A new technology could help researchers better understand subsurface phenomena.
NIKOLAUS NESTLE, THOMAS BAUMANN, AND REINHARD NIESSNER
Imaging Science nce the exclusive tool of medical practitioners, advances in magnetic resonance imaging (MRI) equipment and techniques are increasingly opening up opportunities for technological applications in the geological and environmental sciences. MRI’s noninvasive nature and its potential for generating images mapping a range of different parameters such as flow velocities or content of paramagnetic materials offers interesting possibilities for novel applications, especially studies of subsurface processes. Most MRI environmental science studies reported to date are essentially feasibility studies. Nevertheless, progress is occurring at such a rate that over the next few years, MRI should become a versatile tool, especially for three-dimensional (3-D) visualization of processes involving nonaqueous-phase liquids (NAPLs) in soils and aquifer environments, enabling realistic, in situ studies of remediation techniques in the laboratory. The technology is poised to become an important tool for studying the adsorption of dissolved materials and filtration of colloidal substances in subsurface matrixes. In this article, we explore the current and potential future applications of MRI in environmental science, focusing on subsurface processes such as the transport and dynamics of water, dissolved materials, and NAPLs in soils and sediments. For background, the basic principles (1–8) of MRI are reviewed in the sidebar on pages 158A–159A.
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Developments and applications First demonstrated in the 1970s (9, 10), MRI evolved into a major technique for noninvasive medical diagnostics in the 1980s (11). During the 1990s, MRI’s sensitivity increased significantly, leading to the introduction of magnetic resonance microscopy, with spatial resolutions of a few micrometers (1, 12, 13). Most early MRI work concerned medical and biological research, but there are numerous, recent materials science and engineering applications (2). The experimental challenges of studying environ© 2002 American Chemical Society
mental processes in living macroscopic organisms are comparable to general MRI studies of biological specimens. Recent investigations that reveal the difficulties and complexities of these studies include an investigation of the effect of toxic substances in rats (14), use of MRI for studying plant specimens (15, 16), and a study of plant tissue changes caused by environmental poisoning (17). MRI also has been used to study heavy metal ion exchange in algal biomass—see Figure 1 and Supporting Information (http://pubs.acs.org/est) (18). MRI subsurface environmental studies typically involve analyzing soil, unconsolidated sediment, aquitard materials (low-permeability materials that slow down the flow of groundwater), and fractured rock that has a high mineral content. Compared with typical biological and medical samples, the nuclear magnetic resonance (NMR) properties of such materials are much more complex. For example, liquid and mobile phases (which lead to a detectable MRI signal in standard MRI while spins in rigid solid phases do not) make up only about 25% of the sample volume compared with >90% in most biological and medical samples. Thus, the signal-to-noise ratio at the same spatial resolution is much lower in environmental samples. Moreover, spin relaxation times in the mobile phase tend to be much shorter than in standard medical MRI. This poses a problem because sample imaging requires more intensive magnetic field gradients, which must be switched in shorter times and require more sophisticated gradient control hardware than do medical applications. In addition, spin relaxation (which is responsible for the image contrast in many MRI methods and limits the signal intensity available at given echo and repetition times) tends to become strongly nonexponential in many environmental materials due to such factors as wide pore size distribution in soils and sediments and local variations in the content of paramagnetic materials. The nonexponential relaxation can lead to images that are not representative of the whole fluid content in the sample; in certain soil samples, the detected NMR signal can APRIL 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1
