Fast Field-cycling Relaxometry Research Articles

Universal 1H Spin-Lattice NMR Relaxation Features of Sugar-A Step towards Quality Markers.

1H fast field-cycling and time-domain nuclear magnetic resonance relaxometry studies have been performed for 15 samples of sugar of different kinds and origins (brown, white, cane, beet sugar). The extensive data set, including results for crystal sugar and sugar/water mixtures, has been thoroughly analyzed, with a focus on identifying relaxation contributions associated with the solid and liquid fractions of the systems and non-exponentiality of the relaxation processes. It has been observed that 1H spin-lattice relaxation rates for crystal sugar (solid) vary between 0.45 s-1 and 0.59 s-1, and the relaxation process shows only small deviations from exponentiality (a quantitative measure of the exponentiality has been provided). The 1H spin-lattice relaxation process for sugar/water mixtures has turned out to be bi-exponential, with the relaxation rates varying between about 13 s-1-17 s-1 (for the faster component) and about 2.1 s-1-3.5 s-1 (for the slower component), with the ratio between the amplitudes of the relaxation contributions ranging between 2.8 and 4.2. The narrow ranges in which the parameters vary make them a promising marker of the quality and authenticity of sugar.

Vegan and sugar-substituted chocolates: assessing physicochemical characteristics by NMR relaxometry, rheology, and DSC

The main physicochemical characteristics of novel artisanal chocolates (both dark and milky) intended for vegan consumers or for those requiring assumption of fewer simple sugars, were analysed. Replacement of milk (with coconut copra, almonds, and soy protein isolates), and sucrose (with coconut sugars, stevia and erythritol, respectively) in dark chocolate, were accounted for by means of texture analysis, rheology, water activity, fatty acid composition, differential scanning calorimetry (DSC) and fast field cycling (FFC) nuclear magnetic resonance (NMR) relaxometry. The vegan sample (i.e., the milk-less one) showed lower values of hardness and adhesiveness as well as a larger peak in the melting behavior at the calorimetric evaluation (DSC). Moreover, the absence of milk resulted in the halving of the yield stress and a decrease in both the apparent and Casson’s viscosity. In the sample of chocolate with less sucrose, the peak temperatures measured at the DSC indicate crystallization of cocoa butter in its best form (Vβ2), unlike in dark chocolate, due to the different sugar composition. Similarly, the Casson yield stress (τ0), increased significantly (almost 70%), with the substitution of sugar. Finally, the results of NMR FFC relaxometry made it possible to identify aggregates of different sizes, laying the basis for its use as a rapid, non-destructive method for chocolate analysis.

Radicals on the silica surface: probes for studying dynamics by means of fast field cycling relaxometry and dynamic nuclear polarization

Determining the dynamics of adsorbed liquids on nanoporous materials is crucial for a detailed understanding of interactions and processes on the solid-liquid interface in many materials and porous systems. Knowledge of the influence of the presence of paramagnetic species on the surface or within the porous matrices is essential for fundamental studies and industrial processes such as catalysts. Magnetic resonance methods, such as electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR) and dynamic nuclear polarization (DNP), are powerful tools to address these questions and to quantify dynamics, electron-nuclear interaction features and their relation to the physical-chemical parameters of the system. This paper presents an NMR study of the dynamics of polar and nonpolar adsorbed liquids, represented by water, n-decane, deuterated water and nonane-d20, on the native silica surface as well as silica modified with vanadyl porphyrins. The analysis of the frequency dependence of the nuclear spin-lattice relaxation time is carried out by separating the intra- and intermolecular contributions, which were analyzed using reorientations mediated by translational displacements (RMTD) and force-free-hard-sphere (FFHS) models, respectively.

Slow water dynamics in polygalacturonate hydrogels revealed by NMR relaxometry and molecular dynamics simulation

Diffusions in gels are of prime importance, but their measurements are mainly focused on the diffusion in the pores or through the mesh of the gels. In this study, we performed a deeper dynamic analysis of the water in close interaction with the fibers structuring two heterogeneous polygalacturonate (polyGalA) hydrogels formed by Ca and Zn ions (crosslinking agents). Nuclear magnetic resonance dispersion (NMRD) profiles recorded in-situ by fast-field cycling relaxometry allow to observe the very slow dynamics of water within the gels. Two distinct interpretations of the NMRD profiles are discussed, the first in regard of rotational and translational dynamics in the fibers and the second with respect to a Levy-walk on the fibers' surface. These discussions are confronted with molecular dynamics simulations on a model Ca-polyGalA fiber.

