Radiofrequency Field Gradients Research Articles

A dual-core NMR system for field-cycling singlet assisted diffusion NMR.

Long-lived singlet spin order offers the possibility to extend the spin memory by more than an order of magnitude. This enhancement can be used, among other applications, to assist NMR diffusion experiments in porous media where the extended lifetime of singlet spin order can be used to gain information about structural features of the medium as well as the dynamics of the imbibed phase. Other than offering the possibility to explore longer diffusion times of the order of many minutes that, for example, gives unprecedented access to tortuosity in structures with interconnected pores, singlet order has the important advantage to be immune to the internal field gradients generated by magnetic susceptibility inhomogeneities. These inhomogeneities, however, are responsible for very short T2 decay constants in high magnetic field and this precludes access to the singlet order in the first instance. To overcome this difficulty and take advantage of singlet order in diffusion experiments in porous media, we have here developed a dual-core system with radiofrequency and 3-axis pulsed field gradients facilities in low magnetic field, for preparation and manipulation of singlet order and a probe, in high magnetic field, for polarisation and detection. The system operates in field-cycling and can be used for a variety of NMR experiments including diffusion tensor imaging (both singlet assisted and not). In this paper we present and discuss the new hardware and its calibration, and demonstrate its capabilities through a variety of examples.

A robust Freeman-Hill-inspired pulse protocol for ringdown-free T1 relaxation measurements

A new difference-spectroscopy method is introduced for measuring T1 relaxation times. It is inspired by the earlier work of Freeman and Hill and eliminates the need for recording signal intensities at thermodynamic equilibrium. The new method is termed SIP-R (Split-Inversion Pulse and Recovery) and reduces the number of refinable parameters in the curve fitting process of relaxation-delay-dependent signal intensities by using two instead of the three parameters typically used in the standard inversion-recovery sequence. The SIP-R method preserves the dynamic range of measurement of the standard inversion-recovery method but converts the rise-to-maximum mathematical functionality of the recorded data into a decay-to-zero functionality. The decay-to-zero functionality renders the SIP-R sequence advantageous for inverse Laplace transformation numerical optimizations. The new technique proves to be extremely robust with respect to pulse imperfections, pulse-power changes during the pulse sequence, pulse-width miscalibrations, resonance offsets, and radiofrequency field variations. It also compensates for acoustic ring-down effects and proves reliable for measurements with inhomogeneously broadened signals up to several kilohertz linewidth. 1H NMR experiments with methane gas at pressures up to 50 atm in toroid-cavity pressure vessel probes and in the presence of the methane-to-methanol conversion catalyst Cu-ZnO/Al2O3 are used to show the usefulness of the new method for relaxation time investigations under pressure, at strong radiofrequency field gradients, and in the presence of paramagnetic materials.

Design of a three-loop asymmetric coil producing a homogeneous radiofrequency B1 field gradient along the axis of a vertical sample tube

A coil system generating a vertical radio-frequency (rf) field gradient (B1 gradient) has been built for surrounding, in a horizontal magnet, a vertical sample (object) of axial symmetry. The system comprises three coaxial loops with an overall shape either spherical or ellipsoidal. The geometry has been theoretically and experimentally devised for producing a very uniform gradient (cancellation of B1 derivatives from second order up to sixth order) in the central region where a vertical receiver/transmitter coil is installed. The latter is of the saddle-shaped type and is geometrically and electrically decoupled from the gradient coil system. This receiver/transmitter coil not only ensures an optimal signal reception but, in addition, is able to deliver perfectly homogeneous rf hard pulses which are mandatory in most NMR experiments. In its present design, the system delivers a uniform gradient in a limited region but could be extended at will. Its main advantages over static field gradients (B0 gradients) appear clearly in the case of very short transverse relaxation times. This property has been emphasized in the case of experiments leading to the measurement of diffusion coefficients. Also, this system would be suitable for chemical shift imaging (CSI) experiments as confirmed by a preliminary test experiment.

