A new technology

could help researchers

better understand

subsurface phenomena.

N I K O L A U S N E S T L E ,

T H O M A S B A U M A N N ,

A N D R E I N H A R D N I E S S N E R

© 2002 American Chemical Society APRIL 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY � 155 A

Once the exclusive tool of medical prac-titioners, advances in magnetic reso-nance imaging (MRI) equipment andtechniques are increasingly opening upopportunities for technological appli-

cations in the geological and environmental sciences.MRI’s noninvasive nature and its potential for gener-ating images mapping a range of different parame-ters such as flow velocities or content of paramagneticmaterials offers interesting possibilities for novel ap-plications, especially studies of subsurface processes.

Most MRI environmental science studies report-ed to date are essentially feasibility studies. Never-theless, progress is occurring at such a rate that overthe next few years, MRI should become a versatiletool, especially for three-dimensional (3-D) visual-ization of processes involving nonaqueous-phaseliquids (NAPLs) in soils and aquifer environments,enabling realistic, in situ studies of remediation tech-niques in the laboratory. The technology is poised tobecome an important tool for studying the adsorp-tion of dissolved materials and filtration of colloidalsubstances in subsurface matrixes.

In this article, we explore the current and poten-tial future applications of MRI in environmental sci-ence, focusing on subsurface processes such as thetransport and dynamics of water, dissolved materials,and NAPLs in soils and sediments. For background,the basic principles (1–8) of MRI are reviewed in thesidebar on pages 158A–159A.

Developments and applicationsFirst demonstrated in the 1970s (9, 10), MRI evolvedinto a major technique for noninvasive medical di-agnostics in the 1980s (11). During the 1990s, MRI’ssensitivity increased significantly, leading to the in-troduction of magnetic resonance microscopy, withspatial resolutions of a few micrometers (1, 12, 13).Most early MRI work concerned medical and biolog-ical research, but there are numerous, recent mate-rials science and engineering applications (2).

The experimental challenges of studying environ-

mental processes in living macroscopic organismsare comparable to general MRI studies of biologicalspecimens. Recent investigations that reveal the dif-ficulties and complexities of these studies include aninvestigation 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 byenvironmental poisoning (17). MRI also has beenused to study heavy metal ion exchange in algal bio-mass—see Figure 1 and Supporting Information(http://pubs.acs.org/est) (18).

MRI subsurface environmental studies typicallyinvolve analyzing soil, unconsolidated sediment, aqui-tard materials (low-permeability materials that slowdown the flow of groundwater), and fractured rockthat has a high mineral content. Compared with typ-ical biological and medical samples, the nuclear mag-netic resonance (NMR) properties of such materialsare much more complex. For example, liquid and mo-bile phases (which lead to a detectable MRI signal instandard MRI while spins in rigid solid phases do not)make up only about 25% of the sample volume com-pared with >90% in most biological and medical sam-ples. Thus, the signal-to-noise ratio at the same spatialresolution is much lower in environmental samples.Moreover, spin relaxation times in the mobile phasetend to be much shorter than in standard medicalMRI. This poses a problem because sample imagingrequires more intensive magnetic field gradients,which must be switched in shorter times and requiremore sophisticated gradient control hardware than domedical applications. In addition, spin relaxation(which is responsible for the image contrast in manyMRI methods and limits the signal intensity availableat given echo and repetition times) tends to becomestrongly nonexponential in many environmental ma-terials due to such factors as wide pore size distribu-tion in soils and sediments and local variations in thecontent of paramagnetic materials. The nonexpo-nential relaxation can lead to images that are not rep-resentative of the whole fluid content in the sample;in certain soil samples, the detected NMR signal canDI

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ImagingScienceImagingScience

156 A � ENVIRONMENTAL SCIENCE & TECHNOLOGY / APRIL 1, 2002

be as small as 0.2% of the overall liquid content (19).Therefore, to avoid misinterpreting MRI results, know-ledge of the relaxation properties of individual sam-ple components is required. Studies of spin relaxationfor a specific type of sample can be performed onmuch simpler equipment than MRI, as no gradientpulses are needed to do this. Qualitative rules-of-thumb for relaxation in porous systems are given inthe NMR text sidebar.

