Spatially resolved nuclear magnetic resonance studies of planar samples

Progress in Nuclear Magnetic Resonance Spectroscopy 48 (2006) 161–181 www.elsevier.com/locate/pnmrs

Spatially resolved nuclear magnetic resonance studies of planar samples J. Mitchell a, P. Blu¨mler b, P.J. McDonald a,* a

School of Electronics and Physical Sciences, University of Surrey, Guildford, Surrey GU2 7XH, UK b ICG-III, Phytosphere, Forschungszentrum Ju¨lich, 52425 Ju¨lich, Germany Received 7 March 2006 Available online 6 June 2006

Keywords: Planar samples; NMR; STRAFI; GARField; NMR-MOUSE

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Stray-field imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Resolution and gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Gradient designs—the GARField magnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Unilateral devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. NMR-MOUSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Surface-GARField . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Signal-to-noise ratio (SNR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Accessible NMR parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Measurement techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Transverse (T2) relaxation time measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. The Hahn echo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Quadrature echoes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3. CPMG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Longitudinal relaxation time measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. T1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. T1r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Double quantum filtered signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Thin films and film formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1. Curing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2. Swelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3. Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Cross-link density and curing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2. Profiling of polymer products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3. Degradation and aging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4. Solvent ingress and swelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.5. Stress and strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

* Corresponding author. Tel.: C44 1483 68 6800; fax: C44 1483 68 6781. E-mail address: [email protected] (P.J. McDonald).

0079-6565/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.pnmrs.2006.04.001

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4.3.

5.

Building materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Cement hydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. Surface treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3. Cultural heritage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.4. Water ingress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.5. Wood as a building material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Paper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Bio-systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1. Tendons (in vivo) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2. Skin (in vitro and in vivo) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3. Vessel walls (in vivo) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Although we are now celebrating 60 years of Nuclear Magnetic Resonance (NMR) success (1946–2006), the topic of this review—spatially resolved NMR studies of planar samples—is a fairly recent development that extends back little more than a decade. Conventional Magnetic Resonance Imaging (MRI) has been developing since 1973 [1,2], predominantly in the field of medical research and diagnosis where three-dimensional anatomical images are now routinely obtained. The images are required to show a high level of image contrast between different tissue types but only more rarely are accurate quantitative measurements of the parameters (relaxation times, etc.) characterising the tissue useful. In materials science studies, there is often less intrinsic value in obtaining three-dimensional images. Rather one seeks to quantitatively measure properties of the sample with sufficient spatial localisation so as to characterise either the sample or a sample-process. To that end, it is often only generally necessary to conduct an experiment in one-dimension. This review focuses on an even smaller class of materials systems for which one-dimensional profiling is invariably entirely adequate: planar samples. In this context, the term ‘planar samples’ is taken to include both layered materials such as paints, and buried planes beneath the surface of larger bodies, such as cement blocks. A number of different magnet designs have been constructed specifically for use in such studies and they have one property in common: a spatially varying magnetic field to provide resolution in one-dimension. Strong, stable magnetic field gradients suitable for planar imaging are most readily associated with the stray-fields of magnets, whether super-conducting coils, electromagnets, or permanent magnetic material. Profiling is generally achieved in one of two distinctly different ways: (a) thin samples such as films and coatings can be imaged in single-shot Fourier transform measurements, as in conventional MRI, or (b) a ‘sweet-spot’ within an inhomogeneous magnetic field is used to target a sensitive plane in a sample and the plane is stepped through the sample to provide a profile. The latter technique can also be applied to arbitrarily shaped or large samples by using unilateral magnets to target spatially localised near-surface planes. Again, profiles into these surfaces can be achieved by

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moving the sensitive plane relative to the sample and acquiring signal from the volume-slice at each depth. In some respects profiling rather than imaging simplifies the experimentation, although this is offset by the fact that most samples of interest are solid, semi-solid, or solid-like (that is, characterised by a short signal lifetime, T2
), unlike the soft tissues studied in medical imaging. The broad linewidths (and hence short life-times) of NMR signals associated with solids makes the imaging process far more difficult. In the later 1980s, early 1990s, STRAy Field Imaging (STRAFI) [3] was one of the first techniques to routinely and reliably provide profiles and images of solids. The stable, high magnetic field gradient found in the fringe fields of conventional super-conducting NMR spectroscopy or imaging magnets is sufficient to overcome the broad linewidths of the solid materials. This gradient is far greater than could be reliably generated inside an imaging magnet using pulsed field gradient technology at that time. STRAFI typically allowed samples to be profiled with a resolution on the order of a few micrometers to a few tens of micrometers. STRAFI was (and is) used as a materials characterisation tool to study samples ranging from soils to dental resins (see reviews in Refs. [4,5]). However, its application to planar samples has been limited. The term ‘planar samples’ encompasses a wide range of materials including paints, polymer films, and human skin. In recent years, advances in NMR equipment and magnet designs have provided the opportunity to explore these thin samples. The curvature in the magnetic flux across the region of interest in a STRAFI experiment limits the maximum obtainable resolution. For this reason, a new permanent magnet was designed to provide high-resolution profiles of planar samples [6]. This magnet, later named GARField (gradient at right-angles to the field), utilises the concepts of STRAFI and improves upon them: curved pole pieces on the magnets provide lines of magnetic flux with constant magnitude (but not direction) parallel to the sensitive plane with an orthogonal gradient in the field strength. GARField has been used to provide profiles through planar samples such as paints [6] and skin [7] with a resolution as high as 5 mm. A second GARField magnet has also been developed [8] to allow the profiling of larger samples and also in vivo imaging of human skin [9]. Elsewhere, GARField designs have been implemented by adapting an