Heavy metal exchange in algal biomass This sequence of pictures shows nuclear magnetic resonance (NMR) microimaging of copper biosorption in strips of a Laminaria japonica frond: (a) fresh sample; (b) after ~90 min; and (c) after ~5-h exposure to a flowing solution containing 1 mM Cu2+ (see reference (18) for details). The direction of the solution flow is perpendicular to the image plane. Because of the high flow velocities in (a) and (b), the flowing water in the center of the image is dark. When image (c) was taken, the flow velocities in the biosorbent column were lower because of some shrinking of the frond strips. At low copper saturations, the frond tissue signal intensity increases—see (a) and (b), and with increasing copper saturation, the transverse relaxation time of frond strips decreases strongly, and the fronds become dark as in (c). In this latter image, the frond skin and central region exhibit shorter relaxation times—probably due to higher copper binding—than the tissue in between. Note the inhomogeneous intrusion profile of the copper ions into the frond biomass due to an uneven flow distribution and the frond anatomy. The bright bar corresponds to 1 mm. These images are part of an NMR movie, which is provided in the Supporting Information along with another movie on rare earth ion binding in algal tissue. a
b
be as small as 0.2% of the overall liquid content (19). Therefore, to avoid misinterpreting MRI results, knowledge of the relaxation properties of individual sample components is required. Studies of spin relaxation for a specific type of sample can be performed on much simpler equipment than MRI, as no gradient pulses are needed to do this. Qualitative rules-ofthumb for relaxation in porous systems are given in the NMR text sidebar. Another concern is that spatially varying concentrations of paramagnetic materials and local magnetic susceptibility variations in samples may cause severe NMR image distortions and strong signal losses due to self-diffusion of the pore liquid in the internal magnetic field gradients. The latter problem is partially resolved by using a spectrometer that allows fast switching of strong gradient pulses to keep the time intervals during which the spin magnetization is sensitive to field inhomogeneities as short as possible. Observing water flow in environmental MRI is a greater experimental challenge than in medical MRI. Compared with human blood flow of several centimeters per second and faster, typical flow velocities of water in sediments can be ~100 µm/s or less. Therefore, measuring flow phenomena under natural environmental conditions requires a combination of long time intervals for spin manipulation and strong gradient pulses. Unfortunately, the time available for spin manipulation is limited by the longitudinal relaxation time (T1) of the water. Again, reducing the times for spin manipulation requires stronger gradient pulses, but this option is limited for technical and cost reasons. Furthermore, discriminating between flow and diffusive movement in environmental samples becomes 156 A
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increasingly difficult, as the flow and the diffusive displacements of water fall within the same range (1, 20). Finally, in certain cases, the maximum sample size suitable for an MRI experiment is significantly smaller than the representative elementary volume of the sample material of interest, which, as a rule of thumb, is about 10 times greater than the largest grain sizes in the medium. On the other hand, the spatial resolution possible at this sample size may not be sufficient to directly explore pore-scale phenomena. Thus, as for other laboratory-scale measurement problems, the question of upscaling and downscaling of MRI experiments must be considered. Despite these difficulties and limitations, successful MRI studies of the properties of environmentally relevant sediment materials and model systems—for example, the propagation of dissolved paramagnetic ions such as Ni2+ (21) and Cu2+ (22) through sandy aquifer materials—have been reported. The solutionphase ion concentration in the experiments cited was roughly several millimolar, which is rather high compared with typical levels in the environment or in waste waters. However, lower concentrations of dissolved paramagnetic ions typically fail to provide reliable relaxation time contrast in the NMR images. The situation is different if chemical interactions (such as adsorption) occur between the flowing water and the column medium. In this case, the enriched ion concentrations in the column medium lead to considerable relaxation time contrast, even for ion concentrations of 1 mM or less in the solution (18). Knowledge of the behavior of NAPL phases in sediments is important for predicting potential hazards due to the infiltration of NAPLs from broken fuel and
