NMR Imaging Techniques Research Articles

Fluid Transport in Porous Rocks. II. Hydrodynamic Model of Flow and Intervoxel Coupling

In a preceding paper [P. Mansfield and B. Issa,J. Magn. Reson. A122, 137–148 (1996)], a stochastic model of fluid flow in porous rocks based upon the experimental observation of water flow through a Bentheimer sandstone core was proposed. The flow maps were measured by NMR-imaging techniques. The stochastic theory led to a Gaussian velocity distribution with a mean value in accord with Darcy's law. Also predicted was a linear relationship between flow variance and mean fluid flow through rock, the Mansfield–Issa equation, originally proposed as an empirical relationship. In the present work a flow coupling mechanism between voxels is proposed. Examination of the flow coupling between isolated voxel pairs leads to a complementary explanation of the Gaussian velocity distribution, and also gives further details of the Mansfield–Issa equation. These details lead to a new expression for the connectivity, 〈C〉, between voxels with an experimental value of 〈C〉 = 5.64 × 10−9for Bentheimer sandstone.

NMR imaging studies of transport heterogeneity and anisotropic diffusion in porous alumina pellets

One- and two-dimensional NMR imaging techniques have been used to investigate the influence of pore structure on mass transfer through porous alumina catalyst pellets. Studies are reported for two batches of pellets. In the first study it is shown that radial variations in voidage and pore size directly influence mass transfer. In the second study, a relatively homogeneous cylindrical tablet is studied. Effective diffusivity, and hence tortuosity, is observed to differ significantly with respect to axial and radial transient diffusion processes. This observation is correlated with a similar difference in fractal dimension describing the pore structure in these two directions. NMR imaging is shown to be a useful technique for gaining insight into the effect of manufacturing process on the mass transfer properties of porous catalyst pellets.

Magic echoes and NMR imaging of solids

Solid state NMR imaging techniques based on magic echoes are reviewed. The theoretical background of magic echoes in general and their spatial encoding in particular is treated. The magic-echo imaging pulse sequences presently in use are described and discussed. Particular emphasis is devoted to those techniques, which allow the incorporation of spectroscopic or parameter-selective information. The applicability of these methods for the investigation of materials is demonstrated and perspectives for materials science are outlined.

Quantitative determination of porosity: A local assessment by NMR imaging techniques

The local determination of porosity is an extremely valuable target because of different applications in petrophysics. In fact, obtaining a reliable profile of porosity or saturations could considerably improve the evaluation of transport properties of porous media, especially if applied to multiphase flow tests (i.e., for relative permeability characterisation). However, the best procedure to adopt for these kind of studies is currently under debate, involving different experimental choices both for acquisition and hardware solutions. The choice of a reliable procedure could be particularly important especially if heterogeneous samples (i.e., vugular or fractured carbonates) are approached. We have investigated selected carbonate cores, characterised by fractures and large vugs, using different instrumental solutions in order to assess usefulness of application of NMR imaging methods as a porosity measurement tool for heterogeneous samples. Local porosity data have been obtained and discussed relative to conventional measurements and petrographical aspects of rock samples.

Hadamard NMR imaging with slice selection

Stochastic NMR imaging is one of the less common NMR imaging techniques. Nevertheless, stochastic rf excitation is characterized by some remarkable features: the rf excitation power is at least two orders of magnitude lower in comparison to conventionally pulsed NMR imaging schemes. 1–3 Thus, the technique is of interest for imaging of large objects. The systematic noise inherent in images obtained with random noise excitation has been eliminated by using pseudorandom noise together with Hadamard transformation for data evaluation. Data acquisition times are comparable to those of ultrafast imaging techniques. For slice selection, z magnetization is destroyed outside the slice region with specially designed low power pulses. Thus, gradient switching times are only limited by T 1 and not T 2 ∗ . Images are reconstructed by the backprojection algorithm. We have set up a stochastic imaging procedure on a conventional Bruker MSL 300 spectrometer, and have drawn a comparison between images obtained by the pseudorandom noise excitation and by conventional Fourier imaging.