A Systematic Method to Explore Radio-Frequency Non-Thermal Effect on the Growth of Saccharomyces Cerevisiae

Radio frequency (RF) non-thermal (NT) bio-effects have been a subject of debate and attracted significant interests due to potential health risks or beneficial applications. In this work, we report a systematic method for broadband investigation of RF NT effects on <i>Saccharomyces cerevisiae</i> yeast growth. The method includes a transverse electro-magnetic (TEM) device and a dielectric spectroscopy technique for RF frequency selection. A stripline-based TEM device has two 240-<i>&#x03BC;</i>L chambers 3D printed for cell cultures. The fabricated device operates up to a few GHz and produces uniform RF fields for cell exposure testing. A vector network analyzer (VNA) was used to provide &#x2212;20 dBm continuous-wave (CW) RF power. The heating effects on cell growth were estimated to be negligible. Frequency regions, where large permittivity differences between the medium and yeast cultures were obtained and used to select RF testing frequencies, <i>e.g.,</i> 1.0 MHz, 3.162 MHz, 10 MHz. These differences may indicate RF field gradients near cell membrane, and the gradients may affect local nutrient transport. Additionally, RF at 905 MHz is tested for comparison purpose. Yeast cells in the exponential growth phase were examined at four RF frequencies and compared with two controls. One control device held at the same temperature as the test device, while the other control was held at a temperature 1 &#x00B0;C higher. The results showed that the RF fields at 3.162 MHz reduced yeast growth rates by 15.1&#x0025;; however, the RF fields at 1.0 MHz enhanced cell growth by 13.7&#x0025;, while the observed 4.3&#x0025; growth rate increase at 10 MHz is insignificant and the RF fields at 905 MHz had no effects on the cell growth. These results showed a clear RF NT effects on <i>S. cerevisiae</i> growth that was frequency dependent. The hypothesized mechanisms of these effects, i.e., non-uniform RF fields near cell membranes and fluidic diodes in cell membrane ion channels may play important roles in nutrient transport, need to be further investigated.

A Statistical Robust Approach to Design Parallel Transmit Radiofrequency Excitations in MRI

In MRI, at ultrahigh field, the use of parallel transmit radiofrequency (RF) arrays is very beneficial to better control spin excitation spatially. In that framework, the so-called “universal pulse” technique, proposed recently for head imaging at 7 tesla, gives access to “plug-and-play” nonadiabatic solutions exhibiting good robustness against intersubject variations in the resonant transmit fields. This new type of solution has been defined so far as the result of numerical pulse optimizations performed across a collection of RF field maps acquired on a small population sample (pulse design database). In this work, we investigate an alternative universal pulse design approach in the linear small tip angle regime whereby the database of RF field maps is first transformed into a second-order statistical model and which then exploits a statistical robust design formalism for the optimization of the RF and magnetic field gradient waveforms. Experimental validation with an eightfold transmit RF coil for 7 tesla brain imaging shows that this new approach brings some benefit in terms of computational efficiency. Hence, for a design database composed of 35 maps, the computation time initially of 50 min could be reduced down to 3 min. The proposed statistical approach thus enables integration of large databases, presumably necessary to ensure robust solutions. Finally, it provides means to compute flip angle statistics and, along with it, simple performance metrics for quality assurance (RF pulse performance) or guidance in the optimization of TX array architectures.