Another concern is that spatially varying concen-trations of paramagnetic materials and local magnet-ic susceptibility variations in samplesmay cause severeNMR image distortions and strong signal losses due toself-diffusionof thepore liquid in the internalmagneticfield gradients. The latter problem is partially resolvedby using a spectrometer that allows fast switching ofstrong gradient pulses to keep the time intervals dur-ing which the spin magnetization is sensitive to fieldinhomogeneities as short as possible.

Observing water flow in environmental MRI is agreater experimental challenge than in medical MRI.Compared with human blood flow of several cen-timeters per second and faster, typical flow velocitiesof water in sediments can be ~100 µm/s or less. There-fore, measuring flow phenomena under natural en-vironmental conditions requires a combination oflong time intervals for spin manipulation and stronggradient pulses. Unfortunately, the time available forspin manipulation is limited by the longitudinal relax-ation time (T1) of the water. Again, reducing the timesfor spin manipulation requires stronger gradient puls-es, but this option is limited for technical and cost rea-sons. Furthermore, discriminating between flow anddiffusivemovement inenvironmental samplesbecomes

increasingly difficult, as the flow and the diffusive dis-placements of water fall within the same range (1, 20).

Finally, in certain cases, the maximum sample sizesuitable for an MRI experiment is significantly small-er than the representative elementary volume of thesample material of interest, which, as a rule of thumb,is about 10 times greater than the largest grain sizesin the medium. On the other hand, the spatial reso-lution possible at this sample size may not be suffi-cient to directly explore pore-scale phenomena.Thus,as for other laboratory-scale measurement problems,the question of upscaling and downscaling of MRIexperiments must be considered.

Despite these difficulties and limitations, success-ful MRI studies of the properties of environmentallyrelevant sediment materials and model systems—forexample, the propagation of dissolved paramagneticions such as Ni2+ (21) and Cu2+ (22) through sandyaquifer materials—have been reported. The solution-phase ion concentration in the experiments cited wasroughly several millimolar, which is rather high com-pared with typical levels in the environment or inwaste waters. However, lower concentrations of dis-solved paramagnetic ions typically fail to provide re-liable relaxation time contrast in the NMR images.The situation is different if chemical interactions (suchas adsorption) occur between the flowing water andthe column medium. In this case, the enriched ionconcentrations in the column medium lead to con-siderable relaxation time contrast, even for ion con-centrations of 1 mM or less in the solution (18).

Knowledge of the behavior of NAPL phases in sed-iments is important for predicting potential hazardsdue to the infiltration of NAPLs from broken fuel and

Heavy metal exchange in algal biomassThis sequence of pictures shows nuclear magnetic resonance (NMR) microimaging of copper biosorption in stripsof a Laminaria japonica frond: (a) fresh sample; (b) after ~90 min; and (c) after ~5-h exposure to a flowing solutioncontaining 1 mM Cu2+ (see reference (18 ) for details). The direction of the solution flow is perpendicular to the imageplane. Because of the high flow velocities in (a) and (b), the flowing water in the center of the image is dark. Whenimage (c) was taken, the flow velocities in the biosorbent column were lower because of some shrinking of the frondstrips. At low copper saturations, the frond tissue signal intensity increases—see (a) and (b), and with increasingcopper saturation, the transverse relaxation time of frond strips decreases strongly, and the fronds become darkas in (c). In this latter image, the frond skin and central region exhibit shorter relaxation times—probably due tohigher copper binding—than the tissue in between. Note the inhomogeneous intrusion profile of the copper ionsinto the frond biomass due to an uneven flow distribution and the frond anatomy. The bright bar corresponds to1 mm. These images are part of an NMR movie, which is provided in the Supporting Information along with anothermovie on rare earth ion binding in algal tissue.

FIGURE 1

a b c

APRIL 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY � 157 A

solvent tanks or from contaminated industrial or mil-itary sites, and for planning remediation measures.The migration of oil through estuarine sediment hasbeen studied (5), as have coal tar wastes both aloneand in a sediment matrix (23). In the latter study, thecontrasting possibilities between water and partiallyhalogenated hydrocarbons were explored. The dis-solution behavior of NAPLs by water flowing throughcolumns has been studied on a glass bead model sed-iment (24), and several groups are working on simi-lar studies in more realistic sediments.