solvent tanks or from contaminated industrial or military sites, and for planning remediation measures. The migration of oil through estuarine sediment has been studied (5), as have coal tar wastes both alone and in a sediment matrix (23). In the latter study, the contrasting possibilities between water and partially halogenated hydrocarbons were explored. The dissolution behavior of NAPLs by water flowing through columns has been studied on a glass bead model sediment (24), and several groups are working on similar studies in more realistic sediments. NMR imaging of NAPLs in environmental samples differs from experiments with NAPLs in water in two respects: NAPL relaxation times in most matrixes are much longer than those in water, and the spin density—the product of the number of NMR-active nuclei in the molecule and the number density of molecules in the liquid—is significantly different for some environmentally relevant classes of NAPLs (for example, halogenated (23), aromatic, and nitro solvents; see Table 1). Image contrast can be increased by doping either the NAPL or water phase with an MRI tracer (for example, a paramagnetic salt); by using special imaging sequences providing an appropriate relaxation time or diffusion contrast; and through the use of fluorinated NAPLs (for example, hexafluorobenzene), which can be imaged using fluorine MRI and thus discriminated from the water phase, which in turn can be studied independently using proton MRI. An important question concerning image contrasts due to doping is whether and how the possible contrast is affected in mixtures of small droplets of the two liquid phases. In nonimaging test experiments, we observed no notable relaxation time reductions for both polar and nonpolar NAPLs when they were mixed in a fine sand matrix together with a 0.1 M Cr(NO3)3 solution. This indicates that the suppression of the water signal by paramagnetic doping does not affect the NAPL signals even for fine droplets. Although long NAPL relaxation times are favorable for detecting these nonaqueous liquids, even under unfavorable matrix conditions, the long spin–lattice relaxation times, especially with aromatic or halogenated NAPLs (T1 > 10 s), can lead to quite long measuring times. Despite this limitation, MRI is one of the few options available for in situ studies of such liquids in sediments. Studies of water transport in unsaturated sediment are relevant both to environmental and agricultural investigations. However, as previously stated, signal losses in soils due to short relaxation times reduce the signal-to-noise ratio and may lead to water signals that are not representative of the overall water, making mapping of the water content of soil specimens with standard MRI protocols difficult (19). In unsaturated sandy sediments, conditions are more favorable for MRI studies than in soils because relaxation times for low water contents are not so strongly reduced. MRI has been used in model systems to study water transfer between soil and active plant roots (25). In actual soil materials, the main focus of MRI work has been to study water infiltration into partially dried soil material (26). One possible approach for overcoming relaxation time filtering effects in natural soil
materials is to use specialized imaging techniques that allow NMR image acquisition at very short echo times with only minor relaxation time filtering. In MRI studies of NAPL or water transport, the NMR signal originates directly from the liquid phase of interest. Other transport processes can be studied indirectly via their influence on the relaxation properties of the pore liquid. This option enables study of solution−matrix interactions such as adsorption or filtration. Studies of ion adsorption from the solution phase to the matrix can be performed for paramagnetic ions. Filtration of colloidal particles in the sediment also can be observed for many nonmagnetic mineral particles. The filtered particles lead to an increase in the surface of the sediment matrix, which in turn leads to a decrease in the relaxation times that can be exploited for the image contrast. Qualitative mapping of filtrate concentrations can be performed without special calibration, but a quantitative analysis requires additional studies of the relaxation time dependence of the filtrate of interest in its respective matrix. Suspended nonmagnetic colloids have only a minor influence on the relaxation time of the pore water, so that the method is selective to actually filtered material. Organic colloids, such as microorganisms or colloidal dead biomass, do not lead to TA B L E 1
Solvent proton densities and relative NMR signal amplitudes Although most aliphatic hydrocarbons exhibit proton densities similar to those in water, values for halogenated (blue), nitro (yellow), and aromatic (gray) solvents are significantly lower. Most of these solvents have relevance as soil contaminants, and the strong signal intensity difference may be used as an NMR contrast to monitor the distribution of the respective phases in sediments. Liquid
Proton density (mol/L)