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existing electromagnet [10]. A discussion of the GARField magnets can be found in Ref. [11]. Another area we will discuss in this review is the measurement of NMR signals from planar volume-slices near to the surface of large samples. Profiling of samples either too bulky (e.g. building materials [12]) or unsuitable (e.g. ferromagnetic materials [13]) to be placed in a conventional MRI system offers a different challenge that has been met by unilateral NMR magnets. One of the first and most successful of these was the NMR-MOUSE (mobile universal surface explorer) [14], originally designed to conduct in situ investigations of rubber products (e.g. car tyres). The NMR-MOUSE is invariably constructed from permanent magnetic material, although it has passed through a series of incarnations from its inception as a pair of semi-cylindrical magnets [14]. These include the bar-magnet NMR-MOUSE [15] and the most common variation: the U-shaped NMR-MOUSE [16]. The NMR-MOUSE is most suited to the study of polymer materials such as natural rubber [17] due to the ideal combination of long transverse relaxation times and slow molecular diffusion that they exhibit. More recently, the applications of the NMRMOUSE have been diversified to include paper in historical documents [18] and frescos in ancient monuments [19]. As well as depth profiling [20,21], the NMR-MOUSE has even been used for near-surface imaging [22] and three-dimensional imaging [23]. Recently, microscopic resolution has been obtained with the NMR-MOUSE by mounting the device on a mobile platform, akin to STRAFI, except that the magnet is now moved relative to the sample [24] rather than vice versa. This NMR-MOUSE has already been used to fulfil the roles of STRAFI and GARField in profiling solvent ingress into polymers, cultural heritage paintings and in vivo skin measurements [25]. It has also now been demonstrated that spectroscopic information can be obtained from a unilateral device [26]. The unilateral approach of the NMR-MOUSE and the welldefined gradients of GARField have been combined by Marble et al. [27]. By placing shaped pole pieces on permanent magnetic blocks the size and shape of the traditionally irregular sensitive region of a NMR-MOUSE-like device can be controlled. A similar concept being developed is the SurfaceGARField magnet [28] for specifically profiling moisture content in the top 50 mm of cement and concrete structures in the built environment. This design involves moving the magnet relative to the surface of the sample in order to step the sensitive volume-slice through the material. The SurfaceGARField has also been shown to provide analysis of cementbased materials beyond the basic profiles [29]. Using twodimensional relaxation correlation analysis [30] additional information can be obtained on the surface interactions and pore structure in cements [31]. A major use of unilateral magnets, which has seen both considerable instrumentation and technique innovations is the field of oil-well logging by NMR. Specially designed magnets are lowered into bore holes to provide structural information on the rocks, and the oil and water contained therein. In several areas, oil-well logging concepts pre-date analogous

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developments for other planar samples. However, it is an extensive topic in its own right and we will not elaborate further on oil-well logging here. There are already review publications covering this subject: see Ref. [32] and other articles in the same volume for more information. 2. Instrumentation In this section, we describe magnets that have been specially designed for NMR studies of planar samples or of a planar slice though a large sample. Of course such geometries can also be studied with more standard MRI equipment, but the high planar (typically only one-dimensional) resolution makes designs that utilise a strong spatial variation of the magnetic field though the sample very favourable. Furthermore, the designs must be distinguished between magnets that surround the sample and unilateral or single-sided magnet set-ups, which detect NMR signals from regions relatively close to the surface of a potentially very large sample. 2.1. Stray-field imaging A method which comprises of most of these aspects is the acquisition of NMR signals in the stray-field of commercial super-conducting magnets, therefore, nicknamed STRAFI (stray-field imaging) [3,4]. As originally envisaged, the method consists of obtaining spatially localised NMR signals. A finite radio frequency pulse has insufficient frequency bandwidth, Dup, to excite nuclei across the full length of a sample in a large gradient [33]. Rather nuclei are in a slice—the STRAFI plane, Fig. 1(a)—of thickness Dr are selectively excited. The

Fig. 1. Schematic representation of the STRAFI experiment showing: (a) field of a super-conducting coil and STRAFI plane. Shown are flux lines, and magnitude, jB0j, as shades of grey; (b) orientation of the magnetic fields and gradients relative to the sample and gravity.

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spatial resolution is given by

Dr Z

Dup gG

(1)

where g is the gyromagnetic ratio of the nucleus being investigated and G is the gradient of the magnetic field in the direction parallel to the main polarizing field, B0 (typically the z-direction, with values of GZ50–100 T mK1 [5]). The method was intended for solid-state imaging due to the fact that the spatial variation of the magnetic field was strong enough to dominate the broad lines in solids, DuL, which impose an ultimate limit on the resolution never achieved in practice. A profile may be built up by mechanically stepping the sample through the sensitive slice and acquiring data from each location in turn. Since most materials science applications only require one-dimensional profiles, the time-consuming mechanical scanning can be kept to a minimum. However, three-dimensional data acquisition and back-projection of profiles is possible if samples are additionally rotated [34] although relatively few applications for this imaging technique have been devised. Increasingly interest focused on planar samples for which Dr as defined in Eq. (1) exceeds the sample thickness. In such cases, it is possible to acquire an echo in the normal fashion with G as the read gradient during data acquisition and follow this by the Fourier transform of the data to provide the profile through the sample layer. The method became known as FT-STRAFI [35,36]. As an aside, many of these samples are soft solid or liquids in solids (e.g. porous media). Since they flow, the orientation of gravity is important, see Fig. 1(b). The drawback of this technique is that it can be applied only to very thin samples because the excitation and detection bandwidth of the radio frequency (RF) pulses and spectrometer, respectively, are typically limited to a few mega Hertz even with modern solid-state NMR instrumentation. Although the STRAFI and, to a lesser extent, FT-STRAFI approaches are slow and need the construction of an accurate mechanical repositioning device, the high planar resolution together with the robustness and simplicity make these methods suitable for quantitative investigations in materials science. Since the resolution is independent of the linewidth, STRAFI can be equally applied to solids, semi-solids and liquids (within the limits of diffusion), allowing such studies as solid/liquid phase transitions on planar surfaces. STRAFI can therefore be seen as a way to utilise existing equipment for new experiments and is very suited for flat and thin samples, like films or planar surfaces. An interesting question is: what spatial resolution can be obtained in practice with this method? The inherent sensitivity problem of magnetic resonance techniques, particularly of small samples or poor fill factors, will be discussed later. For the moment, the focus should be on the properties of the gradient, which so far has only been discussed as a strong spatial variation of Bz.

2.1.1. Resolution and gradients Generally, the spatially varying part of the magnetic flux is described by the following tensor: 3 2 vBx vBy vBz 6 vx vx vx 7 7 6 7 6 6 vBx vBy vBz 7 7 6 (2) G Z 6 vy vy vy 7 7 6 7 6 6 vB vB vB 7 y z5 4 x vz vz vz This tensor can be superimposed on a main flux component, e.g. the strong magnetic flux component, B0, along the z-direction: BðrÞ Z B0 C G,r

(3)

If B0 is much stronger than the spatial extension of this tensor inside the sample, the limited bandwidth of the NMR experiment reduces this tensor to a gradient vector
 vBz vBz vBz ; ; (4) h ½Gx ;Gy ;Gz  h G; vx vy vz because, to a good approximation, the difference between the B field vector with and without this tensor is negligible. Preferably, the G-vector should have constant components that do not vary over the volume of interest, i.e. ðvBz ðx;y;zÞÞ=vxZ Gx Z constant. For the STRAFI method, this has the following consequence: the so-called STRAFI plane, the chosen xy-plane of the experiments in the stray-field (see Fig. 1(a)), does not necessarily coincide with the position where the gradient, Gz, is strongest, but where it is most homogeneous over the sample. Ideally this is vBz ðx;y;zS Þ vBz ðx;y;zS Þ Z Z0 vx vy