NMR imaging of NAPLs in environmental samplesdiffers from experiments with NAPLs in water in tworespects: NAPL relaxation times in most matrixes aremuch longer than those in water, and the spin densi-ty—the product of the number of NMR-active nucleiin the molecule and the number density of moleculesin the liquid—is significantly different for some en-vironmentally relevant classes of NAPLs (for example,halogenated (23), aromatic, and nitro solvents; seeTable 1). Image contrast can be increased by dopingeither the NAPL or water phase with an MRI tracer(for example, a paramagnetic salt); by using specialimaging sequences providing an appropriate relax-ation time or diffusion contrast; and through the useof fluorinated NAPLs (for example, hexafluoroben-zene), which can be imaged using fluorine MRI andthus discriminated from the water phase, which inturn can be studied independently using proton MRI.

An important question concerning image contrastsdue to doping is whether and how the possible con-trast is affected in mixtures of small droplets of thetwo liquid phases. In nonimaging test experiments,we observed no notable relaxation time reductions forboth polar andnonpolarNAPLswhen theyweremixedin a fine sand matrix together with a 0.1 M Cr(NO3)3solution. This indicates that the suppression of thewater signal by paramagnetic doping does not affectthe NAPL signals even for fine droplets. Although longNAPL relaxation times are favorable for detectingthese nonaqueous liquids, even under unfavorablematrix conditions, the long spin–lattice relaxationtimes, 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 optionsavailable for in situ studies of such liquids in sediments.

Studies of water transport in unsaturated sedimentare relevant both to environmental and agriculturalinvestigations. However, as previously stated, signallosses in soils due to short relaxation times reducethe signal-to-noise ratio and may lead to water sig-nals that are not representative of the overall water,making mapping of the water content of soil speci-mens with standard MRI protocols difficult (19). Inunsaturated sandy sediments, conditions are morefavorable for MRI studies than in soils because re-laxation times for lowwater contents are not so strong-ly reduced.

MRI has been used in model systems to studywater transfer between soil and active plant roots (25).In actual soil materials, the main focus of MRI workhas been to study water infiltration into partially driedsoil material (26). One possible approach for over-coming relaxation time filtering effects in natural soil

materials is to use specialized imaging techniquesthat allow NMR image acquisition at very short echotimes with only minor relaxation time filtering.

In MRI studies of NAPL or water transport, theNMR signal originates directly from the liquid phaseof interest. Other transport processes can be studiedindirectly via their influence on the relaxation prop-erties of the pore liquid. This option enables study ofsolution−matrix interactions such as adsorption orfiltration. Studies of ion adsorption from the solutionphase to the matrix can be performed for paramag-netic ions. Filtration of colloidal particles in the sed-iment also can be observed for many nonmagneticmineral particles. The filtered particles lead to an in-crease in the surface of the sediment matrix, whichin turn leads to a decrease in the relaxation times thatcan be exploited for the image contrast. Qualitativemapping of filtrate concentrations can be performedwithout special calibration, but a quantitative analy-sis requires additional studies of the relaxation timedependence of the filtrate of interest in its respectivematrix. Suspended nonmagnetic colloids have only aminor influence on the relaxation time of the porewater, so that the method is selective to actually fil-tered material. Organic colloids, such as microor-ganisms or colloidal dead biomass, do not lead to

TA B L E 1

Solvent proton densities and relative NMR signalamplitudesAlthough most aliphatic hydrocarbons exhibit proton densities similar tothose in water, values for halogenated (blue), nitro (yellow), and aromatic(gray) solvents are significantly lower. Most of these solvents have rele-vance as soil contaminants, and the strong signal intensity differencemay be used as an NMR contrast to monitor the distribution of the re-spective phases in sediments.

Proton density NMR intensityLiquid (mol/L) relative to water

Hexadecane 116.061 1.045Octanol 114.017 1.026Water 111.111 1.000Cyclohexane 110.932 0.998Triolein 106.882 0.962Ethanol 105.492 0.949Methyl tert-butyl ether 100.737 0.907Methanol 98.752 0.889Tetrahydrofuran 98.405 0.886Mesitylene 86.356 0.777Dimethyl sulfoxide 84.475 0.760Ethyl acetate 81.716 0.735Acetone 81.715 0.735Toluene 75.537 0.680Benzene 67.588 0.6083-Nitrotoluene 58.954 0.531Nitromethane 55.537 0.500Chlorobenzene 49.129 0.442Trichlorobenzene 23.974 0.216Trichloromethane 12.397 0.112Tribromomethane 11.414 0.103Trichlorethylene 11.142 0.100

