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.     

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.     

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.     

Self-diffusion measurements by a mobile single-sided NMR sensor with improved magnetic field gradient

A simple and fast method of measuring self-diffusion coefficients of protonated systems with a mobile single-sided NMR sensor is discussed. The NMR sensor uses a magnet geometry that generates a highly flat sensitive volume where a strong and highly uniform static magnetic field gradient is defined. Self-diffusion coefficients were measured by Hahn- and stimulated echoes detected in the presence of the uniform magnetic field gradient of the static field. To improve the sensitivity of these experiments, a Carr-Purcell-Meiboom-Gill pulse sequence was applied after the main diffusion-encoding period. By adding the echo train the experimental time was strongly shortened, allowing the measurement of complete diffusion curves in less than 1min. This method has been tested by measuring the self-diffusion coefficients D of various organic solvents and poly(dimethylsiloxane) samples with different molar masses. Diffusion coefficients were also measured for n-hexane absorbed at saturation in natural rubber with different cross-link densities. The results show a dependence on the concentration that is in good agreement with the theoretical prediction. Moreover, the stimulated-echo sequence was successfully used to measure the diffusion coefficient as a function of the evolution time in systems with restricted diffusion. This type of experiment proves the pore geometry and gives access to the surface-to-volume ratio. It was applied to measure the diffusion of water in sandstones and sheep Achilles tendon. Thanks to the strong static gradient G(0), all diffusion coefficients could be measured without having to account for relaxation during the pulse sequence.     

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.     

Switchable magnetic bottles and field gradients for particle traps

Versatile methods for the manipulation of individual quantum systems, such as confined particles, have become central elements in current developments in precision spectroscopy, frequency standards, quantum information processing, quantum simulation, and alike. For atomic and some subatomic particles, both neutral and charged, a precise control of magnetic fields is essential. In this paper, we discuss possibilities for the creation of specific magnetic field configurations which find application in these areas. In particular, we pursue the idea of a magnetic bottle which can be switched on and off by transition between the normal and the superconducting phase of a suitable material in cryogenic environments, for example, in trap experiments in moderate magnetic fields. Methods for a fine-tuning of the magnetic field and its linear and quadratic components in a trap are presented together with possible applications.     

Signal selection in high-resolution NMR by pulsed field gradients

We describe a new and powerful computer program called TRIPLE GRADIENT which calculates optimized pulsed field gradient sequences for specific coherence pathway selection or rejection. Sequences can be computed for gradient coils acting along one, two, or three perpendicular axes. The program is based on the computational minimization of a penalty function formed from the summed amplitudes of the unwanted signals. The underlying mathematical analysis makes use of a vectorial representation of the way in which a gradient sequence suppresses different signals. It is argued that experiments using well-calculated gradient sequences are quicker and generally perform better than those using extensive phase cycling, especially when suppressing extremely strong solvent signals, and it is shown that in many cases gradient experiments of optimal signal-to-noise ratio can be performed. These claims are illustrated by spectra obtained from an HQQC experiment.     

Multidimensional imaging using combined stray field and pulsed gradients

The paper describes an advance in stray field imaging (STRAFI) whereby images of planar samples can be obtained in the stray field of a superconducting magnet without the need for sample rotation. This is achieved by using the static stray magnetic field gradient in combination with pulsed orthogonal gradients. Results of both two- and three-dimensional implementations of the experiment are presented and discussed. An extension to diffusion-weighted imaging is introduced. The technique is expected to prove particularly useful in experiments where high resolution is required in only one direction while lower resolution is acceptable in the orthogonal directions, such as in studies of the drying and curing of paints and varnishes. Arising from the work, a new method for accurately calibrating the radiofrequency pulse width in stray field is found.     

Interface profile evolution between binary immiscible fluids induced by high magnetic field gradients

A mechanical analysis is done to find the evolution of the interface profile between binary immiscible fluids induced by a three-dimensional orthogonal magnetic field gradient.In the experiments,the changes of the interface profile between four groups of binary immiscible fluids are investigated under the same horizontal magnetic field gradients.The binary immiscible fluids are made of benzene and other liquids,like CuSO4,Fecl3,FeSO4 or Cucl2 aqueous solutions.In addition,the interface profile between the benzene and CuSO4 aqueous solution is examined under different horizontal magnetic field gradients.The experimental results are consistent with the theoretical analysis.This study explains the enhanced Moses effect from a mechanics standpoint.Furthermore,a new method for susceptibility measurement is proposed based on this enhanced Moses effect.     

Anisotropic water diffusion in macroscopically oriented lipid bilayers studied by pulsed magnetic field gradient NMR

The anisotropy, D(parallel)/D( perpendicular ), of water diffusion in fully hydrated bilayers of dimyristoylphosphatidylcholine at 29 degrees C has been measured by pulsed magnetic field gradient (pfg) NMR. By using NMR imaging hardware to produce magnetic field gradients in an arbitrary direction with respect to a stack of macroscopically aligned lipid bilayers, translational diffusion of water was measured as a function of the angle between the direction of the magnetic field gradient and the normal of the lipid membrane. The observed diffusion coefficient is found to depend strongly on this angle. The anisotropy cannot be accurately determined due to the very small value of D( perpendicular ), but a lower limit of about 70 can be estimated from the observed diffusion coefficients. The results are discussed in terms of the relatively low permeability of water across the lipid bilayer, instrumental limitations, and/or possible defects in the lamellae.