Toward nitrogen-14 nuclear quadrupole resonance imaging by nutation experiments performed with a radio-frequency field gradient

Until now, NQR imaging has been considered mainly in the case of Chlorine-35. This is a spin 3/2 resonating at relatively high frequency (around 30MHz) thus affording a favorable sensitivity. Conversely, Nitrogen-14 (spin 1) NQR is much less sensitive because its resonances frequencies are below 6MHz. In contrast to already existing methodologies for object localization by 14N NQR, we present here a new, more straightforward, approach and the principles of space dependent 14N Quadrupole Resonance are laid down. The method is based on nutation curves obtained with an inhomogeneous radio-frequency field produced by a dedicated transmit-receive coil. The gradient created that way does not need to be uniform although more accurate results would be obtained with a uniform gradient. Nutation curves have been simulated by a specially designed algorithm which takes into account the 14N Quadrupole Resonance particularities including the so-called powder average. Preliminary experiments were carried out with the highest resonance frequency (4.64MHz) of sodium nitrite (NaNO2). A cylindrical sample of powder sodium nitrite containing a spacer was used. Simulations of the corresponding nutation curves not only demonstrate the existence of this spacer but, in addition, provide its position and its thickness. This clearly ascertains the feasibility of 14N Quadrupole Resonance imaging.

Mathematical Modeling and Data Analysis of NMR Experiments using Hyperpolarized (13)C Metabolites.

Rapid-dissolution dynamic nuclear polarization (DNP) has made significant impact in the characterization and understanding of metabolism that occurs on the sub-minute timescale in several diseases. While significant efforts have been made in developing applications, and in designing rapid-imaging radiofrequency (RF) and magnetic field gradient pulse sequences, very few groups have worked on implementing realistic mathematical/kinetic/relaxation models to fit the emergent data.The critical aspects to consider when modeling DNP experiments depend on both nuclear magnetic resonance (NMR) and (bio)chemical kinetics. The former constraints are due to the relaxation of the NMR signal and the application of ‘read’ RF pulses, while the kinetic constraints include the total amount of each molecular species present. We describe the model-design strategy we have used to fit and interpret our DNP results. To our knowledge, this is the first report on a systematic analysis of DNP data.

Single-sided radio-frequency field gradient with two unsymmetrical loops: Applications to nuclear magnetic resonance

Magnetic field gradients are nowadays indispensable to most nuclear magnetic resonance experiments and are at the basis of magnetic resonance imaging (MRI). Most of the time, gradients of the static magnetic field are employed. Gradients of the radio-frequency (rf) field may constitute an interesting alternative. Until now, they were produced by a single loop. We demonstrate in this paper how two unsymmetrical series loops can be optimized to produce rf gradients of much better performances. This optimization is based on a thorough theoretical approach and the gradient uniformity is studied through accurate simulations. Two prototypes were devised: one for a 2.34 T horizontal magnet (used in MRI), and the other for a 4.7 T vertical magnet (used for pure spectroscopic applications). These two-loop systems were designed for proton resonance frequencies (100 and 200 MHz, respectively). Performances of both systems were verified (versus theoretical predictions) by means of experiments employing gradients in view of the determination of the self-diffusion coefficients of liquids.

Diffusive diffraction in a porous polymer material by NMR using a radio-frequency field gradient (B1 Gradient). Comparison with the conventional static field gradient (B0 Gradient) method

Diffusive diffraction in a porous polymer material by NMR using a radio-frequency field gradient (B1 Gradient). Comparison with the conventional static field gradient (B0 Gradient) method

Diffusive diffraction phenomenon in a porous polymer material observed by NMR using radio-frequency field gradients

Pulsed field gradient NMR diffusion experiments can, in principle, lead to the "diffusive diffraction" phenomenon. In practice, its observation by gradients of the static magnetic field is difficult in real systems because they involve internal gradients due to the static magnetic field (necessary for polarizing nuclear spins). This latter drawback can be circumvented by using gradients of the radio-frequency (rf) field (the other magnetic field used in any NMR experiment). For the first time, by means of rf gradients, a so-called diffusive diffraction peak has been observed in a real porous system and its position provides a value of the mean distance between pores; this can be further complemented by the mean pore size determined from the dependence of the apparent diffusion coefficient with respect to the diffusion interval.