Low-Field NMR Relaxometry for Intraoperative Tumour Margin Assessment in Breast-Conserving Surgery.

Simple SummaryBreast cancer is the most diagnosed cancer for women, and clear surgical margins in breast-conserving surgery (BCS) are essential for preventing recurrence. In this study, the potential of fast field-cycling 1H-NMR relaxometry as a new tool for intraoperative margin assessment was evaluated. The technique allows the determination of the tissue proton relaxation rates as a function of the applied magnetic field on small tissue samples excised from surgical specimens, at the margins of tumour resection, prior to histopathological analysis. It was found that a good accuracy in margin assessment, i.e., a sensitivity of 92% and a specificity of 85%, can be achieved. The discriminating ability shown by the relaxometric assay relies mainly on the difference of fat/water content between healthy and tumour cells. The information obtained has the potential to support the surgeon in real-time margin assessment during BCS.As conserving surgery is routinely applied for the treatment of early-stage breast cancer, the need for new technology to improve intraoperative margin assessment has become increasingly important. In this study, the potential of fast field-cycling 1H-NMR relaxometry as a new diagnostic tool was evaluated. The technique allows the determination of the tissue proton relaxation rates (R1), as a function of the applied magnetic field, which are affected by the changes in the composition of the mammary gland tissue occurring during the development of neoplasia. The study involved 104 small tissue samples obtained from surgical specimens destined for histopathology. It was found that a good accuracy in margin assessment, i.e., a sensitivity of 92% and a specificity of 85%, can be achieved by using two quantifiers, namely (i) the slope of the line joining the R1 values measured at 0.02 and 1 MHz and (ii) the sum of the R1 values measured at 0.39 and 1 MHz. The method is fast, and it does not rely on the expertise of a pathologist or cytologist. The obtained results suggest that a simplified, low-cost, automated instrument might compete well with the currently available tools in margin assessment.

Molecular-level insights into structure and dynamics in ionic liquids and polymer gel electrolytes

Designing new electrolytes requires a better understanding of the correlation between their transport properties and their molecular structure. In this work, we present a detailed study of ionic liquids and polymer gel electrolytes probing their structure and dynamics by nuclear magnetic resonance (NMR) spectroscopy. In particular, ammonium- and phosphonium-based ionic liquids (ILs) are combined with different LiTFSI concentrations, and then with different ratios of poly(methylmethacrylate) polymer. The temperature dependence of self-diffusion coefficients of mobile species, DLi+, DTFSI- and DP4441+ or DN4441+, measured by pulsed field gradient (PFG) NMR spectroscopy obeys the Arrhenius equation. Diffusivity of [P4441][TFSI] ILs is found to be greater than those of the [N4441][TFSI] ILs in both liquid and gel electrolytes. Solid state NMR experiments including 13C and 19F MAS, 13C{19F},13C{1H} CPMAS probe the local structure and molecular-level interactions between ions and polymer in gel electrolytes, particularly for samples with higher PMMA content (≥25 wt%). Finally, the fast field cycling relaxometry has been used to unveil the rotational and translational dynamics of P4441+ or N4441+ and TFSI− by measuring 1H and 19F R1 relaxation rate profiles at different temperatures. A comprehensive NMR analysis including relaxation studies at low magnetic field provides decisive new insights regarding the formation of ionic clusters and the interaction of ions with the polymer chain in the case of the gel electrolytes.

Rotational correlation times, diffusion coefficients and quadrupolar peaks of the protic ionic liquid ethylammonium nitrate by means of 1H fast field cycling NMR relaxometry

