Fast Field Cycling Research Articles

High Temperature Fast Field Cycling Study of Crude Oil

A set of crude oil samples with different composition and characteristics is studied by means of Fast Field Cycling (FFC) 1H relaxometry, which probes the distribution of longitudinal relaxation times T1 as a function of the Larmor frequency. Investigation of T1 profiles at different temperatures is able to provide an insight into the dynamics and structural changes of oil components, with our particular interest being the high temperature behaviour of asphaltene. It is well-known that asphaltenes tend to form porous clusters in crude oils, which can cause severe problems for the process of oil extraction. Therefore, FFC experiments are conducted on Stelar Spinmaster FFC2000 in the temperature range 203K < T <443K, where the upper limit of 443K is aimed at approximating the typical maximal in-situ well temperatures. FFC relaxometry data of crude oils at such a high temperature are obtained for the first time with the use of a specially modified NMR probe. Inverse Laplace transformation is applied to the longitudinal agnetization decays, yielding T1 distributions at different frequencies. A comparative analysis of these distributions for different Larmor frequencies and temperatures showed that there is a systematic variation of the frequency dependence of T1 correlating with the asphaltene content in the samples, at temperatures similar to the well conditions.

Dynamic Nuclear Polarization Fast Field Cycling Method for the Selective Study of Molecular Dynamics in Block Copolymers.

Dynamic nuclear polarization (DNP) is one of the most useful methods to increase sensitivity in NMR spectroscopy. It is based on the transfer of magnetization from an electron to the nuclear spin system. Based on previous work that demonstrated the feasibility of integrating DNP with fast field cycling (FFC) relaxometry and the possibility to distinguish between different mechanisms, such as the Overhauser effect (OE) and the solid effect (SE), the first FFC study of the differential relaxation properties of a copolymer is presented. For this purpose, concentrated solutions of the polystyrene-block-polybutadiene-block-polystyrene (SBS) triblock copolymer and the corresponding homopolymers were investigated. T1 -T2 relaxation data are discussed in terms of molecular mobility and the presence of radicals. The DNP selective data indicate a dominant SE contribution to the enhancement of the NMR signal for both blocks of the triblock copolymer and for the homopolymer solutions. The enhancement factors are different for both polymer types and in the copolymer, which is explained by the individual 1 H T1 relaxation times and different electron-nucleus coupling strength. The T1 relaxation dispersion measurements of the SE enhanced signal were performed, which led to improved signal-to-noise ratios that allowed the site-specific separation of relaxation times and their dependence on the Larmor frequency with a higher accuracy.

Simple algorithm for the correction of MRI image artefacts due to random phase fluctuations

PurposeFast Field-Cycling (FFC) MRI is a novel technology that allows varying the main magnetic field B0 during the pulse sequence, from the nominal field (usually hundreds of millitesla) down to Earth's field or below. This technique uses resistive magnets powered by fast amplifiers. One of the challenges with this method is to stabilise the magnetic field during the acquisition of the NMR signal. Indeed, a typical consequence of field instability is small, random phase variations between each line of k-space resulting in artefacts, similar to those which occur due to homogeneous motion but harder to correct as no assumption can be made about the phase error, which appears completely random. Here we propose an algorithm that can correct for the random phase variations induced by field instabilities without prior knowledge about the phase error. MethodsThe algorithm exploits the fact that ghosts caused by field instability manifest in image regions which should be signal free. The algorithm minimises the signal in the background by finding an optimum phase correction for each line of k-space and repeats the operation until the result converges, leaving the background free of signal. ConclusionWe showed the conditions for which the algorithm is robust and successfully applied it on images acquired on FFC-MRI scanners. The same algorithm can be used for various applications other than Fast Field-Cycling MRI.