(5)

where zS denotes the position of the STRAFI plane. The symmetry of the field is determined by the super-conducting coil in this case and furthermore has to obey V !B Z 0

(6)

in the absence of further flux sources. Therefore, the condition in Eq. (5) is strictly fulfilled only in a single plane normal to the main field component; otherwise, the flux has a curvature over a finite volume [5]. Although the radius of this curvature is large (in the order of the radius of the super-conducting coil) its effect on the gradient homogeneity is not negligible and limits the resolution of the experiment. Nevertheless, typical resolutions on the order of a few micrometers are still obtainable. 2.2. Gradient designs—the GARField magnet In order to overcome the inherent problem of magnetic field curvature within the sample volume of a STRAFI experiment, a special magnet was designed for analysing planar samples with high spatial resolution. The approach [6] uses a scalar

J. Mitchell et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 48 (2006) 161–181

potential, f, defined by BZ Vf. To relax the constraints of the classical arrangement, the direction of the spatially resolving gradient and the polarizing field are no longer parallel and are ultimately perpendicular within the region of interest, as shown in Fig. 2(c). The potential can be defined by separate components for both directions, leading to a simple set of simultaneous and harmonic differential equations. For the given constrains, a particular solution is fðz;yÞ Z a sinðbzÞexpðKbyÞ

(7)

where a and b are constants. The magnitude of the field and its gradient in the y-direction are then given by jBj Z jVfj Z ab expðKbyÞ Gy Z

vjBj Z ab2 expðKbyÞ vy

and (8)

respectively, which are both independent of z and hence homogeneous over the entire sampled slice. Furthermore, following from Eq. (8), this design gives a fixed ratio of Gy =jBjZKb that no longer depends on any particular position. The position of the sample along the y-axis then not only defines the field but also the gradient strength and hence the maximum achievable resolution, see Eq. (1). The final values of jB0j and a will be determined by the magnetic materials used in the construction. The parameters b and w (where w is the pole separation) will define the shape of the pole-pieces placed

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on top of permanent magnets (see Fig. 2(b)) [6]. This design was later nick-named GARField (gradients at right angles to the field). The constructed device gives excellently resolved planar images even of larger films with a spatial resolution in the order of 5–10 mm. From the orientation of the static magnetic field, B0, and the static field gradient, Gy, it is possible to place a surface RF excitation/detector coil directly below the sample (see Fig. 2(c)). The detected NMR signal will provide a profile vertically through the sample, unlike in the earlier STRAFI measurements. This has the immediate advantage of allowing the measurements of liquids since gravity will act to hold them in place. The small diameter coils and coil orientation in these systems act to provide a higher sensitivity than can be achieved with larger solenoids that surround the sample. The original GARField was constructed using neodynium iron boron (NdFeB) permanent magnets and has a value of G/jB0jZ25 mK1 [6]. A proton resonance frequency of 30 MHz therefore corresponds to a gradient of 17.5 T mK1, defining a useful field of view of approximately 700 mm when using a 1.0 ms excitation pulse. Acquiring 256 points per echo with a sampling interval of 0.4 ms provides a resolution of 13 mm. The second generation GARField magnet [8,9] (nicknamed GARField-II, or the Open-GARField due to its more accessible sensitive plane) is 50% larger than the original design. It is also constructed on a C-frame rather than an H-frame, allowing access to the pole pieces from the sides. The larger pole separation defines a value of G/jB0jZ16.67 mK1 on the upper curvature of the poles and an alternative value of G/jB0jZ 33.33 mK1 on the lower curvature. This magnet design can therefore accommodate a greater range of samples and probe configurations (see Section 4). 2.3. Unilateral devices

Fig. 2. Schematic representation of the GARField design showing: (a) geometry of the permanent magnets and the special pole caps; (b) resulting field of a GARField magnet. Shown are flux lines, and magnitude, jB0j, as shades of grey [6]; (c) orientation of the magnetic fields and gradients relative to the sample and gravity.

2.3.1. NMR-MOUSE Since all the techniques described use strong inhomogeneities in the magnetic field to obtain spatial resolution sufficient for the study of planar samples, it was considered early on that the classical concept of a surrounding magnet can be waived [37], which then allows one to investigate a near-surface region of a large sample. Likewise it was found that the STRAFI plane can lie outside the Dewar on some magnet designs, which in principle allows the same experiments to be conducted on existing equipment although in many cases it would not be practical to bring the sample to the magnet or vice versa. The mobile universal surface explorer [14] (NMR-MOUSE) is probably one of the simplest and most robust NMR devices ever build. It was designed to allow single-sided NMR measurements to be conducted on surfaces of arbitrarily shaped, large samples. As illustrated in Fig. 3(a), the B0 field is generated by two anti-parallel polarized permanent magnets mounted on an iron yoke to increase the flux on the upper side. They produce an inhomogeneous magnetic field (jB0jZ0.4– 0.7 T at the surface, or 17–30 MHz RF frequency for 1H, depending on the material, size, and arrangement of the magnets) whose main component is parallel to the surface and

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2.3.2. Surface-GARField The Surface-GARField [28] is a novel unilateral magnet designed to target a large, uniform sensitive plane within an ‘infinite’ sample volume, specifically in the study of cement and concrete structures in the built environment [29] (see Section 4.3). The design of the magnet was drawn directly from a linear eddy-current brake [40] although some features have properties in common with the NMR-MOUSE and earlier GARField magnets. The magnet is based on an infinite series of repeating permanent magnetic blocks with alternating polarity along the z-direction, see Fig. 4 where the blocks are of thickness d, have magnetisation My, and repeat with an array period of lZ2pa. By carefully selecting the separation of the magnetic blocks all the even Fourier harmonics of the magnetisation as well as the third harmonic can be removed leaving (to pffiffiffi a good approximation) just the fundamental My;1 zð2 3=pÞMy . It can be shown that the magnetic field a small distance above the blocks is approximated by [29]: B Z m0 adMy;1 expðKayÞfcosðazÞj C sinðazÞkg: Fig. 3. Schematic representation of the original U-shaped NMR-MOUSE design showing: (a) geometry of the device including permanent magnets, RF-coil and yoke; (b) resulting field of a NMR-MOUSE. Shown are flux lines, and magnitude, jB0j, as shades of grey; (c) orientation of the magnetic fields and gradients relative to an arbitrarily large sample.