The protic ionic liquid (PIL) ethylammonium nitrate (EAN) is characterized by hydrogen bonding between the NH bonds of the cations and the oxygen atoms of the anions. The three-dimensional H-bond network of EAN very much resembles that of water. Although this first ionic liquid is known for more than 100 years, there is still lack of information about the rotational and translational dynamics, and how both are related to each other. For that purpose, we measured the 1H nuclear magnetic relaxation dispersion curves (NMRD) of EAN by means of Fast Field Cycling (FFC) NMR relaxometry. We were able to analyze the measured 1H spin-lattice relaxation rates R1 covering a large temperature range between 248 and 333 K simultaneously. Applying the well-known Bloembergen-Purcell-Pound (BPP) approach, we decomposed the total relaxation rates into intramolecular and intermolecular contributions, describing the rotational and translational dynamics of the ethylammonium cation. Here, we report rotational correlation times, τR, and translational diffusion coefficients, DT, both derived from the total 1H spin-lattice relaxation rates as a function of temperature and frequency. Self-diffusion coefficients, DT, were also determined from the total relaxation rates in the low frequency range, wherein only the intermolecular relaxation contribution is frequency dependent. Then, DT results from the slope of a linear fit of the spin-lattice relaxation rate R1 as a function of the square root of frequency, ν. The diffusion coefficients obtained from both methods are in good agreement with self-diffusion coefficients measured by pulsed-field-gradient (PFG) NMR. The rotational correlation times, τR, derived from the intramolecular 1H relaxation contribution were compared to correlation times obtained from high-field NMR deuteron relaxation experiments, dielectric spectroscopy (DS) and femto-second-infrared (fs-IR) spectroscopy. For EAN, we also observed a local enhancement of the 1H spin-lattice relaxation rates R1 at high frequencies. This so-called quadrupole relaxation enhancement (QRE) results from the interference of the Zeeman transition energy of the 1H spins and the energy difference of two levels of the quadrupole 14N nuclei. QRE is usually known for solids, but rarely for the liquid state as observed here for EAN. Overall, we show that FFC relaxometry provides access to rotational correlation times, translational diffusion coefficients and quadrupole relaxation enhancement at the same time.

A Photocleavable Contrast Agent for Light-Responsive MRI.

Thanks to its innocuousness and high spatiotemporal resolution, light is used in several established and emerging applications in biomedicine. Among them is the modulation of magnetic resonance imaging (MRI) contrast agents’ relaxivity with the aim to increase the sensitivity, selectivity and amount of functional information obtained from this outstanding whole-body medical imaging technique. This approach requires the development of molecular contrast agents that show high relaxivity and strongly pronounced photo-responsiveness. To this end, we report here the design and synthesis of a light-activated MRI contrast agent, together with its evaluation using UV–vis spectroscopy, Fast Field Cycling (FFC) relaxometry and relaxometric measurements on clinical MRI scanners. The high relaxivity of the reported agent changes substantially upon irradiation with light, showing a 17% decrease in relaxivity at 0.23T upon irradiation with λ = 400 nm (violet) light for 60 min. On clinical MRI scanners (1.5T and 3.0T), irradiation leads to a decrease in relaxivity of 9% and 19% after 3 and 60 min, respectively. The molecular design presents an important blueprint for the development of light-activatable MRI contrast agents.

Probing relaxation models by means of Fast Field-Cycling relaxometry, NMR spectroscopy and molecular dynamics simulations: Detailed insight into the translational and rotational dynamics of a protic ionic liquid

A combination of temperature dependent Fast Field-Cycling (FFC) relaxometry and high-resolution NMR spectroscopy provides more than five orders of magnitude in frequency range for studying translational and rotational dynamics of ionic liquids. However, to make use of this broad frequency range, certain requirements have to be met: The viscosity of the liquid has to be of the right order of magnitude, and the material has to exhibit a sufficiently broad liquid range. In addition, NMR sensitive nuclei have to be present (ideally) on both, cation and anion, and appropriate relaxation models properly describing the molecular dynamics have to be available. For the latter, Molecular Dynamics (MD) simulations are ideally suited to suggest meaningful relaxation models. In this study we employ the protic ionic liquid triethylammonium bis(trifluoromethylsulfonyl)-imide [TEA][NTf2] as model compound to gain insight into details of the molecular dynamical processes. By addressing different NMR sensitive nuclei on both ions, 1H nuclei on the triethylammonium cation, and 19F on the NTf2 anion, we are able to obtain translational dynamics as well as rotational dynamics for both species at the same time. The obtained temperature dependent translatoric diffusion coefficients are consistent with our MD simulations, and are found to be in agreement with data reported in the literature. In addition, two types of NMR relaxation processes are employed to investigate the intramolecular relaxation: (1) dipolar relaxation at low frequencies (employing FFC) addressing 1H and 19F nuclei, and (2) quadrupolar relaxation at high frequencies addressing 2H nuclei. In near quantitative agreement with MD simulations, we show that both, dipolar relaxation obtained via FFC and quadrupolar relaxation obtained via high-field NMR are leading to consistent rotational correlation times, suggesting that the rotational motion of the cation is dominantly isotropic. Hence, the intramolecular 1H relaxation rate of the cation determined via FFC can be appropriately expressed by a single rotational correlation time, compatible with the Bloembergen Purcell Pound (BPP) approach. MD simulations of the NTf2 anion, however, suggest that its reorientational dynamics is strongly anisotropic. Consequently, the proper description of the intramolecular 19F relaxation rate of the anion requires a more complex model beyond the BPP approach employing at least two correlation times: By taking internal rotation of the CF3 groups into account, we are able to determine correlation times compatible with our MD simulations.

