Stray-field Magnetic Resonance Imaging Research Articles

Micrometer scale resolution of materials by stray-field Magnetic Resonance Imaging

As Magnetic Resonance Imaging devices are becoming more and more powerful, resolutions as small as 10 μm can now be obtained. But, this is only possible when systems with slow transverse relaxation rates, like living tissues, are investigated. In this case, the time available for gradients space-encoding is long, and high k values can be reached in the Fourier domain. However, numerous materials have fast relaxation rates, thus limiting the spatial resolution to a few hundreds of microns. The Stray Field Imaging technique has solved this problem by using a very high (typically 5000 G/cm) static gradient. Consequently, the trajectory in k space is accelerated and it is possible, in principle, to reach a micrometer resolution in a few hundreds of microseconds. Most of the time, however, only resolutions in the millimeter range can be achieved due to mispositioning of the sample within the static magnetic field. Here, we show that by finely mapping the magnetic field and precisely positioning the sample, it is possible with a standard spectrometer to reach a micrometer resolution even on very fast relaxing materials.

One-step self-etching adhesive polymerization: Influence of a self-curing activator

Objectives The objectives of this work were to study the spatially resolved extent of photopolymerization and the final volumetric contraction of the self-etching adhesive Xeno IV (Dentsply/Caulk, Milford, DE, USA) in association with the self-curing agent Aurora (Dentsply/Caulk, Milford, DE, USA), using 1H Stray-Field magnetic resonance imaging ( 1H STRAFI-MRI). Methods One-dimensional 1H STRAFI-MRI images (projections along an axis, profiles) were acquired from the adhesive with and without Aurora, before and after solvent removal. Specimens were observed after the irradiation period recommended by the manufacturers and new profiles were recorded. Extent of polymerization and volumetric contraction were obtained by subtracting the profile of the cured adhesive from the profile of the liquid; the profiles were previously normalized using an external reference signal and the highest magnetization intensity recorded from the cured adhesive, respectively. Data were obtained in the presence of oxygen. Solvent evaporation was measured by gravimetry. Results High volumetric contraction was observed for Xeno IV and Xeno IV and Aurora mixture mainly related to solvent evaporation. A drying period favourably influenced the extent of photopolymerization of Xeno IV, which did not increase in the presence of Aurora. Conclusions The incorporation of Aurora reduced the extent of polymerization of the one-step adhesive, probably due to the increase in the solvent content of the mixture. It may be that enhancing solvent evaporation is important in order to reach a higher extent of polymerization during the bonding procedure.

Influence of Grain Size on the Setting of Portland Cement: A Stray-Field Magnetic Resonance Imaging Study

A number of failures of large concrete structures during construction have been reported in the last decades [1]. The overestimation of concrete strength at early ages was one of the reasons for the failures. Consequently, reliable information about early age properties of the material is essential to guarantee life-time performance of structures. Portland cement is a complex heterogeneous particulate material and a full knowledge of kinetics of the hydration reactions, for example, is still missing. Gel constitutes the major phase in the hardening cement paste and the corresponding structure and dynamics represent an important contribution to determine the concrete performance. X-ray diffraction, which is widely used for the study of crystalline cement components, does not give information about the gel, amorphous, phase. Conversely, 1H stray-field magnetic resonance imaging (STRAFI-MRI) technique has proved to be a powerful tool to follow the early hydration and hardening periods of Portland cement (type I) [2-4]. The setting of cement pastes depends on parameters like the initial water/cement ratio, R, or particle size of the powder (G) and the compressive strength can be used to characterize the behaviour of hardening concrete. Water availability at the particle surfaces, which is controlled by R and G, limits cement hydration. At low R, G effects are less important. In general, it is accepted that for R<0.42, unreacted solid remain, as all the free volume is filled with hydration products [5]. For example, hydration of Portland cement pastes as a function of R (0.24-0.48) was studied using by STRAFI-MRI and hydrogen maps, from different types of water (capillary, gel or chemically bound water), enabled a spatially-resolved kinetics to be obtained [4]. Using STRAFI-MRI was now evaluated the influence of G (<70 μm to < 90 μm) on the early stages of hydration and hardening of Portland cement. Portland cement uses extend well beyond construction. For example, a mineral trioxide aggregate is now being applied as a root-end filling material, which was shown to have a similar chemical constitution to that of Portland cement except for the addition of bismuth compounds, seemingly to make the materials radiopaque for dental use [6].