158 A � ENVIRONMENTAL SCIENCE & TECHNOLOGY / APRIL 1, 2002

The physical basis of NMR methods is the Zeeman splitting of the en-ergy levels of nuclear spins in parallel or antiparallel orientation to anexternally applied magnetic field B0. This energy splitting ∆E is pro-portional to the magnetic field:

where γ is the gyromagnetic ratio of the respective nucleus, h isPlanck’s constant, and ν is the resonance frequency. Nuclei such as1H, 19F, and 31P, which have a high natural abundance and reasonablyhigh γ values, are of greatest interest for use in NMR imaging experi-ments. Other important nuclei, such as 13C, occur in lower concentra-tions 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 mag-

netic moment of all stable nuclei and is present in high concentra-tions in natural environments. Furthermore, the 1H nucleus has onlytwo possible spin orientations in an external magnetic field, simplify-ing 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 magneticfields available in special labs are ~40 T. This proton energy splittingis small compared with the 25 meV of room temperature, thermalnoise energy. Thus, the population difference between the differentspin energy levels at thermal equilibrium is in the parts-per-millionrange. As a result, spectrometer electronics having a wide dynamicrange are needed to produce well-defined, high-power radio fre-

quency pulses (up to 1 kW of power) for exciting the NMR signal andto detect the spin magnetization signals in the nanowatt range.The proportionality between magnetic field and resonance fre-

quency in NMR can be used for spatially selective excitation of spinsin a magnetic field gradient and for computing the spatial localizationof the nuclei from a resonance signal acquired in the presence of amagnetic field gradient. The combination of these options makes MRIfeasible. As an example, a typical pulse sequence used for two-dimensional image sections through extended objects is shown inFigure 1. The slice is selected by signal excitation in the presence ofa slice gradient. Spatial localization in one of the two in-plane dimen-sions is provided by reading out the NMR signal in the presence of amagnetic field gradient (the read gradient). The two dashed gradientpulses in the slice and read directions are needed for technical rea-sons in order to compensate for the unwanted side effects of theother gradients. The other in-plane dimension is provided by multipleruns of the sequence in which the phase gradient applied betweenexcitation and signal readout is set to different values. This phasegradient imposes a localization-dependent phase pattern onto theNMR signal, which is determined from the acquired NMR signalalong with the frequency distribution resulting from the read gradient.The actual spatial image can be reconstructed using a two-dimen-sional Fourier transformation of the frequency and phase data setprovided by the signals acquired for different values of the phase gra-dient. Figure 2 provides an example of the Fourier space image andthe actual real-space image obtained on a column packed with dif-ferent sediments. The number of phase gradient steps needed forproducing an image depends on the image size in points. For exam-ple, 128 gradient steps are needed for an image of 128 × 128 pointswithout special data processing.Because of the Fourier image computation in MRI, internal struc-

∆E =γhπ

B0 = hν2(1)

NMR and MRI basics

Spin-echo MRI sequenceA standard spin-echo MRI sequence used for two-dimensionalimaging of sections through an object uses slice, read, and phasegradients (refer to the sidebar text for discussion). Usually, thegradients are applied in orthogonal spatial directions. The valuesof the slice and read gradients are not changed between the differentruns of the sequence. The phase gradient, by contrast, issystematically incremented to cover the whole Fourier space.

Echo time, te

Repetition time, tR

RF,signal

FIGURE 1

Phasegradient

Readgradient

Slicegradient a

b

c

Fourier space data and real-space MRI imageThe images on the left side show (a) a Fourier space signal(absolute values, logarithmic scaling) and (b) a real-space NMRimage after fast Fourier transformation. Images (a) and (b) representa slice through part of a water-saturated sediment column (c)packed coaxially with coarse (~1-mm grain size) and fine (~0.4-mm grain size) sand. The lower signal intensity in the fine sandlayer is due to the shorter transverse relaxation time. AdditionalMRI data obtained on the column are provided in SupportingInformation (http://pubs.acs.org/est).