Nuclear Magnetic Resonance with Fast Field-Cycling Setup: A Valid Tool for Soil Quality Investigation

Nuclear magnetic resonance (NMR) techniques are largely employed in several fields. As an example, NMR spectroscopy is used to provide structural and conformational information on pure systems, while affording quantitative evaluation on the number of nuclei in a given chemical environment. When dealing with relaxation, NMR allows understanding of molecular dynamics, i.e., the time evolution of molecular motions. The analysis of relaxation times conducted on complex liquid–liquid and solid–liquid mixtures is directly related to the nature of the interactions among the components of the mixture. In the present review paper, the peculiarities of low resolution fast field-cycling (FFC) NMR relaxometry in soil science are reported. In particular, the general aspects of the typical FFC NMR relaxometry experiment are firstly provided. Afterwards, a discussion on the main mathematical models to be used to “read” and interpret experimental data on soils is given. Following this, an overview on the main results in soil science is supplied. Finally, new FFC NMR-based hypotheses on nutrient dynamics in soils are described

NMR Relaxometry and Diffusometry Analysis of Dynamics in Ionic Liquids and Ionogels for Use in Lithium-Ion Batteries.

We have investigated the charge transport dynamics of a novel solid-like electrolyte material based on mixtures of the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM] TFSI) and various concentrations of lithium salt bis(trifluoromethylsulfonyl)imide (LiTFSI) confined within a SiO2 matrix, prepared via a sol-gel method. The translational diffusion coefficients of BMIM+, TFSI-, and Li+ in ILs and confined ILs (ionogels, IGs) with different concentrations of lithium salt have been measured at variable temperatures, covering the 20-100 °C range, using nuclear magnetic resonance (NMR) pulsed field gradient diffusion spectroscopy. The mobility of BMIM+, TFSI-, and Li+ was found to increase with the [BMIM] TFSI/LiTFSI ratio, exhibiting an almost liquid-like mobility in IGs. Additionally, the effect of confinement on IL rotational dynamics has been analyzed by measuring 1H, 19F, and 7Li spin-lattice relaxation rate dispersions of IGs at different temperatures, using fast field-cycling NMR relaxometry. The analysis of the experimental data was performed assuming the existence of two fractions of the liquid: a bulk fraction (at least several ionic radii from the silica particles) and a surface fraction (close to the silica particles) and using two different models based on translational and rotational diffusion and reorientation mediated by translational displacements. The existence and weighting of these two fractions of ions were obtained from the direct diffusion measurements. The results show that the ion dynamics slowed only modestly under confinement, which evidences that IGs preserve IL transport properties, and this behavior is an encouraging indication for using IGs as a solid electrolyte for Li+ batteries.

Limits imposed by liquid/surface interactions in the determination of tortuosity in mesopores

Abstract The restricted mobility of different liquids, polar and non-polar, were studied by means of 1H NMR diffusion and Larmor frequency dependent relaxation measurements, upon confinement in mesoporous silica with mean pore sizes ranging from 3–30 nm. The use of alkanes as a reference in the determination of the true tortuosity value was observed to be accurate for pores larger than 6 nm, while for smaller pore sizes geometrical restrictions affect the diffusion even for short chain linear alkanes. 1H NMR fast field cycling relaxometry was used to measure the relaxation times as a function of the Larmor frequency. We observed that the total observable dispersion (TOD), which is a measure of the surface/liquid interaction, is mainly driven by the liquid polarity for pores larger than 10 nm, while less dependence on polarity is observed for smaller pore sizes. Our results indicate that as confinement increases, liquid/surface interactions become more relevant than the structural properties of the porous network, imposing a limit to the use of alkanes to probe the geometrical tortuosity.