Spatially resolved studies on the photopolymerization of dimethacrylate monomers

The photopolymerization of four dimethacrylate monomers under different irradiating conditions was followed in situ by 1H stray-field magnetic resonance imaging (1H STRAFI-MRI). This technique is capable of discriminating local changes at a spatial resolution of tens of micron and is suitable for studying larger samples than infrared spectroscopy and photocalorimetry. The evolution of proton magnetization with irradiation time and intensity was recorded and correlated with volumetric polymerization shrinkage, extent of reaction and spatially resolved reaction rates.

Spatially resolved photopolymerization kinetics and oxygen inhibition in dental adhesives

A comparative study of three commercial dental adhesives was performed by 1H nuclear magnetic resonance spectroscopy and Stray-Field magnetic resonance imaging. Spectroscopic evidence was found for the presence of solvent and unreacted methacrylate groups in photopolymerized adhesives. Spatially resolved photopolymerization kinetics and volumetric contraction (solvent evaporation and polymerization shrinkage) were obtained without solvent removal and in the presence of oxygen from the atmosphere. The oxygen and solvent inhibitor effects in the photopolymerization were found to be higher for water/ethanol based adhesives. However, was one of these adhesives that exhibited less spatially dependent irradiation time to start vitrification, higher concentration and a more uniform spatial distribution of rigid domains at the end of the photopolymerization.

Measurement of the diffusion of liquids into dental restorative resins by stray-field nuclear magnetic resonance imaging (STRAFI)

Objectives. The purpose of this investigation was to determine the diffusion mechanism for water/ethanol mixtures in a diacrylate dental resin by direct observation of the absorbed liquid profiles using NMR microimaging. Methods. Frequency-swept stray-field magnetic resonance imaging (STRAFI) was used. Solutions containing 25–65% by volume ethanol remained in contact with visible light cured 54% TEGDMA: 46% modified TUDMA sheets while measurements were made. The diffusion profiles were recorded periodically for diffusion times up to 10 h, to a depth of 360 μm and with readings taken at 24 μm intervals. Results. For all liquid mixtures, diffusion was found to be Fickian with coefficients that increased progressively and smoothly with alcohol content in the diffusing mixture, from 2.4×10 −13 to 150×10 −13 m 2 s −1. A rule of mixtures approach, as suggested by Kwei and Zupko, gave a satisfactory description of the ethanol fraction dependence of the diffusivity. Significance. Frequency-swept STRAFI offers a new and unique opportunity to produce spatially resolved measurements of the liquids in dental resins to high resolution. In this study, absorption was investigated since an understanding of its mechanism is fundamental to limiting consequent environmental degradation. STRAFI has great potential for other applications, for example drying, liquid exchange, etc. Since STRAFI can discriminate 1H in the liquid from those in softened polymer additional applications are envisaged.

Stray field magnetic resonance imaging: a preliminary study of skin hydration

We report the first use of GARField (STRAFI-like) magnetic resonance profiling (1D-MRI) to study skin hydration, both in vitro and in vivo. It is shown that, for in vitro samples, a high degree of reproducibility is achievable for skin samples from a single donor and that the signal intensity correlates well with equilibrium moisture content in the skin. In vivo, the method allows measurement of the influence of a moisturizer cream on water levels in the skin as a function of time.