decays roughly with yK2 in the normal direction (see Fig. 3(b)). For the generation of the excitation field, a surface coil is placed in the gap between the magnets to produce a RF field, B1, with its main component normal to the surface. As a result, both the B 0 and the B 1 magnetic fields are strongly inhomogeneous, and only the region where they have significant perpendicular components contributes to the sensitive volume from which the NMR signal originates (see Fig. 3(c)). For the U-shaped NMR-MOUSE, these inhomogeneous fields define a complex three-dimensional curved sensitive volume. The decay of B0 normal to the surface can be used again for the investigation of planar samples. Field and gradient strength can principally be chosen by the carrier frequency of the RF excitation, but do not follow simple relations as in the GARField set-up. Therefore, several improvements for less inhomogeneous gradients have been suggested [38]. Profiling through the sample can be accomplished by retuning the transmission/detection circuit to several RF frequencies [39] and hence various positions along y. Alternatively the field can be swept by additional gradient coils [21]. However, both techniques are tedious and need careful calibration. The most promising technique is—like in STRAFI—the mechanical manipulation of the probe position relative to the sample. Although this method does not provide the maximum signalto-noise ratio (SNR) it is the most robust and reliable method with obtainable resolutions comparable to STRAFI and GARField [24].

(9)

This ensures that the modulus of the magnetic field, jB0j, is constant in a plane a distance y above the surface of the magnet. By applying the same theory to the RF coil (using current windings rather than permanent magnetic material) a constant jB1j can be achieved in the same plane. By offsetting the centre of the current windings from the magnetic blocks by a distance equal to l/4, the components of B0 and B1 become everywhere orthogonal. The Surface-GARField magnet as constructed consists of three NdFeB magnetic block segments and two complementary RF coil windings. The entire magnet/RF detector assembly is mounted on a mobile platform allowing the sensitive volumeslice to be moved relative to the surface of the sample. Like the other GARField magnets, this design also has the advantage of a constant G/jB0j ratio. In the specific constructed implementation G/jB0jZ38.46 mK1. By tuning the RF probe to a 1H frequency of 3.2 MHz the magnet targets a region 50 mm away from the surface of the detector over a plane some 130! 100 mm2. The curvature of the field edges defines a maximum resolution of 0.03 mm in samples much larger than the excited volume-slice.

Fig. 4. Schematic representation of the Surface-GARField design: alternating permanent magnets (N/S) provide a plane (dotted line) of magnetic field of constant magnitude within the sample region (shaded) from which the NMR signal originates. The RF sensor coils (grey bars) are situated immediately above the magnets. The RF field (grey lines) is everywhere orthogonal to the permanent magnetic flux lines (arrows).

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2.4. Signal-to-noise ratio (SNR) All of the designs described above do not use the maximum available magnetic field and due to the presence of strong gradients have to acquire very short-lived signals, which requires a broad receiver bandwidth. This reduces the SNR, so that typically only very sensitive and abundant nuclei, like 1H and 19F, can be investigated. Receiving short lived signals at low frequencies with short dead times requires coils with low quality factors, which further decreases the SNR [41]; and finally mechanical positioning might additionally slow down the measurement. In unilateral applications, the fact that the receiving coil is not surrounding the sample reduces the so-called ‘filling factor’ and thus the SNR even more. Since unilateral RF coils tend to receive/detect both above and below the plane of the coil the maximum theoretical ‘filling factor’ is immediately reduced by 50%, and experimentally often much less depending on the geometry of the equipment and sample. The effective excitation range of such a surface coil inside a sample is approximately given by its radius, and limits the penetration depth of single sided applications. Therefore, considerations of reducing noise or enhancing the signal are paramount in planning experiments. While passive shielding and optimisation of cable lengths is considered as a standard procedure, the transmit/receive circuits can profit enormously from separation, e.g. Q-switching [16], special ‘actively shielded’ RF coils [16] and optimised circuits [42]. Finally the mechanical mounting has to be addressed. The reproducible detection of thin slices of a few micrometers over minutes requires a set-up that reduces or compensates sufficiently for vibrations. While such vibrations have frequencies of a few Hertz, the RF component of the device has to be protected from self-induced vibrations in kilo Hertzto mega Hertz-regime due to acoustic ringing [16,43]. 2.5. Accessible NMR parameters All the described systems use static inhomogeneous magnetic fields to excite thin slices or measure highly resolved profiles. Therefore, direct NMR spectroscopy (detection of chemical shifts) is impossible without special modifications to the system. However, the measurement of relaxation times and transport phenomena like self-diffusion is, in principle, not hampered. On the other hand, the intrinsic inhomogeneity of the magnetic fields allows the investigation of objects with strong variations in magnetic susceptibility or even containing ferromagnetic parts (e.g. reinforcing steel in tyres, concrete, belts, etc.) because they impose small variations on the magnetic fields compared to the intrinsic field inhomogeneities. However, the inhomogeneous fields associated with the original design of the NMR-MOUSE cause intrinsic problems in accurate measurements of NMR parameters such as relaxation times [44]. The interference of the inhomogeneous B0 and B1 makes a quantitative interpretation of the measured signal a difficult task. Nevertheless, as long as similar samples

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are compared under otherwise identical conditions, the acquired data are quantitatively very reliable (see Section 4). The degree of B0 inhomogeneity can be somewhat weakened even outside the magnet by producing a saddle point or ‘sweet spot’ in the magnetic flux [27,45–47]. Such designs are very helpful when accurate relaxation measurements are required over a larger volume of the sample, a typical application being geophysical oil-well logging [48,49], or magnetic resonance imaging (MRI) [50,51]. Recently, it was also demonstrated that if the spatial inhomogeneities of B0 and B1 can be matched then even spectroscopy seems possible with unilateral devices [26,52,53]. 3. Measurement techniques The majority of measurements conducted with high gradient and unilateral devices involve little more than obtaining signal intensities that are in some way related to the proton density in a given region of the sample. These measurements usually extend to transverse relaxation time studies and occasionally to longitudinal relaxation times. It is also possible to perform diffusion measurements in the high gradients associated with these stray field techniques. There is now an extensive resource of literature discussing the merits and pitfalls of stray-field relaxation time measurements. Many pulse sequences have been devised to provide true T2 relaxation times from spin-echo trains recorded in high-gradient magnetic fields, as well as determining diffusion coefficients that are independent of relaxation rate. Much of this work has been conducted by the oil industry for use with their NMR oil-well logging tools. Notable publications include: improved CPMG sequences using composite pulses [54], an optimisation of the CPMG sequence [55], and restricted diffusion measurements in the stray field [56–58]. What follows is a brief description of the most basic NMR measurements commonly conducted in stray fields. For more in-depth information on echo shapes, effective relaxation times and data analysis in stray field experiments, consult Refs. [44,59,60]. 3.1. Transverse (T2) relaxation time measurements In stray field techniques, it is not usually possible to record a free induction decay (FID) [61] as the signal decays too rapidly due to phase interference in the presence of grossly inhomogeneous fields. Therefore, echo-techniques are normally used to refocus the spin magnetisation after a suitable delay to overcome experimental dead-times. Additionally, the application of multi-echo pulse sequences allows the measurement of the T2 decay from the echo intensities. In all cases, the echo envelope contains a spread of frequencies excited in the inhomogeneous B0 and B1 magnetic fields. To obtain the signal intensity, the echoes are integrated. The integral of the echo pertains to the on-resonance signal obtained from the centre of the sample volume-slice being investigated. Since these echoes also contain spatial information it is possible to Fourier transform them to provide a profile.