Hexadecane Octanol Water Cyclohexane Triolein Ethanol Methyl tert-butyl ether Methanol Tetrahydrofuran Mesitylene Dimethyl sulfoxide Ethyl acetate Acetone Toluene Benzene 3-Nitrotoluene Nitromethane Chlorobenzene Trichlorobenzene Trichloromethane Tribromomethane Trichlorethylene
116.061 114.017 111.111 110.932 106.882 105.492 100.737 98.752 98.405 86.356 84.475 81.716 81.715 75.537 67.588 58.954 55.537 49.129 23.974 12.397 11.414 11.142
NMR intensity relative to water 1.045 1.026 1.000 0.998 0.962 0.949 0.907 0.889 0.886 0.777 0.760 0.735 0.735 0.680 0.608 0.531 0.500 0.442 0.216 0.112 0.103 0.100
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NMR and MRI basics The physical basis of NMR methods is the Zeeman splitting of the energy levels of nuclear spins in parallel or antiparallel orientation to an externally applied magnetic field B0. This energy splitting ∆E is proportional to the magnetic field: ∆E =
γh B = hν 2π 0
(1)
where γ is the gyromagnetic ratio of the respective nucleus, h is Planck’s constant, and ν is the resonance frequency. Nuclei such as 1H, 19F, and 31P, which have a high natural abundance and reasonably high γ values, are of greatest interest for use in NMR imaging experiments. Other important nuclei, such as 13C, occur in lower concentrations and need to be isotopically enriched for imaging experiments; alternatively, indirect detection schemes may be used (1). Most MRI is performed using 1H, which has both the highest magnetic moment of all stable nuclei and is present in high concentrations in natural environments. Furthermore, the 1H nucleus has only two possible spin orientations in an external magnetic field, simplifying MRI experiments considerably. In proton NMR, the energy splitting of protons as a result of a magnetic field is 0.17 µeV in a field of 1 tesla (T); for comparison, Earth’s magnetic field is ~100 µT, and the highest constant magnetic fields available in special labs are ~40 T. This proton energy splitting is small compared with the 25 meV of room temperature, thermal noise energy. Thus, the population difference between the different spin energy levels at thermal equilibrium is in the parts-per-million range. As a result, spectrometer electronics having a wide dynamic range are needed to produce well-defined, high-power radio fre-
FIGURE 1
quency pulses (up to 1 kW of power) for exciting the NMR signal and to detect the spin magnetization signals in the nanowatt range. The proportionality between magnetic field and resonance frequency in NMR can be used for spatially selective excitation of spins in a magnetic field gradient and for computing the spatial localization of the nuclei from a resonance signal acquired in the presence of a magnetic field gradient. The combination of these options makes MRI feasible. As an example, a typical pulse sequence used for twodimensional image sections through extended objects is shown in Figure 1. The slice is selected by signal excitation in the presence of a slice gradient. Spatial localization in one of the two in-plane dimensions is provided by reading out the NMR signal in the presence of a magnetic field gradient (the read gradient). The two dashed gradient pulses in the slice and read directions are needed for technical reasons in order to compensate for the unwanted side effects of the other gradients. The other in-plane dimension is provided by multiple runs of the sequence in which the phase gradient applied between excitation and signal readout is set to different values. This phase gradient imposes a localization-dependent phase pattern onto the NMR signal, which is determined from the acquired NMR signal along with the frequency distribution resulting from the read gradient. The actual spatial image can be reconstructed using a two-dimensional Fourier transformation of the frequency and phase data set provided by the signals acquired for different values of the phase gradient. Figure 2 provides an example of the Fourier space image and the actual real-space image obtained on a column packed with different sediments. The number of phase gradient steps needed for producing an image depends on the image size in points. For example, 128 gradient steps are needed for an image of 128 × 128 points without special data processing. Because of the Fourier image computation in MRI, internal struc-
FIGURE 2
Spin-echo MRI sequence
Fourier space data and real-space MRI image
A standard spin-echo MRI sequence used for two-dimensional imaging of sections through an object uses slice, read, and phase gradients (refer to the sidebar text for discussion). Usually, the gradients are applied in orthogonal spatial directions. The values of the slice and read gradients are not changed between the different runs of the sequence. The phase gradient, by contrast, is systematically incremented to cover the whole Fourier space.