Correction of environmental magnetic fields for the acquisition of Nuclear magnetic relaxation dispersion profiles below Earth’s field

T1 relaxation times can be measured at a range of magnetic field strengths by Fast Field-Cycling (FFC) NMR relaxometry to provide T1-dispersion curves. These are valuable tools for the investigation of material properties as they provide information about molecular dynamics non-invasively. However, accessing information at fields below 230 μT (10kHz proton Larmor frequency) requires careful correction of unwanted environmental magnetic fields.In this work a novel method is proposed that compensates for the environmental fields on a FFC-NMR relaxometer and extends the acquisition of Nuclear Magnetic Relaxation Dispersion profiles to 2.3μT (extremely low field region), with direct application in the study of slow molecular motions. Our method is an improvement of an existing technique, reported by Anoardo and Ferrante in 2003, which exploits the non-adiabatic behaviour of the magnetisation in rapidly-varying magnetic fields and makes use of the oscillation of the signal amplitude to estimate the field strength. This increases the accuracy in measuring the environmental fields and allows predicting the optimal correction values by applying simple equations to fit the data acquired. Validation of the method is performed by comparisons with well-known dispersion curves obtained from polymers and benzene.

Towards a Model-Based Field-Frequency Lock for NMR

Nuclear Magnetic Resonance (NMR) relaxometry is a powerful technique that allows to investigate the properties of materials. More advanced NMR relaxometry techniques such as Fast Field-Cycling (FFC) require the magnetic field to reach any desired value in a very short time (few milliseconds) and field oscillations to stay within few ppms. Such specifications call for the introduction of a suitable Field Frequency Lock (FFL) system. FFL relies on an indirect measure of the magnetic field which can be obtained by performing a parallel NMR experiment with a known sample. In this paper we propose a PID controller to guarantee field fluctuations to stay below the desired level and short settling time. The tuning of the controller is based on a mathematical description of the entire process, which is validated by performing real experiments. Numerical simulations show promising results that we expect to be confirmed by real experiments.

Segmental dynamics of polyethylene-alt-propylene studied by NMR spin echo techniques.

Segmental dynamics of a highly entangled melt of linear polyethylene-alt-propylene with a molecular weight of 200 kDa was studied with a novel proton nuclear magnetic resonance (NMR) approach based upon 1H → 2H isotope dilution as applied to a solid-echo build-up function ISE(t), which is constructed from the NMR spin echo signals arising from the Hahn echo (HE) and two variations of the solid-echo pulse sequence. The isotope dilution enables the separation of inter- and intramolecular contributions to this function and allows one to extract the segmental mean-squared displacements in the millisecond time range, which is hardly accessible by other experimental methods. The proposed technique in combination with time-temperature superposition yields information about segmental translation in polyethylene-alt-propylene over 6 decades in time from 10-6 s up to 1 s. The time dependence of the mean-squared displacement obtained in this time range clearly shows three regimes of power law with exponents, which are in good agreement with the tube-reptation model predictions for the Rouse model, incoherent reptation and coherent reptation regimes. The results at short times coincide with the fast-field cycling relaxometry and neutron spin echo data, yet, significantly extending the probed time range. Furthermore, the obtained data are verified as well by the use of the dipolar-correlation effect on the Hahn echo, which was developed before by the co-authors. At the same time, the amplitude ratio of the intermolecular part of the proton dynamic dipole-dipole correlation function over the intramolecular part obtained from the experimental data is not in agreement with the predictions of the tube-reptation model for the regimes of incoherent and coherent reptation.

Dynamics and wettability of petroleum fluids in shale oil probed by 2D T1-T2 and fast field cycling NMR relaxation

We present nuclear magnetic relaxation dispersion of longitudinal relaxation rates 1/T1 (NMRD) and 2D spin-correlation T1-T2 at different frequencies for oil and brine confined in shale oil rocks. We describe the nuclear spin relaxation models used for obtaining important dynamical and structural parameters from these experiments. These models allow interpreting the very different T1/T2 ratio observed for these petroleum fluids on the 2D spin-correlation T1-T2 observed at two frequencies. They give also new informations on the dynamics and wettability of the embedded fluids as well as some structural measurement on the kerogen pore size.