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3.1.1. The Hahn echo The basic spin echo, also called a Hahn echo [62] is obtained from the pulse sequence 908x KtK1808x KtKecho

(10)

By varying the time separation of the pulses, t, an estimate of the T2 relaxation time can be obtained by plotting the intensity of each echo. However, this method is extremely sensitive to diffusion attenuation and errors due to imperfect 1808 pulses. It should also be noted that in conventional Hahn echo experiments (conducted with homogeneous magnetic fields) the 1808 pulse is normally twice the length of the 908 pulse. If this technique were used in a stray field experiment, then the different bandwidths of the two pulses would excite a different volume of the sample. Instead, it is preferable to double the power of the 1808 RF pulse in respect to the 908 pulse. This, to a good approximation, maintains an equal excitation volume for each pulse. The main disadvantage of this method is that the requisite reduction in power for the 908 pulse (assuming ‘full power’ for the 1808 pulse) leads to longer pulses, which in turn excite narrower regions of the sample. Whilst in some cases this can be beneficial (experiments where each slice is measured individually and a high resolution is required, e.g. the profiling NMR-MOUSE [24]) it can be less advantageous when the RF pulse is required to excite the entire sample to obtain a single-shot profile (e.g. GARField [6]). In the latter cases, it is better to use quadrature echoes.

the 1808 pulses along Gy each subsequent echo will be unaffected by inaccuracies in the 1808 RF pulse length. Also by placing the echoes reasonably close together, some of the additional decay due to diffusion can be overcome. However, it is generally extremely difficult to measure bulk liquids with large self-diffusion coefficients in high magnetic field gradients. Additionally, as with quadrature echoes care must be taken not to unwittingly spin-lock the system by placing the pulses too close together. Consequently, the observed decay rate is normally specified as an effective transverse relaxation time, T2,eff. As with the Hahn echo sequence mentioned above, it is better to vary the power of the RF pulses and maintain a constant pulse length throughout in order to excite the same volume-slice during the course of the experiment. 3.2. Longitudinal relaxation time measurements 3.2.1. T1 The longitudinal relaxation time—the time required for excited spins to return to equilibrium with the static magnetic field—can be obtained either through inversion or saturation recovery experiments. The saturation recovery sequence is often preferable in stray field measurements since it automatically utilises RF pulses of equal bandwidth. The alternative inversion recovery pulse sequence has the form 1808x=y Kt1 K908x KFID;

(13)

whereas the saturation recovery pulse sequence has the form 3.1.2. Quadrature echoes The first STRAFI experiments [3] utilised a solid-echo sequence [63] to refocus the local inter-nuclear dipolar interactions of the protons (rather than refocusing the spins within the static magnetic field) allowing echoes to be obtained from short T2 materials. Most stray field measurements are no longer conducted on pure solids so it is more convenient to refer to these as quadrature echoes [4] since the majority of the refocused signal will arise simply from the fact that a pair of RF pulses are being applied in an inhomogeneous magnetic field. The pulse sequence 908x KtK½908y KtKechoKtKn

(11)

supplies a train of n echoes. This is also sometimes referred to as the Ostroff–Waugh sequence [64]. Dependent on t and the dipole interaction strength, the observed echo relaxation rate will be a function of both T2 and T1r due to spin locking (see Section 3.2.2). 3.1.3. CPMG To obtain echo trains in homogeneous magnetic fields it is now usual to implement the Carr–Purcell sequence [65] with the Meiboom–Gill modification (CPMG) [66]. This is similar to a series of n Hahn echoes, viz. 8 908x KtK½180Gy KtKechoKtKn

(12)

except that the 1808 pulses are applied aligned with the spins, rotating them around the y-axis. By alternating the direction of

908y Kt1 K908x KFID:

(14)

By recording the signal intensity at the start of the FID as a function of varying t1 times it is possible to follow the longitudinal relaxation. However, in stray field measurements it is not usually possible to measure a FID due to the short T2
decay times inherent to samples within the high magnetic field gradients. Instead, it is often necessary to replace the FID with an echo train acquisition, thus 908y Kt1 K908x KtK½908y KtKechoKtKn

(15)

so that the train of n echoes can be fitted and projected back to provide the signal intensity immediately after the second 908 pulse. 3.2.2. T1r The standard T1 measurement is sensitive to molecular motions in the mega Hertz region. In contrast, the T1r (T1 relaxation in the rotating frame) measurement is sensitive to molecular motions in the kilo Hertz region. Normally, the T1r measurement utilises a long spin-locking pulse in the B1 field (RF) followed by a FID measurement: 908x Kðspin locking pulseÞGy KFID:

(16)

As with the T1 measurement (above), it is advantageous to replace the FID measurement with an echo train when measuring T1r in stray field experiments. An alternative measurement technique is to apply a series of closely spaced

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RF pulses using either the quadrature echo sequence (11) or CPMG sequence (12) rather than applying a spin locking pulse. However, whilst this can be advantageous as it reduces the RF duty cycle, it can be difficult to know whether the spins are retaining coherence and the result can be a complex combination of T1r and T2. 3.3. Diffusion Stray field diffusion measurements offer the advantage of being able to observe molecular diffusion as slow as 10K11 cm2 sK1 [5]. In conventional pulsed field diffusion experiments [67], the applied gradient, G, can be varied in strength and duration to allow the self-diffusion coefficient, D, of the sample to be measured. However, this is not possible in stray field measurements and so the experimental timings must be altered in order to observe the diffusive attenuation of the signal. Unfortunately, this adds the complication of introducing variable signal attenuation due to T1 and T2 relaxation as well. The stimulated echo sequence [62] can be used to obtain both a direct (primary) and stimulated echo 908x Kt1 K908y Kt1 Kechoprimary Kt2 K908y Kt1 Kechostimulated ;