The images on the left side show (a) a Fourier space signal (absolute values, logarithmic scaling) and (b) a real-space NMR image after fast Fourier transformation. Images (a) and (b) represent a slice through part of a water-saturated sediment column (c) packed coaxially with coarse (~1-mm grain size) and fine (~0.4mm grain size) sand. The lower signal intensity in the fine sand layer is due to the shorter transverse relaxation time. Additional MRI data obtained on the column are provided in Supporting Information (http://pubs.acs.org/est).
Echo time, te RF, signal
Repetition time, tR
Slice gradient
Medium sand Inner: fine sand a Outer: fine sand
Read gradient
Coarse sand Gravel
Phase gradient
c b
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tural changes during the acquisition of an image may lead to image artifacts. In MRI on samples undergoing temporal change, one should therefore make sure that the image is acquired in a time interval during which the changes are small. In routine clinical applications, it is possible to obtain two-dimensional slice images with a resolution in the millimeter range in less than 1 s, and three-dimensional images can be obtained in about 1 min. If the signal intensity is lower or one is interested in obtaining images at a high resolution in the submillimeter range, the acquisition of a single high-quality two-dimensional image might take 1 min or even up to 1 h under unfavorable conditions. Unlike the X-ray absorption coefficients or other material constants used in other noninvasive techniques, the NMR signal intensity S(x,y) originating from a given voxel (a three-dimensional volume ele( ment) in the sample depends on the choice of parameters in the applied imaging sequence. For the basic imaging sequence in Figure 1, the signal is t te )] exp(– )[1–exp(– R T2(x,y) T1(x,y) S(x,y) = S0(x,y)sin(ϑ ) (2) tR 1–cos(ϑ )exp(– ) T1(x,y) where ϑ is the excitation angle, S0 is the signal intensity extrapolated to no relaxation (usually called spin density and proportional to the number of excitable spins in the voxel), te is the echo time, tR is the repetition time of the pulse sequence, and T1 and T2 are the longitudinal and transverse relaxation times, respectively. Even when the spin density is approximately uniform over the sample, that is, the water content of the sample is uniform throughout the cross-sectional slice, there may be strong relaxation time variations, allowing discrimination among different sample components. T1 typically is long for rigid
FIGURE 3
Relaxation time variations Reciprocal NMR longitudinal (T1) and transverse (T2) relaxation times can vary widely, depending on the nature of the sample being probed. In this example, the content of a paramagnetic model colloid (iron hydroxide particles, BASF Sicotrans L1916) in a series of water-saturated quartz-sand samples was varied. The field strength used is 0.5 T. Note how, initially, T2 rapidly becomes shorter (1/T2 increases) with added pigment. This effect is due to the action of internal magnetic field gradients.
1/T1 1/T2
350
References
1/T2, 10/T1 (1/s)
300 250 200 150 100 50 0 0
0.5
solid materials and for free, clean liquids (T1 > 1 s), but has much smaller values for molecules contained in porous systems, in soft matter, and in the presence of dissolved or surface-accessible paramagnetic substances (millisecond range, see Figure 3). T2 is extremely short in rigid solids (T2 50 ms—it is hard to image local differences in the chemical shift and coupling in environmental science applications.