Composition and phase changes observed by magnetic resonance imaging during non-solvent induced coagulation of cellulose

The coagulation of cellulose from solution in N-methylmorpholine- N-oxide by water was studied by stray field magnetic resonance imaging. Changes in composition and diffusion were readily observable at different depths within the cellulose gel. It was found that the composition changed relatively quickly, due to diffusive exchange of water and solvent, such that cellulose precipitation occurred deep within the bi-phasic region of the phase diagram. The expected spinodal decomposition mechanism was supported by electron microscopic observations of the cellulose morphology. Large increases in diffusion coefficient and relaxation times were attributable to the phase separation and the progressive development of pores within the gel. Moreover, it appeared that the composition changes and evolution of morphology occurred over separate time-scales.

The study of a commercial dental resin by 1H stray-field magnetic resonance imaging

A dentine/enamel resin containing methacrylate monomers, included in a new generation adhesive system, was used to evaluate the potentialities of recent nuclear magnetic resonance imaging (MRI) techniques to obtain spatially-resolved information on photo-polymerization reaction and subsequent polymerization shrinkage. 1H stray-field (STRAFI)-MRI one-dimensional images (1D profiles) of visible-light cured resins were obtained in the presence of oxygen from the atmosphere, and the variation of magnetization with irradiation time was recorded for each resin slice. The polymerization shrinkage was obtained from 1D profiles. The spatial distribution of the unreacted methyl methacrylate groups was obtained from 3D STRAFI experiments. In particular, the thickness of the surface remaining unpolymerised was measured.

Stray Field Magnetic Resonance Imaging (STRAFI) of Water Transport in Ceramic Substrates

Measurements of water transport in ceramic tiles made using the stray field magnetic resonance imaging technique (STRAFI) are reported. STRAFI represents one method of accomplishing magnetic resonance imaging (MRI) of solids and confined liquids.

Low-, stray-field imaging and spectroscopic studies of the sodium polyacrylate water uptake

Water uptake in sodium polyacrylate was followed by low-field magnetic resonance imaging (LF-MRI), at 0.1T and 5mT/m, and by stray-field magnetic resonance imaging (STRAFI-MRI), at 2.9T and 37.5T/m, over 50h. One-dimensional (1-D) images were recorded and the model for the diffusion of the liquid into the polymer was obtained. The complementarity of the techniques is discussed. 1H, 13C and 23Na spectroscopic NMR data are also presented.

Concentration Profiles in Creaming Oil-in-Water Emulsion Layers Determined with Stray Field Magnetic Resonance Imaging

We report the first use of stray field magnetic resonance imaging in the determination of concentration profiles in layers (with submillimeter thickness) of coarse oil-in-water emulsions that are undergoing creaming. We compare our results for emulsions having various oil contents to the predictions of a numerical model. In emulsions having low oil content (12 and 23 vol %), we find that the model adequately predicts the profile shape and time-dependent change in the lower region of the emulsion. In the cream layer at the top of each of the samples, however, the predictions of the model deviate substantially from the observed profiles. Whereas the model assumes that the concentration of oil in the cream layer is constant as the layer increases in thickness, we observe that there is a concentration gradient in the cream layer and that the concentration of oil in the layer increases with time. In explaining our findings, we consider the effects of polydispersity and the presence of gum xanthan in the contin...

Stray-Field Imaging of Planar Films Using a Novel Surface Coil

A surface-coil approach to stray-field magnetic resonance imaging (STRAFI), which has certain advantages for the profiling of planar films and layers, is described. Frequency-swept and Fourier-transform versions of the experiment, which are particularly suitable for use with the coil, are reported. As an example application, a latex coating is analyzed during the evaporative drying process, in which a colloidal dispersion is transformed to a continuous polymer film. The ultimate achievable resolution is discussed and contrasted with conventional MR microscopy.