(17)

where the ratio of the two echo amplitudes will be independent of T2 but still dependent on D which is measured by varying t1. As long as t2 remains constant, the effects of T1 relaxation will be the same in each experiment [68]. More complex multipulse sequences have been proposed [69,70] to completely remove the relaxation time effects and a discussion of these can be found in Ref. [69]. It is also possible to rapidly obtain selfdiffusion coefficients in stray field experiments [71,72] by simply comparing the echo decay in two CPMG measurements with different t echo spacings. These can be used to provide diffusion-weighted profiles from STRAFI and GARField measurements [73]. 3.4. Double quantum filtered signals Multiple quantum coherence experiments have been shown to allow the measurement of residual dipolar couplings and dynamic order parameters in polymer systems (see [74] and references therein). Double quantum (DQ) build-up and decay curves of 1H in these materials can be related to even very weak dipolar couplings and hence to material properties such as cross-link density. Conventionally, the pulse sequence for a DQ experiment consists of equally matched excitation and re-conversion times. In homogeneous fields such experiments yield an initial increase in the DQ coherence, followed by a maximum and then a decay as the transverse relaxation becomes dominant over the pumping of the multiple quantum coherences. The normalised signal is then [74]   SDQ ðt 0 Þ 3  2  02 f 1K u D t sin4 q (18) 4 SDQ ð0Þ

Fig. 5. Pulse sequence used to generate mismatched excitation and re-conversion (MERE) in double quantum filtered experiments. This pulse sequence is suitable for use in inhomogeneous fields where the arbitrary flip angle, q, will be well-defined over each sample voxel. A schematic representation of the experimental timings is shown in (a) and the actual pulse sequence is shown in (b). Image reproduced from with permission from Ref. [74], copyright (2001), Elsevier.

where t 0 is the difference between the excitation and re-conversion periods in a mismatched excitation and re-conversion (MERE) sequence (see Fig. 5) [75,76], q is the flip angle of the RF pulses, and u D is the total residual dipolar coupling. Hence by plotting the normalised DQ signal against t 0 2, the average square of the residual dipolar couplings, hu 2D i, can be obtained. However, when using this method with unilateral devices such as the NMR-MOUSE, the broad distribution of RF frequencies results in a flip angle distribution that has to be taken into account for quantitative interpretation of the DQ signal [77]. Still, it has been shown that Eq. (18) describes the experimental data well enough to provide a qualitative comparison [74]. 4. Applications 4.1. Thin films and film formation 4.1.1. Curing Films and coatings are perhaps the most obvious planar samples to be analysed using NMR. The high magnetic field gradient present in STRAFI is essential to provide spatial information across these thin samples, if curing and transport properties within the films are to be determined. The GARField magnet has provided an easier alternative to the superconducting magnets conventionally used in STRAFI for the study of film formation since its inception in 1999 [6]. In the original GARField paper, the curing of an alkyd coating with a cobalt catalyst and solvent was observed through a series of relaxation time profiles. By fitting single exponential decays to sets of n consecutive quadrature echoes, the average T2 relaxation time of each pixel was obtained. The profiles had a resolution of 6.5 mm. The solvent evaporation was observed as a thinning of the coating, whilst the curing (increase in polymer cross-link density) was observed by a reduction in T2 relaxation time resulting in a reduction in signal intensity. GARField was used to study the photo-initiated crosslinking in the curing of a poly(vinyl acetate-co-ethylene) latex dispersion [78]. Significantly, the cross-linking process was seen to occur most rapidly at the centre of the coating, see Fig. 6. In these experiments, nZ32 echoes were acquired with

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Fig. 6. Profiles recorded across a latex coating as it cured exposed to light on the top surface (right side) obtained using a quadrature echo pulse sequence in the GARField magnet with a jB0jZ0.7 T and a corresponding field gradient strength of GZ17.5 T mK1 in the sensitive volume. The profiles were recorded after 10 m (dotted line) and after (top to bottom) 30, 60, and 90 m and 2, 3, 6 and 17 h. The 10 m profile had reduced signal intensity compared to the subsequent profile due to T1-weighting. Graph reproduced with permission from Ref. [78], copyright (2000), American Chemical Society.

a separation of 2tZ190 ms. By comparison with a numerical model the result was explained by a combination of light scattering in the turbid latex and the inhibition of the crosslinking reaction by oxygen (both initial molecular oxygen content and ingress of further oxygen from the atmosphere). This influence of oxygen ingress on cross-linking has been observed in solvent and water based alkyd coatings [10]. A further study [79] combined GARField profiling measurement with a quartz crystal microbalance with energy dissipation (QCM-D) in the investigation of cross-linking and drying in waterborne coatings (a latex and alkyd emulsion). Ouriadov et al. [80] successfully observed the polymerisation of a polyurethane coating and a polymer latex emulsion using a local surface coil probe in a regular MR imaging system, where the nominal resolution was about 10 mm. Recently, Erich et al. [81] have demonstrated a good similarity between GARField-like MRI and confocal Raman microscopy (CRM) measurements. The sensitive region of their NMR magnet had a jB0j of 1.4 T (obtained using

electromagnetic coils with curved pole pieces rather than permanent magnetic material) and a corresponding gradient strength of 36.4 T mK1. The measurement parameters provided an optimum resolution of 5 mm. Two alkyd emulsions (organic solvent based and water based) were studied. Fig. 7 shows the curing of the water based alkyd emulsion as an example. In each case, the CRM measured the disappearance of the double bonds of the unsaturated fatty acids, whereas the MRI observed the oxidative cross-linking that results in a decrease of molecular mobility and hence a change in the transverse relaxation time. This work therefore indicates that the disappearance of the double bonds in the fatty acids of the alkyd polymer is related to the formation of cross-links by these fatty acid side chains. 4.1.2. Swelling The GARField magnet has been used to study the behaviour of water in water-swollen cellophane films [73]. The diffusion coefficients were measured from the exponential decay of quadrature echo trains recorded using various t times. Here MR imaging was combined with spatially resolved diffusion measurements to show that the water self-diffusion is constant across the film thickness. Over most of the film thickness, the self-diffusion coefficient of water was found to be approximately 0.9!10K9 m2 sK1, indicating relatively unhindered movement. Slower diffusion was observed in the water in narrow regions near the cellophane surfaces. Due to resolution limitations the narrow regions could not be profiled clearly. The measured self-diffusion coefficient of 0.5!10K9 m2 sK1 is an average of the water in the surface regions and water in the main body of the film. The measurements suggested the presence of a denser, less porous region of cellulose at the film surfaces. STRAFI has also been used to investigate the dissolution interface between soap and water [82]. Whilst the stray field images were sufficient to identify the ingress of water into the surface of the soap as being a Fickian process over the first 4 h of exposure, double quantum filtered deuterium spectroscopy

Fig. 7. Profiles of a water based alkyd emulsion during curing. Profiles were recorded (left) by confocal Raman spectroscopy and (right) magnetic resonance imaging. The MRI profiles were obtained from quadrature echoes measured using a 1.4 T magnet (of a similar design to GARField) with a gradient of 36.4 T mK1. The individual profiles were recorded after 4, 30, 96, 150, 198, and 270 h in each graph, in the direction indicated by the arrows. Graphs reproduced with permission from Ref. [81], copyright (2005), Elsevier.