Optimal conditions for two-point estimation of self-diffusion coefficients through rf gradient NMR experiments

We address the problem of measuring accurately and as quickly as possible a self-diffusion coefficient by the so-called pulsed gradient stimulated echo experiment or, equivalently, by using a sequence involving radio-frequency field gradients. The discussion deals with the smallest diffusion coefficient which can be measured with a reasonable accuracy, possibly taking into account the values of longitudinal and transverse relaxation times along with the nature of gradients. It is shown theoretically and experimentally that the diffusion coefficient can be measured accurately via only two experiments provided that the ratio of the signal amplitudes lies in the range 0.2–0.6.

NMR Microscopy of an “Anisotropic” Polymer

AbstractTwo different samples of high‐density polyethylene (HDPE) have been studied. One (isotropic) is extracted from the material core whereas the other (anisotropic) involves two sides which have been in contact with the injection mold. It is observed by NMR microscopy (using radiofrequency field gradients) that these two sides favor toluene penetration into the material. The distribution of toluene nuclear spin relaxation times (extracted from proton T1 and T2 images) exhibits likewise important differences between the two samples. These differences can be accounted for by partial molecular ordering at the vicinity of the “mold sides”. Finally, in investigating the anisotropic sample (without solvent), three different phases (two amorphous and one crystalline) are revealed by 13C chemical shift imaging experiments (performed with radiofrequency field gradients under CP/MAS conditions). Each amorphous component is preferentially present at one of the two “mold sides”.

Self-diffusion measurements with chemical shift resolution in inhomogeneous magnetic fields

A methodology for chemical shift resolved molecular self-diffusion measurements in time-independent static and radiofrequency field gradients is demonstrated. Diffusion encoding is provided by a stimulated echo sequence with additional z-storage that allows for a change of diffusion time without affecting the relaxation weighting. The signal is acquired stroboscopically between the pulses of a train of adiabatic double passages that induces a z-rotation counteracting the phase spread resulting from precession in the inhomogeneous static field, as demonstrated in recent approaches to the goal of high-resolution “ex situ” NMR. Simulations of the pulse sequence show that the acquired signal results from the desired coherence pathway. Successful demonstrations of the experiment were performed on a mixture of water and isopropanol.

Rotating-frame spin–lattice relaxation time imaging by radio-frequency field gradients: visualization of strained crosslinked natural rubbers

NMR imaging by radio-frequency field gradients ( B 1 gradients) is especially convenient for heterogeneous samples and/or in the case of relatively short transverse relaxation times. The method has been combined with the application of two spin-lock periods of different duration so as to produce rotating-frame spin–lattice relaxation time ( T 1 ρ ) images. In the case of natural rubber samples with different crosslink densities, such images are not only characteristic of the crosslink density but also reveal the way in which the material has been stressed. The strained parts can be visualized either directly or through histograms showing the T 1 ρ distribution over the whole sample.

Detection of acoustic waves by NMR using a radiofrequency field gradient

A B 1 field gradient-based method previously described for the detection of mechanical vibrations has been applied to detect oscillatory motions in condensed matter originated from acoustic waves. A ladder-shaped coil generating a quasi-constant RF-field gradient was associated with a motion-encoding NMR sequence consisting in a repetitive binomial 1 3 ̄ 3 1 ̄ RF pulse train (stroboscopic acquisition). The NMR response of a gel phantom subject to acoustic wave excitation in the 20–200 Hz range was investigated. Results showed a linear relationship between the NMR signal and the wave amplitude and a spectroscopic selectivity of the NMR sequence with respect to the input acoustic frequency. Spin displacements as short as a few tens of nanometers were able to be detected with this method.

Cross-correlation suppressed T1 and NOE experiments for protein side-chain 13CH2 groups.