The visualization of spatial gradients in polymer and solvent dynamics for mixed solvents ingressing poly(methyl methacrylate) using stray field magnetic resonance imaging

Stray-field magnetic resonance imaging (m.r.i.) has been used to study the ingress of methanol and methanol-acetone vapour mixtures into poly(methyl methacrylate), PMMA. For pure methanol, the polymer concentration and 1H spin-spin relaxation time is found to be constant across the swollen region. The relaxation time is, however, greater by a factor of 1.1 compared to the rigid polymer. For mixed solvents, the polymer chain spin-spin relaxation time is greater by factors ranging from 1.1 to 1.4 across the swollen region in samples exposed to solvent for up to 1 week, providing evidence for a spatial gradient in the polymer chain dynamics. Small polymer concentration gradients are also seen in samples exposed to methanol-acetone mixtures. In further experiments, stray field imaging and gradient spin echo diffusion techniques have been combined for the first time in order to make high spatial resolution measurements of the solvent self-diffusion coefficient. These experiments yield complementary spatial gradients in the diffusion coefficient such that the diffusion coefficient typically varies by a factor of 2 across any one sample, ranging from 0.15 to 0.30 × 10 −6 cm 2 s −1 over 0.8 mm for pure methanol in PMMA and from 1.8 to 3.2 × 10 −6 cm 2 s −1 over 1.5 mm for 70% methanol-30% acetone. These results are discussed in the context of previous liquid-state m.r.i. investigations of the solvent alone in similar systems.

Stray-field magnetic resonance imaging of solid materials

NMR imaging has experienced a tremendous increase in its use in a diverse range of investigations, particularly in the field of medical diagnostics. With regard to NMR imaging of solid materials, a number of challenges must be overcome before solid-state NMR imaging can experience widespread use. To date, a variety of strategies has been proposed for obtaining solid-state NMR images. In this paper, we will review the literature dealing with a specific solid-state NMR imaging method, stray-field imaging. We will describe our implementation of a two-dimensional version of the stray-field imaging method on a commercial instrument capable of collecting wideline NMR data and the use of this method in the examination of solid materials of interest in the aerospace industry.

A Stray Field Magnetic Resonance Imaging Study of the Drying of Sodium Silicate Films

Stray field magnetic resonance imaging (STRAFI) is shown to be highly suited to the study of drying processes in thin films. Sodium silicate films have been chosen as a model system exhibiting many of the properties of film drying in general. Films have been dried, as a function of temperature in the range 22 to 62°C, down to water contents of order 28% by weight, at which stage the film is glassy. The experimental results have been quantitatively analyzed by treating the drying film as a colloidal solution. The results suggest that the localized hydrogen spin–spin relaxation time, and hence the mobility of the water in the films is independent of the drying regime and depends primarily on the local water concentration.

The application of broad line MRI to the study of porous media

Experiments to visualise water diffusion into dry sandstone rock plugs at low saturation levels, hydrophobic polymer coating diffusion into mortar and methanol/acetone diffusion into PMMA using broad line gradient echo and stray field magnetic resonance imaging are described.

Profile Amplitude Modulation in Stray-Field Magnetic-Resonance Imaging

It is shown that the amplitudes of successive echoes in a stray-field magnetic resonance imaging (STRAFI) experiment are strongly modulated by the intense magnetic-field gradient used and by sample movement during the experiment. In this paper the modulation is analyzed so that quantitative interpretation of STRAFI images reconstructed from multiple-echo data can be made. Spin-spin relaxation is included. Theoretical predictions of relative echo intensities are compared with data obtained from an experiment to visualize acetone penetration into PVC.

Stray field magnetic resonance imaging of the diffusion of acetone into poly(vinyl chloride)

Stray field nuclear magnetic resonance imaging has been used to study the diffusion of acetone vapour into poly(vinyl chloride) (PVC). For the first time in a n.m.r imaging experiment the penetrant, the swollen polymer and the rigid polymer have been simultaneously visualized. Results are presented for a range of acetone vapour activities and sample preparation temperatures, and also for a sample prepared with deuterated acetone. The profiles show that the diffusion is of Case II type. A sloping profile is observed behind the penetrant front and is interpreted in terms of a softening of the swollen polymer, which occurs over a period of hours after penetration by the acetone. The softening rate has been measured as a function of temperature.

The visualization of the ingress of polymer treatment coatings into porous building materials by stray-field magnetic resonance imaging

The visualization of the ingress of polymer treatment coatings into porous building materials by stray-field magnetic resonance imaging