetic fields up to 30.4 T focused on ¹H and ⁹³Nb nuclei are reported. Here we discuss the advantage and limitation of pulsed field NMR and why this technique is able to become a promising research tool.     

Stray field measurements of flow displacement distributions without pulsed field gradients

The probability distribution P(zeta) of diffusive and advective molecular displacements is determined using a fixed field gradient (FFG) pulse sequence, on fluid flow through a Bentheimer sandstone, in the grossly inhomogeneous stray field of a super-conducting magnet. Two decades of q-space are scanned with stimulated echoes, using the gradient of the stray field and variable encoding times delta. The strength of the gradient permits the use of short encoding times, which is desirable for limiting the distorting effects produced by flow displacements through susceptibility induced field inhomogeneities. CPMG and CP echo trains are used to refocus separately the real and imaginary parts of the stimulated echo, for experimental efficiency.