Simulating MRI flow maps in porous rocks: A new approach

The Mansfield and Issa model of voxel pair coupling is extended using electrical circuit simulation to interacting voxel clusters comprising all configurations of contiguous voxels up to and including four voxels. This information is used to simulate flow through porous rock samples by calculating the expected velocity distributions found in porous rocks using the PEPI rapid NMR imaging technique.

NMR imaging of aluminum oxide catalyst spheres

Spatially resolved NMR Imaging techniques have been applied in the study of the three-dimensional structure of aluminum oxide catalyst spheres, since the possible heterogeneities have been found to affect strongly the performance of porous materials as catalysts or catalyst carriers. A special form of activated alumina suitable for Claus catalytic converters was studied. The catalyst spheres were found to exhibit a uniform ball structure with spherically layered structures and voids, ensuring for example optimum flow conditions through the converters.

A fast inversion recovery NMR imaging technique for mapping two-dimensional tracer diffusion and dispersion in heterogeneous media

A nuclear magnetic resonance imaging method is demonstrated for measuring diffusion of a paramagnetic tracer in an aqueous gel, and the hydrodynamic dispersion of the same tracer in a porous medium. The fast inversion recovery method for determining the longitudinal relaxation times T1 is used in conjunction with a standard Fourier imaging scheme to obtain images with null contours, which track particular tracer concentrations. The fast inversion recovery method offers considerable saving of experimental time relative to the standard inversion recovery protocol, but without introducing dependences on transverse relaxation time T2, which would be hard to quantify. With correct parameter choice, null contours can be localized in the images within two pixels and the corresponding tracer concentrations identified from longitudinal relaxation data only. In a cylindrical gel sample, concentration fronts are tracked and yield diffusivities in agreement with results previously reported for a similar one-dimensional method; in a core of natural sedimentary rock, we image clearly the markedly irregular advance of a concentration front caused by natural heterogeneity in the medium. The method is applicable to a variety of fields involving diffusion or dispersion in microscopically or macroscopically heterogeneous media.

Ultra-rapid gradient echo imaging.

A novel ultra-rapid gradient echo (URGE) NMR imaging technique is introduced, which is capable of continuous high resolution 3D scanning while neither subject to fast gradient switching nor excessive RF power deposition. Sampling free induction decays instead of creating spin echoes enables maintaining a workable steady state magnetization. Due to segmented k-space acquisition, chemical shift, diffusion, and field inhomogeneity effects do not present major problems. We report on implementations acquiring from 32 x 64 x 64 partial-Fourier image sets in 0.72 s, allowing for single-shot magnetization-prepared 3D imaging, to 128 x 128 x 128 image sets in 13.3 s and 21.5 s on a standard MRI scanner.

Volume and contrast development in the Rose Bengal model of cerebral infarction

The development of the Rose Bengal model of cerebral infarction in the rat is monitored using rapid NMR imaging techniques. The lesion volume,T1,T2, and the apparent diffusion coefficient (ADC) are measured in the time interval between 24 and 168 h postinduction. VolumetricT1-weighted 3D imaging of the lesions produced by this model is shown to have a standard error of about 5% over the first 3 days postinduction, rising to about 15% at 1 week. Both theT1 andT2 contrasts are found to be more durable than those in ADC and are, hence, more suitable for volumetric monitoring over this time period. An elevation in ADC is also observed in tissue outside the lesion and this is interpreted as being caused by the transport of edema away from the lesion.

Gastric motility by tagged EPI

The high speed echo-planar NMR imaging technique is particularly suitable for visualizing human gut motility and transit. To enable a quantitative study of the rheology of the stomach contents, the magnetization can be prepared using a SPAMM (SPAtial Modulation of Magnetization) tagging sequence prior to imaging. Movement of stomach contents has been measured and analyzed temporally and in the Fourier domain and correlated with stomach motility. Increased stomach activity combined with retropulsive motion were observed at about 25 min following a porridge meal, indicating mixing of the meal and inhibition of gastric emptying.