Probing into the mesoporous structure of carbon xerogels via the low-field NMR relaxometry of water and cyclohexane molecules

The liquid morphology of water and cyclohexane molecules, under partially saturated conditions inside the porous structure of two mesoporous carbon xerogels, was investigated using low-field nuclear magnetic resonance (NMR) relaxometry. The two xerogels were prepared under the same conditions, but with a different resorcinol-to-catalyst ratio. Two techniques were used in our investigations: the well-known Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence for transverse relaxation measurements and the Fast Field-Cycling (FFC) relaxometry technique for longitudinal relaxation dispersion measurements. It was found a stronger interaction of cyclohexane molecules with the pore surface as compared with water molecules. This leads to a uniform coverage of cyclohexane molecules on the pore surface for lower filling degrees while, in the case of water filled carbon xerogels, the distribution was non-uniform. Due to the stronger adsorption of cyclohexane molecules on the pore surface, it was possible to identify the micropores of the carbon xerogels predicted by nitrogen adsorption data. The NMR relaxation data have clearly demonstrated that, by increasing the resorcinol-to-catalyst ratio, the size of mesopores increases. Moreover, the enhanced adsorption affinity of carbon xerogels for cyclohexane shows that this type of materials can be employed as adsorbents for the removal of similar organic contaminants from water.

Ultra-wide range field-dependent measurements of the relaxivity of Gd1\u2212xEuxVO4 nanoparticle contrast agents using a mechanical sample-shuttling relaxometer

The current trend for Magnetic Resonance Imaging points towards higher magnetic fields. Even though sensitivity and resolution are increased in stronger fields, T1 contrast is often reduced, and this represents a challenge for contrast agent design. Field-dependent measurements of relaxivity are thus important to characterize contrast agents. At present, the field-dependent curves of relaxivity are usually carried out in the field range of 0 T to 2 T, using fast field cycling relaxometers. Here, we employ a high-speed sample shuttling device to switch the magnetic fields experienced by the nuclei between virtually zero field, and the center of any commercial spectrometer. We apply this approach on rare-earth (mixed Gadolinium-Europium) vanadate nanoparticles, and obtain the dispersion curves from very low magnetic field up to 11.7 T. In contrast to the relaxivity profiles of Gd chelates, commonly used for clinical applications, which display a plateau and then a decrease for increasing magnetic fields, these nanoparticles provide maximum contrast enhancement for magnetic fields around 1–1.5 T. These field-dependent curves are fitted using the so-called Magnetic Particle (MP) model and the extracted parameters discussed as a function of particle size and composition. We finally comment on the new possibilities offered by this approach.

Influence of underlying local organisations in typical nematic phase of 6CHBT and induced nematic phase of bicomponent liquid crystalline systems 4DBT–12CB – a Fast Field Cycling NMR investigation

ABSTRACTVariable-temperature proton magnetic relaxation dispersion (PMRD) profiles are collected in the induced nematic phases of the binary liquid crystalline mixtures composed of smectic mesogens: 5-butyl-2-(4-isothiocyanatophenyl)-1,3-dioxane (4DBT) and 4′-dodecyl-4-cyanobiphenyl (12CB) and the low viscous nematogen 4-(trans-4′-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT), with a view to assess the influence of local organisations on the power spectrum of director fluctuation modes. Two distinct compositions of the binary mixture i.e. 50 mol% of 12CB (without a smectic phase) and 70 mol% of 12CB (with a smectic phase) are used. Fast Field Cycling nuclear magnetic resonance relaxometry is employed to measure proton spin-lattice relaxation rates (R1) as a function of Larmor frequency (10 kHz–30 MHz). PMRD data analysis with the choice of a suitable model indicated nematic clusters of moderate size (~200 Å) found in the broad nematic region of 50 mol% 12CB, whose size is almost invariant with temperature. On the other hand, cybotactic clusters, i.e. local smectic organisations of relatively larger size (~2000–3000 Å), are observed near the nematic–smectic transition (T > TAN) of 70 mol% 12CB. Interestingly, the shape geometry of such local organisations accessed from PMRD analysis is weakly anisotropic near TAN, while being isotropic near TNI for 70 mol% 12CB.