FIGURE 2

Medium sand

Inner: fine sand

Outer: fine sand

Coarse sand

Gravel

APRIL 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY � 159 A

tural changes during the acquisition of an image may lead to imageartifacts. In MRI on samples undergoing temporal change, one shouldtherefore make sure that the image is acquired in a time interval dur-ing which the changes are small.In routine clinical applications, it is possible to obtain two-dimen-

sional slice images with a resolution in the millimeter range in lessthan 1 s, and three-dimensional images can be obtained in about1 min. If the signal intensity is lower or one is interested in obtainingimages at a high resolution in the submillimeter range, the acquisitionof a single high-quality two-dimensional image might take 1 min oreven up to 1 h under unfavorable conditions.Unlike the X-ray absorption coefficients or other material con-

stants used in other noninvasive techniques, the NMR signal intensityS(x,y) originating from a given voxel (a three-dimensional volume ele-ment) in the sample depends on the choice of parameters in the ap-plied imaging sequence. For the basic imaging sequence in Figure 1,the signal is

where ϑ is the excitation angle, S0 is the signal intensity extrapolatedto no relaxation (usually called spin density and proportional to thenumber of excitable spins in the voxel), te is the echo time, tR is therepetition time of the pulse sequence, and T1 and T2 are the longitudi-nal and transverse relaxation times, respectively. Even when the spindensity is approximately uniform over the sample, that is, the watercontent of the sample is uniform throughout the cross-sectional slice,there may be strong relaxation time variations, allowing discrimina-tion among different sample components. T1 typically is long for rigid

solid materials and for free, clean liquids (T1 > 1 s), but has muchsmaller values for molecules contained in porous systems, in softmatter, and in the presence of dissolved or surface-accessible para-magnetic substances (millisecond range, see Figure 3). T2 is extreme-ly short in rigid solids (T2 << 100 µs) and cannot be detected withstandard MRI techniques. Its range increases to a few to severalhundred milliseconds in porous materials and soft matter and can be-come as long as T1 in clean, free liquids.Relaxation times in porous systems are much shorter than in the

free liquid for systems in which the liquid phase strongly adsorbs tothe surface, for example, with water in mineral matrixes or in hy-drophilic polysaccharides. Furthermore, the relaxation times, whichdepend on the pore size, typically decrease with decreasing poresize. For nonpolar liquids in mineral matrixes, much smaller changesin the relaxation behavior are observed in porous systems, helping todistinguish between water and NAPLs in environmental samples (2).In addition to relaxation time contrasts, the detected signal intensi-

ty can be weighted according to the flow and diffusion behavior ofthe molecules in the sample by applying additional gradient pulses tothe sample. These pulses impose a position-dependent phase shift tothe precessing spin magnetization, which is compensated for in staticmaterial but leads to a phase shift in coherent flow motion (3–5) andto signal attenuation in incoherent diffusive motion (3, 4, 6). The appli-cation of the gradient pulses for measuring flow or diffusion requiresa sufficiently long time, during which the transverse magnetizationcan be manipulated. Therefore, such methods are only applicable forsamples with a sufficiently long T2 of at least 10 ms (relaxation timesof 50 ms or more allow better velocity resolution). There is an inversecorrelation between the expected phase shift of the molecules andthe required duration and intensity of the gradient pulses. Additionalcomplexity arises from internal magnetic field gradients, caused bymagnetic impurities present in most samples of geologic origin orvariations of the susceptibility, such as air bubbles, for example, inplant tissues. Magnetic field heterogeneities within the sample causesignal attenuation, which is hard to eliminate (7). Furthermore, the re-sulting values for diffusion or velocity are also distorted. Some ofthese distortions can be compensated for using specialized NMR se-quences, which again increase the demand on MRI hardware perfor-mance (8).In principle, chemical information can be encoded in NMR images

(3, 4) based on local differences in the chemical shift and coupling.As these imaging sequences are even more sensitive to internalmagnetic field gradients and to short relaxation times—conditions forresolving a chemical shift difference of 1 ppm require a magneticfield homogeneity within the sample better than 1 ppm, and T2 > 50ms—it is hard to image local differences in the chemical shift andcoupling in environmental science applications.

References(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.