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was required to map the water and mesophase distribution over the first 120 h of exposure. 4.1.3. Drying The drying of thin films, as well as the curing, can also be monitored using MRI profiling. Hughes et al. [83] performed one of the first of these studies using STRAFI to observe the drying of sodium silicate films. Spatial resolution was obtained by moving the sample vertically in steps concomitant with the pulse parameters that defined a resolution of 100 mm in the gradient of 38 T mK1. In this work, the transverse relaxation time (being a measure of molecular mobility) of the water in the thin film was seen to be independent of the drying conditions and varied only with the local water concentration. Magnetic resonance microscopy of water loss from alkyd oil-in-water emulsion drops on flat surfaces [84] showed that lateral gradients in water concentration occur across a drop. A model was proposed, similar to that in Ref. [85], which includes lateral water diffusion to minimise the gradients in concentration and was found to agree with the experimental analysis. This model less successfully describes the behaviour observed in drying of drops consisting of dispersions of hard colloidal particles. The model from Ref. [85] was further explored through a series of profiles monitoring the lateral drying of colloidal films with varying initial film thickness, particle size, and evaporation rate [86]. These parameters are encapsulated in the expression for the reduced capillary pressure, pc [85]   20 3gh0 1=2 að1Kfm Þ ; (19) pc Z 75 E mf2m H where h0 is the zero-shear-rate viscosity of the dispersion, E is the water evaporation rate, m is the viscosity of the continuous liquid, fm is the volume fraction of solids at close-packing, a is the particle radius, g is the surface energy of the liquid, and H is the initial film thickness. The time until the water recedes from the edge of a waterborne colloidal dispersion, called the open time, was found to increase greatly for small changes in pc around a critical value that is dependent on the lateral length scale and shape of the drop [86]. It was also observed that more uniform drying could be obtained with larger particles, thinner films, and slower evaporation rates as expected. The vertical distribution of water in drying colloidal films was explored using GARField profiling measurements [87]. For the vertical drying of colloidal films, a Peclet number, Pe [88], can be used as an indicator of the drying mechanism. This is defined as PeZHE/D0 where H is the initial film thickness, E is the evaporation rate, and D0 is the Stokes–Einstein diffusion coefficient. For a Pe[1, evaporation dominates in the drying process and particles are predicted to accumulate near the air surface. For a Pe/1, diffusion dominates and the particles are predicted to remain uniformly distributed. From the GARField profile measurements, these predictions were confirmed. However, the additional prediction that wet sintering should occur immediately when the alkyd particles come into contact was disproved. A skin-like layer (low water content) was

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observed to form on the upper surface of the film when PeO1, but the alkyd particles had not coalesced into a continuous structure. The observation was explained as the presence of surfactant initially preventing direct contact. A Peclet number was also used to describe layers of poly(vinyl alcohol) (PVOH) cast from aqueous solution [89]. Again GARField measurements were used to determine the drying mechanism and physical structure of the layers. Here, in contrast to the alkyd emulsions, a continuous crystalline skin was seen to develop under slow drying conditions characterised by a high Peclet number. GARField profile measurements of drying waterborne acrylic pressure-sensitive adhesives (PSAs) [90] suggest that the capillary pressure drives the transport of surfactant, watersoluble polymer, ions, and other species to the surface of the film. An excess of surfactant at the surface of these PSA films had been observed using atomic force microscopy (AFM) and Rutherford backscattering spectroscopy (RBS): a result that is not consistent with the predicted process of film formation at that time. The MR measurements revealed that in the later stages of drying the water concentration is very low near the film surface and increases with depth into the adhesive. The measured water profiles indicated that water-filled capillaries exist at particle boundaries, see Fig. 8. In the work by Bennett et al. [8], the curing of glue layers was investigated. Three glues were studied under different curing conditions and the water transport properties of the dry glue layers were investigated. The three types of glue were found to behave differently. GARField profiling was demonstrated as a valid non-destructive method of determining the curing time of wood glue [8]. Drying times of adhesives can also be measured using the NMR-MOUSE [91], although currently only through relaxation time measurements without the additional information available from profiles. 4.2. Polymers 4.2.1. Cross-link density and curing The first and still one of the main applications of the NMRMOUSE is the study of polymer materials, particularly rubbers. The first publication incorporating the NMR-MOUSE [14] mentioned the linear relationship observed between the crosslink density of vulcanised natural rubber (determined by the concentration of peroxide cross-linker) and the measured T2,eff relaxation time. Further to this, correlations between the transverse relaxation time and the sulphur content and the glass transition temperature, Tg, were observed [39]. See Fig. 9 as an example. In the same work [39], the industrial applicability of the MOUSE was demonstrated by observing the progress of the cross-linking process (i.e. curing) of carbon black filled natural rubbers. As the cross-linking occurs, the polymer in the rubber loses mobility and the measured relaxation time, T2,eff, decreases. The results were confirmed by comparing the cross-link density measured with a NMR-MOUSE at 17.5 MHz (proton frequency) and a conventional spectrometer at 300 MHz [92]. Work has also been carried out to study the thermo-oxidative aging of similar carbon black filled natural

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rubbers using a temperature controlled sample holder on a NMR-MOUSE [93]. Later, double-quantum (DQ) filtered NMR signals were demonstrated to offer a potentially more accurate method of observing cross-link density [77,94] and the sulphur accelerator content [74] in polymers. Unlike transverse relaxation time measurements, the DQ measurements are independent of the magnetic field inhomogeneity. The molecular mobility of polymers in elastomers can be assessed by relating properties such as mechanical relaxation time and molecular jump frequency to the measured NMR transverse relaxation time [95]. The NMR results were compared to dynamic–mechanical thermal analysis (DTMA) measurements and both methods lead to consistent con-