Relaxation measurements of side-chain 13CH2-groups of uniformly 13C labeled human ubiquitin were performed at 600 MHz and 800 MHz magnetic field strength at 30 degrees C. Dipole-dipole cross-correlated relaxation effects in T1 experiments were suppressed by the combination of radio-frequency pulses and pulsed field gradients during the relaxation delay leading to monoexponential relaxation decays that allow a more accurate extraction of the 13C T1 relaxation times. Heteronuclear [1H]-13C NOEs obtained by using different proton saturation schemes indicate that the influence of cross-correlation is small. The experimental T1 and NOE data were interpreted in a model-free way in terms of a generalized order parameter and an internal correlation time.

HR‐DOSY experiments with radiofrequency field gradients (RFG) and their processing according to the HD method

AbstractThe usual method for probing translational motions is the pulsed gradient spin‐echo (PGSE) experiment introduced by Stejskal and Tanner. It employs gradients of the static magnetic field. Most of the drawbacks from which this basic experiment suffers have been successfully circumvented by several modifications and compensations. Thus, it is now possible to obtain very clean experimental results amenable to optimal data processing so that they can be presented as 2D (or 3D) diffusion ordered spectra (DOSY). In first part of this paper, it is shown that radiofrequency field gradients (RFG) are immune to the drawbacks inherent to the basic PGSE sequence, and can give clean, high‐resolution diffusion data in a straightforward manner. In the second part, an alternative data processing method for reconstructing DOSY spectra based on the Hankel diagonalization (HD) is introduced. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Simulations of NMR experiments employing static and/or radio-frequency field gradients: application to slice selection by the inhomogeneous radio-frequency field of a standard coil

A general computer program has been developed in order to simulate any nuclear magnetic resonance (NMR) experiment. It includes, in addition to the action of radio-frequency (rf) pulses and of gradients of both the static and the rf magnetic fields, the effects of inhomogeneity of the latter. It has been used here for devising a slice selection procedure on the basis of the inhomogeneity of the rf field delivered by a standard coil (e.g., a saddle-shaped coil). Two sequences have been investigated, a DANTE-like pulse train and a very simple one (named S2P for slice selection with 2π pulses), (2π - τ)n, where 2π corresponds to the flip angle of the region to be selected, whereas τ has to be chosen according to the relaxation time values;n, the number of cycles, must be sufficiently large, its actual value being uncritical. Simulations show that performances (in terms of selectivity) of both sequences are comparable, while experimental verifications favor S2P for its robustness and for the absence of any signal loss.

MR detection of mechanical vibrations using a radiofrequency field gradient.

A new method for NMR characterization of mechanical waves, based upon radiofrequency field gradient for motion encoding, is proposed. A binomial B1 gradient excitation scheme was used to visualize the mobile spins undergoing a periodic transverse mechanical excitation. A simple model was designed to simulate the NMR signal as a function of the wave frequency excitation and the periodicity of the NMR pulse sequence. The preliminary results were obtained on a gel phantom at low vibration frequencies (0-200 Hz) by using a ladder-shaped coil generating a nearly constant RF field gradient along a specific known direction. For very small displacements and/or B1 gradients, the NMR signal measured on a gel phantom was proportional to the vibration amplitude and the pulse sequence was shown to be selective with respect to the vibration frequency. A good estimation of the direction of vibrations was obtained by varying the angle between the motion direction and the B1 gradient. The method and its use in parallel to more conventional MR elastography techniques are discussed. The presented approach might be of interest for noninvasive investigation of elastic properties of soft tissues and other materials.

Analysis of solvent diffusion in high‐density polyethylene using NMR imaging techniques

AbstractTwo different samples of high‐density polyethylene (HDPE) were studied. One (isotropic) was extracted from the material core, whereas the other (anisotropic) involved two sides that were in contact with the injection mold. Using radiofrequency field gradients, it was observed by NMR microscopy that these two sides favor toluene penetration into the material. Solvent diffusion in both samples could be successfully modeled, as demonstrated by the comparison between experimental NMR images and simulated images. Weight measurements appear to be consistent with the quantitative conclusions derived from NMR microscopy data. © 2001 John Wiley &amp; Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2781–2792, 2001