Molecular dynamics-based selectivity for Fast-Field-Cycling relaxometry by Overhauser and solid effect dynamic nuclear polarization

In the last decade nuclear spin hyperpolarization methods, especially Dynamic Nuclear Polarization (DNP), have provided unprecedented possibilities for various NMR techniques by increasing the sensitivity by several orders of magnitude. Recently, in-situ DNP-enhanced Fast Field Cycling (FFC) relaxometry was shown to provide appreciable NMR signal enhancements in liquids and viscous systems. In this work, a measurement protocol for DNP-enhanced NMR studies is introduced which enables the selective detection of nuclear spin hyperpolarized by either Overhauser effect or solid effect DNP. Based on field-cycled DNP and relaxation studies it is shown that these methods allow for the independent measurement of polymer and solvent nuclear spins in a concentrated solution of high molecular weight polybutadiene in benzene doped with α,γ-bisdiphenylene-β-phenylallyl radical. Appreciable NMR signal enhancements of about 10-fold were obtained for both constituents. Moreover, qualitative information about the dynamics of the radical and solvent was obtained. Selective DNP-enhanced FFC relaxometry is applied for the measurement of the 1H nuclear magnetic relaxation dispersion of both constituents with improved precision. The introduced method is expected to greatly facilitate NMR studies of complex systems with multiple overlapping signal contributions that cannot be distinguished by standard methods.

Influence of Adsorbed Water on the Activation Energy of Model Photocatalytic Reactions

Two commercial (Merck and Sigma-Aldrich) and two home prepared (HP05 and HP05C) powdered TiO2 photocatalysts were investigated by fast field cycling nuclear magnetic resonance experiments in order to explore the nature of the interactions between water and the solid surfaces. The results were related to the activation energies determined at temperatures ranging from 303 to 353 K for the photocatalytic oxidation in water of three model molecules presenting different interactions with the solid surface (catechol, phenol, and metylbenzoate). The photoactivity results at different temperatures were comparable to each other because the runs were carried out while keeping constant the concentration of O2 in the suspensions. The study highlights the influence of the adsorbed water on the activation energy of a photocatalytic reaction. In particular, we showed that competition between water and substrate adsorption is a determining factor in the activation energy of a photocatalytic oxidation reaction.

Dynamics of poly(vinyl butyral) studied using dielectric spectroscopy and 1H NMR relaxometry.

Dielectric Spectroscopy (DS) and 1H Fast Field-Cycling (FFC) NMR relaxometry were applied for understanding the dynamic behavior of the amorphous ter-polymer poly(vinyl butyral) (PVB) across the glass transition temperature (Tg = 70 °C by Differential Scanning Calorimetry). Above Tg, main chain segmental motions (α relaxation) were detected and characterized using both DS and FFC NMR relaxometry. The correlation times extracted by the analysis of DS and FFC NMR relaxometry data agreed within a factor of three and showed a Vogel-Fulcher-Tammann temperature dependence, with an associated Tg of 69 °C and a fragility of 155 for PVB glass. Below Tg, a secondary process (β relaxation) was revealed by DS, and was ascribed to reorientations of the vinyl alcohol dipoles due to local twisting motions with an associated activation barrier of 11 kcal mol-1. The β process was also found to contribute to 1H NMR relaxation above Tg.

Investigation of Dynamics in BMIM TFSA Ionic Liquid through Variable Temperature and Pressure NMR Relaxometry and Diffusometry

A comprehensive variable temperature, pressure and frequency multinuclear (1H, 2H, and 19F) magnetic resonance study was undertaken on selectively deuterated 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (BMIM TFSA) ionic liquid isotopologues. This study builds on our earlier investigation of the effects of increasing alkyl chain length on diffusion and dynamics in imidazolium-based TFSA ionic liquids. Fast field cycling 1H T1 data revealed multiple modes of motion. Through calculation of diffusion coefficient (D) values and activation energies, the low- and high-field regimes were assigned to the translational and reorientation dynamics respectively. Variable-pressure 2H T1 measurements reveal site-dependent interactions in the cation with strengths in the order MD3 > CD3 > CD2, indicating dissimilarities in the electric field gradients along the alkyl chain, with the CD2 sites having the largest gradient. Additionally, the α saturation effect in T1 vs. P was observed for all three sites, suggesting significant reduction of the short-range rapid reorientational dynamics. This reduction was also deduced from the variable pressure 1H T1 data, which showed an approach to saturation for both the methyl and butyl group terminal methyl sites. Pressure-dependent D measurements show independent motions for both cations and anions, with the cations having greater D values over the entire pressure range.