1
1.5
2
2.5
3
(1) Kunze, C.; Kimmich, R. Magn. Reson. Imag. 1994, 12, 805–810. (2) Chudek, J. A.; Reeves, A. D. Biodegradation 1998, 9, 443–449. (3) Callaghan, P. T. Principles of Magnetic Resonance Microscopy; Clarendon Press: Oxford, U.K., 1991. (4) Blümich, B. NMR Imaging of Materials; Clarendon Press: Oxford, U.K., 2000. (5) Pope, J. M.; Yao, S. Concepts Magn. Reson. 1993, 5, 281–302. (6) Stallmach, F.; Kärger, J. Adsorption 1999, 5, 117–133. (7) McCarthy, M. J.; Zion, B.; Chen, P.; Ablett S.; Darke, A. H.; Lillford, P. J. J. Sci. Food Agric. 1995, 67, 13–20. (8) Cotts, R. M.; Hoch, M. J. R.; Sun, T.; Markert, J. T. J. Magn. Reson. 1989, 83, 252–266.
weight % Sicotrans
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similarly strong changes in the relaxation behavior of the pore water. However, it is possible to tag materials of biological origin with magnetically marked antibodies or by incorporating paramagnetic ions in the microorganisms and thereby establish an MRI contrast.
Technological innovation Aquitard materials are used as mineral barriers at waste disposal sites and in unsaturated soil, where the heterogeneity of the flow pathways influences contaminant propagation. The major challenge with studying aquitard materials by MRI is overcoming their relatively short relaxation times, which limit the time available for spin manipulation to encode spatial information by the MRI sequence. Specialized MRI techniques with short echo times such as STRAFI (STRAy Field Imaging) (27, 28) or SPI (Single Point Imaging) are promising approaches. Using such techniques, a spatial resolution of 150 µm has been achieved for a soil sample with an iron content of 2% (28). SPRITE, a recently developed fast variant of SPI (29), allows reasonably fast 3-D imaging of materials with transverse relaxation times as short as 100 µs and with a spatial resolution on the order of 100 µm—performance using various samples, ranging from hydrated cement with transverse relaxation times of about 50 µs and a longitudinal relaxation time of 500 µs to pharmaceutical drug delivery systems with a similarly short transverse relaxation time, but a 100-ms longitudinal relaxation time, has been reported (30). Although using medical equipment is an option for initiating MRI environmental science studies, dedicated imaging equipment will probably be the option of choice in the future for several reasons. The increasing signal losses due to diffusion in the internal magnetic field gradients that are present in many environmental matrixes tend to cannibalize the signal intensity gain provided by the high magnetic fields of 1.5 tesla (T) or more, which for medical MRIs are now standard. An MRI system operating at a lower external magnetic field strength (typically 0.05–0.5 T) is less sensitive to those artifacts. Furthermore, low-field MRI allows direct comparison with field data from NML (nuclear magnetic resonance well-logging) tools (31). However, low-flow velocities in the environment create a need to use stronger magnetic field gradients of at least several hundred milliteslas/meter (mT/m), which is more than those available in common clinical MRI systems (40 mT/m or less). Working with stronger magnetic field gradients often imposes additional restrictions on sample sizes compared with standard MRI methods, and the electrical power required for generating sufficiently strong magnetic field gradients for studying larger samples becomes unrealistically high when using conventional equipment. This is especially the case with SPRITE and with sequences for mapping small flow velocities or small self-diffusion coefficients. For the same reason, other specialized MRI protocols such as STRAFI or SPI cannot be run on clinical MRI hardware without costly modifications. In summary, for MRI to become routinely available as a tool for environmental science studies, a developmental pathway similar to that which ultimately led 160 A
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to maturation of medical MRI will have to be followed. This will involve development of easy-to-use, robust MRI hardware and imaging sequences for addressing specific environmental questions and correlating NMR data with the results of other noninvasive measurements. Case studies of distinct sample systems, systemic improvement of MRI, and specific calibration of MRI pulse sequences can contribute to this objective.
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Nikolaus Nestle is a postdoctoral researcher, Thomas Baumann is a lecturer, and Reinhard Niessner is a full professor and the director at the Technical University of Munich, Institute for Hydrochemistry, in Germany (nikolaus.nestle@ ch.tum.de).