Determining fluid saturations during multiphase flow experiments by NMR imaging techniques

AbstractThe quantitative determination of porosity and saturation distributions in porous media using unclear magnetic resonance imaging techniques is addresed. A method based on multiple image acquisitions with extrapolation within a model of transverse relaxation was investigated for new experimental situations. The determination of saturation was validated with limestone and sandstone samples. Experimental evidence for dependencies of transverse relaxation and linewidth on saturation is discussed, as well as their impact on image resolution and quantitation. An automatic production measurement device for independent verification of average saturations during dynamic experiments is investigated.

Investigation of the Diffusion Process in Cross-Linked Polystyrenes by Means of NMR Imaging and Solid-State NMR Spectroscopy

The diffusion process of dioxane in cross-linked polystyrenes can be monitored by combined NMR imaging and solid-state NMR techniques. NMR imaging data reveal a strong dependence of the diffusion behavior upon the cross-link density: in the case of lowest cross-linking (1%), a case II diffusion process takes place, whereas in the case of highest cross-linking (5%), Fickian diffusion occurs. The 2.5% cross-linked polystyrene shows intermediate behavior. Solid-state NMR relaxation parameters indicate great differences between the unswollen and the swollen state

Laundering and Deinking Applications of 1H NMR Imaging

Abstract One-dimensional 1 H NMR imaging techniques are used to visualize oil removal from fabrics and paper fibers immersed in aqueous solutions of nonionic detergents. The method provides a unique approach to the study of oil-removal kinetics in nonionic detergent systems where traditional optical techniques fail due to solution turbidity. The only requirement of the NMR experiment is the use of deuterated water in order to selectively image the hydrocarbon phase. Preliminary applications to laundering and paper deinking are discussed.

Probing the structure of porous pellets: An NMR study of drying

Partially saturated porous catalyst support pellets have been studied using NMR relaxation, PGSE, and imaging techniques. Spatially nonresolved PGSE data were acquired as a function of the fractional saturation of the pellets during drying. Combination of these data with an estimate of the width of the pore-size distribution, as determined by mercury porosimetry, and the results of numerical simulations of diffusion in a random pore network enabled predictions of the connectivity of the pore structure to be determined. NMR imaging was shown to be effective in discriminating between different models of drying; even when the bulk drying curves are similar, significantly different moisture redistribution phenomena are seen to occur.

Solvent-Probe NMR Imaging of Low-Density Polyethylene Rods: A Layered Structure Formed during the Molding Process

Abstract The spatial inhomogenuity of a linear low-density polyethylene rod shaped with a metallic mold was clearly imaged by the solvent-probe NMR imaging technique. Images with T2 contrast were obtained using the T2-flash sequence, which reflects the spatially resolved mobility of the solvent molecules absorbed in the polymer. The NMR image was very much dependent on the thermal history both during and after the molding process. These results were interpreted with the aid of the nuclear relaxation parameters of the matrix polyethylene determined by the high-resolution solid state NMR technique. It is suggested that the solvent-probe NMR imaging technique provides valuable information concerning the micro structure (domain structure) and its spatial distribution of a crystalline polymer.

In vivo NMR imaging and spectroscopic investigation of renal pathology in lean and obese rat kidneys

Diabetic nephropathy is a major cause of end-stage renal failure. While our understanding of the pathogenesis of nephropathy is incomplete, progressive glomerular injury appears to play a significant role in the decline of renal function. Proton NMR spectroscopy and imaging techniques were used to address changes in renal pathology associated with glomerular mesangial expansion in vivo in kidneys from spontaneously obese and lean (control) littermate Zucker rats. Fully functioning rat kidneys were surgically exposed and externalized for direct NMR signal detection via a coil placed around the organ. High-resolution (78 microns in plane) proton images were obtained at 4.7 T magnetic field strength revealing fine structure within the well-defined cortical and medullary regions. The obese rat kidney images were distinct in appearance from the lean kidney images and exhibited marked cortical expansion as well as increased overall kidney size. Enlargement of mean glomerular diameter was verified histologically in the obese kidneys as compared with the lean kidneys. Proton T1 and T2 relaxation times were determined from the entire kidney using standard spectroscopic techniques, and from specific regions within the kidney from multiple T1- and T-2 weighted images. Additionally, image contrast enhancement resulting from saturation transfer between protons in restricted-mobility environments and mobile water protons within the kidney was investigated in the lean and obese rat kidneys using magnetization-transfer imaging techniques. At the early stage of renal injury examined in this study, diseased and healthy kidneys could not be differentiated on the basis of relaxation times alone. The magnitude of saturation transfer obtained in cortical tissue in the lean and obese kidneys was also not statistically significantly different. However, the magnitude of saturation transfer achieved in the medullary tissue of obese kidneys was statistically significantly less than that achieved in lean kidneys.