S(x,y) =T2(x,y) T1(x,y)

exp(– )te [1–exp(– )]

tR

tR

T1(x,y)

ϑS0(x,y)sin( )ϑ1–cos( )exp(– )

(

(2)

350

300

250

200

150

100

50

0

1/T 2

,10/

T 1(1

/s)

1/T11/T2

0 0.5 1 1.5 2 2.5 3

weight % Sicotrans

Relaxation time variationsReciprocal NMR longitudinal (T1) and transverse (T2) relaxationtimes can vary widely, depending on the nature of the samplebeing probed. In this example, the content of a paramagneticmodel colloid (iron hydroxide particles, BASF Sicotrans L1916) ina series of water-saturated quartz-sand samples was varied. Thefield strength used is 0.5 T. Note how, initially, T2 rapidly becomesshorter (1/T2 increases) with added pigment. This effect is dueto the action of internal magnetic field gradients.

FIGURE 3

similarly strong changes in the relaxation behaviorof the pore water. However, it is possible to tag ma-terials of biological origin with magnetically markedantibodies or by incorporating paramagnetic ions inthe microorganisms and thereby establish an MRIcontrast.

Technological innovationAquitard materials are used as mineral barriers atwaste disposal sites and in unsaturated soil, where theheterogeneity of the flow pathways influences conta-minant propagation.The major challenge with study-ing aquitard materials by MRI is overcoming theirrelatively short relaxation times, which limit the timeavailable for spin manipulation to encode spatial in-formation by the MRI sequence. Specialized MRI tech-niques with short echo times such as STRAFI (STRAyField Imaging) (27, 28) or SPI (Single Point Imaging)are promising approaches. Using such techniques, aspatial resolution of 150 µm has been achieved for asoil sample with an iron content of 2% (28). SPRITE,a recently developed fast variant of SPI (29), allowsreasonably fast 3-D imaging of materials with trans-verse relaxation times as short as 100 µs and with aspatial resolution on the order of 100 µm—perfor-mance using various samples, ranging from hydratedcement with transverse relaxation times of about 50 µsand a longitudinal relaxation time of 500 µs to phar-maceutical drug delivery systemswith a similarly shorttransverse relaxation time, but a 100-ms longitudinalrelaxation time, has been reported (30).

Althoughusingmedical equipment is an option forinitiating MRI environmental science studies, dedi-cated imaging equipment will probably be the optionof choice in the future for several reasons. The in-creasing signal losses due to diffusion in the internalmagnetic field gradients that are present in many en-vironmental matrixes tend to cannibalize the signalintensity gain provided by the high magnetic fields of1.5 tesla (T) or more, which for medical MRIs are nowstandard. An MRI system operating at a lower externalmagnetic field strength (typically 0.05–0.5T) is less sen-sitive to those artifacts. Furthermore, low-field MRI al-lows direct comparison with field data from NML(nuclear magnetic resonance well-logging) tools (31).However, low-flow velocities in the environment cre-ate a need to use stronger magnetic field gradients ofat least several hundred milliteslas/meter (mT/m),which is more than those available in common clini-cal MRI systems (40 mT/m or less). Working withstronger magnetic field gradients often imposes addi-tional restrictions on sample sizes comparedwith stan-dard MRI methods, and the electrical power requiredfor generating sufficiently strong magnetic field gradi-ents for studying larger samples becomes unrealisti-cally high when using conventional equipment. Thisis especially the case with SPRITE and with sequencesformapping small flowvelocities or small self-diffusioncoefficients. For the same reason, other specializedMRI protocols such as STRAFI or SPI cannot be run onclinical MRI hardware without costly modifications.

In summary, for MRI to become routinely availableas a tool for environmental science studies, a devel-opmental pathway similar to that which ultimately led

to maturation of medical MRI will have to be followed.This will involve development of easy-to-use, robustMRI hardware and imaging sequences for addressingspecific environmental questions and correlatingNMRdata with the results of other noninvasive measure-ments. Case studies of distinct sample systems, sys-temic improvement of MRI, and specific calibration ofMRI pulse sequences can contribute to this objective.