Measurement of the bending elastic modulus in unilamellar vesicles membranes by fast field cycling NMR relaxometry

The elastic properties of lipid membranes can be conveniently characterized through the bending elastic modulus κ. Elasticity directly affects the deformability of a membrane, morphological and shape transitions, fusion, lipid-protein interactions, etc. It is also a critical property for the formulation of ultradeformable liposomes, and of interest for the design of theranostic liposomes for efficient drug delivery systems and/or different imaging contrast agents. Measurements of κ in liposome membranes have been made using the fast field cycling nuclear magnetic relaxometry technique. We analyze the capability of the technique to provide a consistent value of the measured quantity under certain limiting conditions. Relaxation dispersions were measured acquiring a minimal quantity of points, within a reduced Larmor frequency range and, under inferior experimental conditions (in the presence of magnetic field in-homogeneity and lower power supply stability). A simplified model is discussed, showing practical advantages when fitting the data within the reduced frequency range. Experiments are contrasted with standard measurements performed in a state-of-the-art relaxometer. The methodology was tested in samples of 1,2-dimyristoyl-sn-glycero-3-phosphocholine with different percentiles of cholesterol. We observe a tendency to a decrease in κ with increasing temperature, and a tendency to increase with the cholesterol percentile.

Fast-Field Cycling MRI: A new tool for enhanced diagnosis

Fast Field-Cycling MRI (FFC MRI) is a major shift in MRI technology. It aims to explore how relaxation rates change with the magnetic field strength, an idea that has been successfully exploited in NMR for more than half a century and which is known to provide unique structural information on materials, non-invasively. Scaling up FFC NMR to whole-body MRI systems is a difficult technical challenge that has maintained a lock on this area of research for many years. Our research group has successfully lifted this lock and developed two whole-body FFC MRI scanners: one with a field range of 0.1 mT to 59 mT that has been used in clinical trials for several years and the other with a range of 20 uT to 0.2 T still under development. Our pilot studies, which use both FFC MRI and FFC NMR, have discovered new biomarkers in a range of diseases such as osteoarthritis, breast carcinoma, musculoskeletal sarcoma, obesity, liver fibrosis, thrombosis and others. These FFC MRI biomarkers can differentiate tissues that appear similar on conventional MRI devices thanks, in part, to the greater endogenous contrast present at lower magnetic fields but also due to patterns that emerge from the overall relaxation dispersion curves of these tissues. The latter provides unique insight into the structure of materials and is, we believe, a promising tool to characterise tissue remodelling. This presentation will focus on FFC MRI technology, how it differs from conventional MRI and the results obtained so far from our pilot studies. Fast Field-Cycling MRI (FFC MRI) is a major shift in MRI technology. It aims to explore how relaxation rates change with the magnetic field strength, an idea that has been successfully exploited in NMR for more than half a century and which is known to provide unique structural information on materials, non-invasively. Scaling up FFC NMR to whole-body MRI systems is a difficult technical challenge that has maintained a lock on this area of research for many years. Our research group has successfully lifted this lock and developed two whole-body FFC MRI scanners: one with a field range of 0.1 mT to 59 mT that has been used in clinical trials for several years and the other with a range of 20 uT to 0.2 T still under development. Our pilot studies, which use both FFC MRI and FFC NMR, have discovered new biomarkers in a range of diseases such as osteoarthritis, breast carcinoma, musculoskeletal sarcoma, obesity, liver fibrosis, thrombosis and others. These FFC MRI biomarkers can differentiate tissues that appear similar on conventional MRI devices thanks, in part, to the greater endogenous contrast present at lower magnetic fields but also due to patterns that emerge from the overall relaxation dispersion curves of these tissues. The latter provides unique insight into the structure of materials and is, we believe, a promising tool to characterise tissue remodelling. This presentation will focus on FFC MRI technology, how it differs from conventional MRI and the results obtained so far from our pilot studies.