Measuring the Spin-Lattice Relaxation Magnetic Field Dependence of Hyperpolarized [1-13C]pyruvate.

The fundamental limit to in vivo imaging applications of hyperpolarized 13C-enriched compounds is their finite spin-lattice relaxation times. Various factors affect the relaxation rates, such as buffer composition, solution pH, temperature, and magnetic field. In this last regard, the spin-lattice relaxation time can be measured at clinical field strengths, but at lower fields, where these compounds are dispensed from the polarizer and transported to the MRI, the relaxation is even faster and difficult to measure. To have a better understanding of the amount of magnetization lost during transport, we used fast field-cycling relaxometry, with magnetic resonance detection of 13C nuclei at ~0.75 T, to measure the nuclear magnetic resonance dispersion of the spin-lattice relaxation time of hyperpolarized [1-13C]pyruvate. Dissolution dynamic nuclear polarization was used to produce hyperpolarized samples of pyruvate at a concentration of 80 mmol/L and physiological pH (~7.8). These solutions were rapidly transferred to a fast field-cycling relaxometer so that relaxation of the sample magnetization could be measured as a function of time using a calibrated small flip angle (3°-5°). To map the T1 dispersion of the C-1 of pyruvate, we recorded data for different relaxation fields ranging between 0.237 mT and 0.705 T. With this information, we determined an empirical equation to estimate the spin-lattice relaxation of the hyperpolarized substrate within the mentioned range of magnetic fields. These results can be used to predict the amount of magnetization lost during transport and to improve experimental designs to minimize signal loss.

X-nuclei hyperpolarization for studying molecular dynamics by DNP-FFC.

Dynamic Nuclear Polarization methods are used for improving the quality of the NMR data, opening new possibilities by increasing both the sensitivity and the selectivity in NMR relaxation experiments. Recently, Fast Field Cycling relaxometry combined with DNP was introduced, demonstrating that molecular dynamics studies in the presence of natural or artificial radicals are indeed feasible under conditions where the signal-to-noise ratio is frequently critical. In this work, the extension of NMR relaxation dispersion beyond 1H NMR, by hyperpolarization of X-nuclei, is demonstrated. Overhauser effect via nitroxide radicals in simple (low viscous) liquids and saline solutions was observed for 2H, 7Li and 13C nuclei at ambient temperature. Substantial NMR signal enhancement up to several hundred was achieved for the studied samples. An advanced approach for reconstructing of the original relaxation dispersion of pure substances is used to eliminate the effect of the additional radical relaxivity of the X-nuclei.

Relaxivity of Gd‐Based MRI Contrast Agents in Crosslinked Hyaluronic Acid as a Model for Tissues

The efficiency of MRI contrast agents depends on the relaxation rate enhancement that they can induce at imaging fields. It is well known that, at these fields, large relaxation rates are obtained by binding of gadolinium(III) ions to large molecules. By the same token, the interaction of the gadolinium(III) complexes with macromolecules that are found in biological tissues can be responsible for an increase of the relaxation rate with respect to the value observed in liquids. We investigate here the relaxation enhancement of gadoteridol (Gd-HP-DO3A) in crosslinked hyaluronic acid, taken as model tissue, using fast field-cycling relaxometry. The analysis of the relaxation profiles as a function of the magnetic fields indicates that a sizable increase in the relaxation rates is due to a modest interaction of the contrast agent with the hydrogel and to the slower mobility of the water molecules outside the first-coordination sphere of the gadolinium(III) ion.

Distribution of Gadolinium in Rat Heart Studied by Fast Field Cycling Relaxometry and Imaging SIMS.

Research on microcirculatory alterations in human heart disease is essential to understand the genesis of myocardial contractile dysfunction and its evolution towards heart failure. The use of contrast agents in magnetic resonance imaging is an important tool in medical diagnostics related to this dysfunction. Contrast agents significantly improve the imaging by enhancing the nuclear magnetic relaxation rates of water protons in the tissues where they are distributed. Gadolinium complexes are widely employed in clinical practice due to their high magnetic moment and relatively long electronic relaxation time. In this study, the behavior of gadolinium ion as a contrast agent was investigated by two complementary methods, relaxometry and secondary ion mass spectrometry. The study examined the distribution of blood flow within the microvascular network in ex vivo Langendorff isolated rat heart models, perfused with Omniscan® contrast agent. The combined use of secondary ion mass spectrometry and relaxometry allowed for both a qualitative mapping of agent distribution as well as the quantification of gadolinium ion concentration and persistence. This combination of a chemical mapping and temporal analysis of the molar concentration of gadolinium ion in heart tissue allows for new insights on the biomolecular mechanisms underlying the microcirculatory alterations in heart disease.