References(1) Callaghan, P. T. Principles of Magnetic Resonance Mi-

croscopy; Clarendon Press: Oxford, U.K., 1991.(2) Blümich, B. NMR Imaging of Materials; Clarendon Press:

Oxford, U.K., 2000.(3) Kunze, C.; Kimmich, R. Magn. Reson. Imag. 1994, 12,

805–810.(4) Kumar, A.; Welti, D.; Ernst, R. R. J. Magn. Reson. 1975, 18,

69–83.(5) Chudek, J. A.; Reeves, A.D.Biodegradation1998, 9, 443–449.(6) Pope, J. M.; Yao, S. Concepts Magn. Res. 1993, 5, 281–302.(7) Stallmach, F.; Kärger, J. Adsorption 1999, 5, 117–133.(8) McCarthy, M. J.; Zion, B.; Chen, P.; Ablett S.; Darke, A. H.;

Lillford P. J. J. Sci. Food Agric. 1995, 67, 13–20.(9) Damadian, R.; Minkoff, L.; Goldsmith, M.; Standford, M.;

Koutcher, J. Science 1976, 194, 1430–1432.(10) Lauterbur, P. C. Nature 1973, 242, 190–191.(11) Vlaardingerbroek, M.T.; de Boer, J. A.Magnetic Resonance

Imaging; Springer: Heidelberg, Germany, 1999.(12) Dhenain, M.; Ruffins, S. W.; Jacobs, R. E. Dev. Biol. 2001,

232, 458–470.(13) Lee, S. C.; Kim, K.; Kim, J.; Lee, S.; Yi, J. H.; Kim, S. W.; Ha,

K. S.; Cheong, C. J. Magn. Reson. 2001, 150, 207–213.(14) Kinoshita,Y.; Ohnishi, A.; Kohshi, K.;Yota, A. Environ.Res.

A 1999, 80, 348–354.(15) Chudek, J. A.; Hunter, G. Prog. Nucl. Magn. Reson. Spec-

trosc. 1997, 31, 43–62.(16) Ishida, N.; Koizumi, M.; Kano, H. Annal. Bot. 2000, 86,

259–278.(17) Campbell, T. A. J. Plant Nutr. 1999, 22, 827–834.(18) Nestle, N.; Kimmich, R. Biotechnol. Bioeng. 1996, 51,

538–543.(19) Hall L. D.; Amin, M. H. G.; Dougherty, E.; Sanda, M.;

Vortubova, J.; Richards, K. S.; Chorley, R. J.; Cislerova, M.Geoderma 1997, 80, 431–448.

(20) Baumann,T.; Petsch, R.; Niessner, R. Environ. Sci. Technol.2000, 34, 4242–4248.

(21) Pearl, Z.; Magaritz, M.; Bendel, P. J.Magn.Reson.1991,95,597–602.

(22) Oswald, S.; Kinzelbach, W.; Greiner, A; Brix, G. Geoderma1997, 80, 417–429.

(23) Pervizpour, M.; Pamukcu, A.; Moo-Young, H. J. Comput.Civ. Eng. 1999, 13, 96–102.

(24) Johns, M. L.; Gladden, L. F. J. Coll. Interf. Sci. 1999, 210,261–270.

(25) MacFall, J. S.; Johnson, G. A.; Kramer, P. J. Proc.Natl. Acad.Sci. U.S.A. 1990, 87, 1203–1207.

(26) Amin, M. H. G.; Hall, L. D.; Chorley, R. J.; Richards, K. S.Prog. Phys. Geog. 1998, 22, 135–165.

(27) Samoilenko, A. A.; Artemov, D. Y.; Sibeldina, L. A. JETPLett. (Engl. Transl.) 1988, 47, 417–419.

(28) Randall, E.W.; Mathieu, N.; Ivanova, G. I.Geoderma 1997,80, 307–325.

(29) Balcom, B. J.; MacGregor, R. P.; Beyea, S. D.; Green, D. P.;Armstrong, R. L.; Bremner, T. W. J. Magn. Reson., Ser. A1996, 123, 131–134.

(30) Szomolanyi, P.; Goodyear, D.; Balcom, B.; Matheson, D.Magn. Reson. Imag. 2001, 19, 423–428.

(31) Kleinberg, R. In Encyclopedia of NMR;Grant, D. M., Harris,R. K., Eds.; Wiley: New York, 1996; p. 4960.

Nikolaus Nestle is a postdoctoral researcher, Thomas Bau-mann is a lecturer, and Reinhard Niessner is a full profes-sor and the director at the Technical University of Munich,Institute for Hydrochemistry, in Germany (nikolaus.nestle@ch.tum.de).

160 A � ENVIRONMENTAL SCIENCE & TECHNOLOGY / APRIL 1, 2002