Oligonucleotide phosphorofluoridates and fluoridites.

AbstractFluoro derivatives of phosphorus are of great importance in the chemistry and biochemistry of both elements. The incorporation of fluorine into biomolecules has frequently resulted in a remarkable change of biological properties. Simple phosphofluoridates and their structural analogs are the classical inhibitors of serine proteases (1a). It can be envisaged that a combination of nucleoside fragments with the phosphorofluoridate moiety could result in new properties with respect to selectivity of interaction with the active site of the serine hydroxy group. On the other hand, nucleotides containing a P—F linkage when incorporated into oligonucleotides could be used in the studies of biological functions of nucleic acids, including possibilities connected with the chirality of the phosphorofluoridate moiety. New phosphorofluoridate analogs of nucleotides could also be useful in studying the metabolic processes (1b) by noninvasive NMR imaging techniques taking advantage of 100% abundance of 19F nucleus and its high inherent NMR sensitivity.KeywordsFluoride AnionPhosphorus CenterTrimethylsilyl EsterFluoro DerivativeSnake Venom PhosphodiesteraseThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Water penetration in nylon 6,6: Absorption, desorption, and exchange studied by NMR microscopy

AbstractThe uptake of water by nylon 6,6 [42DB Adipure (trade name of Dupont Canada Inc.)] at 100°C has been monitored by a combination of one‐dimensional proton NMR spectroscopy, relaxation time (T1 and T2) measurements and proton microscopic NMR imaging techniques. The relaxation times of the water absorbed into the nylon matrix are very short at room temperature, (T2 < 1 ms and T1 ≈ 1 s) indicating that the water is located in a highly restricted environment and suggesting that strong interactions exist between the absorbed water and the polymer. The diffusion profiles measured at room temperature indicate that the diffusion of water into nylon 6,6 at 100°C is Case I Fickian diffusion. The spatial dependence of the T2 relaxation time constant and its variation with the water content was also examined. The results reveal that both T2 and T2* decrease toward the center of the sample in samples that have a concentration gradient of sorbed water. In fully saturated samples, no spatial dependence was observed. The overall values of T2 and T2* are also observed to increase as a function of exposure time. An evaluation of the desorption process at room temperature and at 100°C was performed. A continuous, exponentially decreasing solvent profile was observed for the desorption process which again indicates Case I Fickian kinetics. The exchange process of external bulk and atmospheric water with deuterium oxide (D2O) saturated nylon rods has also been studied using the microscopic imaging technique. © 1993 John Wiley & Sons, Inc.

NMR Fresnel transform imaging technique using a quadratic nonlinear field gradient

A new NMR imaging technique using a quadratic nonlinear field gradient is described. The center position of the nonlinear field gradient where the field intensity is minimum is moved spatially at a fixed amount in each imaging pulse sequence to produce nonlinear phase encoding as a function of position. A NMR image is reconstructed from the collected data by the computer data processing. The principle of this technique and many experimental results using a low field imaging system operating at 0.0192 T are presented. Theoretical and experimental comparison of the S/N of the image in this technique with that of the conventional Fourier imaging technique are made and the results show that both techniques have almost the same S/N. Moreover, the theoretical and experimental results show that this technique has a feasibility of a spatially localized imaging without using a conventional localization technique such as the use of the selective excitation in the presence of a linear field gradient.