Exploring the tumour extracellular matrix by in vivo Fast Field Cycling relaxometry after the administration of a Gadolinium-based MRI contrast agent.

1 H Fast Field Cycling NMR (FFC-NMR) relaxometry is proposed as a powerful method to investigate tumour stroma in vivo upon the administration of a Gd-based contrast agent. To perform this study, an FFC-NMR equipment endowed with a wide bore magnet was used for the acquisition of Nuclear Magnetic Resonance Dispersion profiles on healthy muscle and tumour tissue in living mice. At magnetic field strengths < of ca. 1MHz, the differences in the relaxation rates of the intra and extracellular compartment become of the same order of magnitude of the exchange rate across the cellular membranes. Under this condition, the water exchange rate between the two compartments yields to a biexponential magnetization recovery that can be analysed by fitting the experimental data with the two-Site eXchange (2SX) model. Using this model, it was possible to obtain, for the two compartments, both relaxation properties and water kinetic constants for water exchange across cell membranes. The method allowed us to determine the effect of the "matrix" on the water proton relaxation times and, in turn, to get some insights of the composition of this compartment, till now, largely unknown.

Theory of fast field-cycling NMR relaxometry of liquid systems undergoing chemical exchange

ABSTRACTThe time evolution of the nuclear magnetisation of chemically exchanging systems in liquids is calculated for the pre-polarised fast field-cycling sequence of nuclear magnetic resonance (NMR) relaxometry. The obtained parameter expressions of the magnetisation allow one to derive the longitudinal relaxation rates and the residence times of the exchanging sites from the experiment. In the particular cases of slow and fast exchange, approximations leading to simple analytic expressions are derived. The theory takes account of the delay time necessary to ensure that the field for acquiring the signal is stable enough after its rapid jump from its relaxation value. The domains of mono-exponential or bi-exponential relaxation of the magnetisation are displayed in a concise way through 3D and 2D logarithmic plots of the population ratio of the exchanging sites and of their intrinsic relaxation times. The influence of the acquisition delay on the fitted values of the populations, residence times, and intrinsic relaxation times of the sites is emphasised in the case of the bi-exponential water proton relaxation observed in a tumour tissue.

Overhauser DNP FFC study of block copolymer diluted solution

Overhauser dynamic nuclear polarization (DNP) is the dominating hyperpolarization technique to increasing the nuclear magnetic resonance signal in liquids and diluted systems. The enhancement obtained depends on the overall mobility of the radical-carrying molecule but also on its specific interaction with the host molecules. Information about the nature of molecular and radical dynamics can be identified from determining the nuclear T1 as a function of Larmor frequency by Fast Field Cycling (FFC) relaxometry. In this work, DNP and FFC methods were combined for a detailed study of 1H Overhauser DNP enhancements at 340 mT (X-band) and 73 mT (S-band) for diluted solutions of a block-copolymer with and without the addition of TEMPO radicals. NMR relaxation dispersions of these solutions are measured at thermal polarization and DNP conditions in the X-band, and the obtained DNP data were analyzed by a model of electron-nucleus interactions modulated by translational diffusion. The coupling factors for the two different blocks of the copolymer are obtained independently from DNP and NMRD experiments. An additional contribution from scalar interactions was found for polystyrene blocks.

Characterisation of magnetic resonance imaging (MRI) contrast agents using NMR relaxometry

ABSTRACTThis contribution describes the use of Fast Field-cycling relaxometry (FFC-NMR) for the characterisation of Gd(III)- and Mn(II)-based contrast agents for MRI. Through a series of selected examples, we analyse the role of different structural and dynamic parameters on 1H relaxivity and on the shape of the 1H Nuclear Magnetic Relaxation Dispersion (NMRD) profiles. The amplitude and shape of the profiles is affected by the number of water molecules coordinated to the metal ion, the water exchange rate, the rotational correlation time of the complex and the relaxation of the electron spin. As a result, 1H NMRD profiles represent a powerful tool for the understanding of the properties of MRI contrast agent candidates at the molecular level.