Conventional and Fast Field Cycling Relaxometry Study of the Molecular Dynamics in Polymer Nanocomposites for Use as Drug Delivery Systems

Conventional and Fast Field Cycling Relaxometry Study of the Molecular Dynamics in Polymer Nanocomposites for Use as Drug Delivery Systems

Dry-cured ham tissue characterization by fast field cycling NMR relaxometry and quantitative magnetization transfer.

Fast field cycling (FFC) and quantitative magnetization transfer (qMT) NMR methods are two powerful tools in NMR analysis of biological tissues. The qMT method is well established in biomedical NMR applications, while the FFC method is often used in investigations of molecular dynamics on which longitudinal NMR relaxation times of the investigated material critically depend. Despite their proven analytical potential, these two methods were rarely used in NMR studies of food, especially when combined together. In our study, we demonstrate the feasibility of a combined FFC/qMT-NMR approach for the fast and nondestructive characterization of dry-curing ham tissues differing by protein content. The characterization is based on quantifying the pure quadrupolar peak area (area under the quadrupolar contribution of dispersion curve obtained by FFC-NMR) and the restricted magnetization pool size (obtained by qMT-NMR). Both quantities correlate well with concentration of partially immobilized, nitrogen-containing and proton magnetization exchanging muscle proteins. Therefore, these two quantities could serve as potential markers for dry-curing process monitoring. Copyright © 2016 John Wiley & Sons, Ltd.

Water Dynamics and Its Role in Structural Hysteresis of Dissolved Organic Matter.

Knowledge of structural dynamics of dissolved organic matter (DOM) is of paramount importance for understanding DOM stability and role in the fate of solubilized organic and inorganic compounds (e.g., nutrients and pollutants), either in soils or aquatic systems. In this study, fast field cycling (FFC) (1)H NMR relaxometry was applied to elucidate structural dynamics of terrestrial DOM, represented by two structurally contrasting DOM models such as Suwanee River (SRFA) and Pahokee peat (PPFA) fulvic acids purchased by the International Humic Substance Society. Measurement of NMR relaxation rate of water protons in heating-cooling cycles revealed structural hysteresis in both fulvic acids. In particular, structural hysteresis was related to the delay in re-establishing water network around fulvic molecules as a result of temperature fluctuations. The experiments revealed that the structural temperature dependency and hysteresis were more pronounced in SRFA than in PPFA. This was attributed to the larger content of hydrogel-like structure in SRFA stabilized, at a larger extent, by H-bonds between carboxylic and phenolic groups. Moreover, results supported the view that terrestrial DOM consist of a hydrophobic rigid core surrounded by progressively assembling amphiphilic and polar molecules, which form an elastic structure that can mediate reactivity of the whole DOM.

Sample Shuttling Relaxometry of Contrast Agents: NMRD Profiles above 1 T with a Single Device.

Nuclear magnetic relaxation dispersion (NMRD) profiles are essential tools to evaluate the efficiency and investigate the properties of magnetic compounds used as contrast agents for magnetic resonance imaging (MRI), namely gadolinium chelates and superparamagnetic iron oxide particles. These curves represent the evolution of proton relaxation rates with the magnetic field. NMRD profiles are unparalleled to probe extensively the spectral density function involved in the relaxation of water in the presence of the paramagnetic ion or the magnetic nanoparticles. This makes such profiles an excellent test of the adequacy of a theoretical relaxation model and allow for a predictive approach to the development and optimization of contrast agents. From a practical point of view they also allow to evaluate the efficiency of a contrast agent in a certain range of magnetic fields. Nowadays, these curves are recorded with commercial fast field cycling devices, often limited to a maximum Larmor frequency of 40 MHz (0.94 T). In this article, relaxation data were acquired on a wide range of magnetic fields, from 3.5 × 10−4 to 14 T, for a gadolinium-based contrast agent and for PEGylated iron oxide nanoparticles. We show that the low-field NMRD curves can be completed with high-field data obtained on a shuttle apparatus device using the superconductive magnet of a high-field spectrometer. This allows a better characterization of the contrast agents at relevant magnetic fields for clinical and preclinical MRI, but also refines the experimental data that could be used for the validation of relaxation models.Electronic supplementary materialThe online version of this article (doi:10.1007/s00723-015-0751-7) contains supplementary material, which is available to authorized users.