Fig. 8. Simulated profiles of the formation of latex films by (a) dry sintering and (b) capillary action, with each profile separated by 80 s (arrow indicates direction of increasing time). In each graph, the bold profile represents the transition from initial drying to particle deformation. The measured MRI profiles (c), obtained using a GARField magnet (where jB0jZ0.7 T in the sensitive volume), more closely match the capillary action curing process modelled in (b). Graphs reproduced with permission from Ref. [90], copyright (2002), American Chemical Society.

clusions. An empirical power law was devised to predict the temperature dependence of the transverse relaxation time in the rubber. Many of these results were summarised or discussed in more detail in Refs. [91,96]. 4.2.2. Profiling of polymer products The original industrial application of the NMR-MOUSE was the in situ measurement of high performance car tyres [14]. In the early experiments, the steel belts below the rubber tyre treads were observed to act like a yoke under certain conditions and they homogenise the magnetic field in the sensitive volume, improving the detected signal. Studies comparing twodimensional MRI cross-sectional images of segments of car tyre with NMR-MOUSE relaxation measurements as a function of depth [39] further demonstrate the information obtainable in this application. The depth resolution was obtained from the NMR-MOUSE by varying the proton frequency in increments of 1 MHz, corresponding to approximately 1 mm steps. A softened region of rubber near the nylon cord in the tyre visible in the MRI image (see Fig. 10(a), centre of image) was also detected by an increase in the relaxation time measured by the NMR-MOUSE at the same depth position. This is in contrast to the tyre segment in Fig. 10(b), which has short T2 relaxation times. By attaching a larger version of the NMR-MOUSE to a mechanical lift, profiles have been obtained through planar samples with a resolution of better than 5 mm [24]. A practical application was the cross-sectional imaging of a multi-layer wall of a gasoline tank. The wall contained a layer of ethylene/ vinylalcohol co-polymer (EVOH) that acts as a barrier to reduce vapour emission. The EVOH layer was glued between layers of high density poly(ethylene) and regrind using resin.

Fig. 9. Correlations observed between the measured T2 and the glass transition temperature in a range of polymeric rubber materials using a NMR-MOUSE with jB0jZ0.5 T close to the magnet surface. The samples of butadiene rubber (cis-BR, types A and B), isoprene butadiene rubber (I-BR), natural rubber (NR), styrene butadiene rubber (SBR), nitrile styrene butadiene rubber (NSBR), and isoprene rubber (3,4IR) all have different molecular mobility. Technical details and chemical structures of these rubber samples can be found in Ref. [95]. Graph reproduced with permission from Ref. [95], copyright (2002), Springer Science and Business Media.

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Fig. 11. One-dimensional profile obtained from the wall of a gasoline tank using a NMR-MOUSE with jB0jZ0.4 T in the sensitive volume, mounted on a mobile platform. The wall structure is shown above the graph. Figure reproduced with permission from [24], copyright (2005), Elsevier.

Fig. 10. T2 spin weighted images of cross-sections through car tyres obtained by conventional (destructive) MRI. The T2 depth profiles obtained from the NMRMOUSE (jB0jZ0.5 T close to the magnet surface) are overlaid. In (a), a soft material (light grey) is seen at the centre of the image, corresponding to a high T2 as measured by the NMR-MOUSE. This is not observed in (b). Images reproduced with permission from Ref. [39], copyright (1998), Elsevier.

As shown in Fig. 11, all five layers can be clearly identified due to the relaxation time weighting of the profile. This technique can be used to locate weak points in the resin or gaps in the EVOH barrier. It can equally be applied to other polymer barriers, such as plastic film wrappings used in food industries. 4.2.3. Degradation and aging The NMR-MOUSE can also be used to observe the degradation of polymers with age. Polyvinylidendifluorine (PVDF) samples were artificially aged by heating in oil at different temperatures [92] before the transverse relaxation time of the polymer was measured. The T2 relaxation time of the polymer (measured at room temperature) was seen to increase with oil temperature. Guthausen et al. suggested that the increased polymer molecular mobility was due to either an increase in the frequency of chain scissions occurring at higher temperatures, or diffusion of oil into the polymer, or the oil acting as a plasticiser. An industrial application of this study was the observation of aging in fast clutches [94]. The clutches

are exposed to temperatures of 140 8C during normal operation and this can lead to thermal oxidative aging of the rubber component. A specially shaped NMR-MOUSE was constructed to measure the transverse relaxation time at the surface of the rubber cylinder, where the aging first occurs. The relaxation time was seen to decrease with exposure to high temperature corresponding to a loss in molecular mobility as the rubber hardened. The NMR-MOUSE has been used to study the weathering of polyvinylchoride (PVC) coatings on iron sheets [13,97] where, despite the presence of the ferromagnetic iron, differences in the relaxation time measurements from the plastic were observed. As the duration of weathering increases, the molecular diffusion is enhanced and the volatile components in the PVC escape. The result is a hardening of the plastic that reduced the measured T2 relaxation time. 4.2.4. Solvent ingress and swelling The ingress of solvent contaminants into thin layers of polymeric materials can be observed with NMR imaging. Applications of these studies range from biomedical implants to large-scale construction projects; see Ref. [98] and references therein for further information. STRAFI profiling was used to observe the ingress of acetone into polyvinylchloride (PVC) [99]. The acetone was applied as a vapour of varying activity for 48 h (vapour activity was defined as aZ f expð1K fÞ where f is the acetone to PVC ratio). In all cases, a Fickian precursor was observed. At higher vapour activities a CaseII-like diffusion front was observed ingressing linearly with time but the Fickian precursor remained, travelling ahead of the sharp diffusion front. The shape of the precursor was found to be time independent; see Fig. 12 as a typical example where the profile was acquired at a proton frequency of 160 MHz from

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Marble et al. [27] have developed a variation on the NMRMOUSE design and used it to determine the moisture content in composite sandwich panels. The panels consisted of an aluminium honeycomb core between graphite epoxy composite skins [105]. Water trapped inside the aluminium was successfully detected despite the RF shielding effect of the metal. Earlier [106], a regular NMR-MOUSE had been used in an attempt to measure thermally damaged reinforced epoxy resins used in aircraft wings. Unfortunately, the only relaxation time that is sensitive to the degree of damage is T1r and the signal-to-noise ratio of the NMR-MOUSE was insufficient to provide a reliable T1r measurement. With the continuing improvements currently being made to the NMR-MOUSE design, such measurements may soon be possible.

Fig. 12. Profiles of PVC exposed to acetone vapour by placing above an acetone reservoir for 48 h at 20 8C. The profiles were obtained using the STRAFI technique in the fringe field of a super-conducting magnet with the sample position corresponding to a proton resonance frequency of 160 MHz, jB0jw4 T, and a gradient GZ50 T mK1. The acetone has ingressed into the PVC from the left. See discussion in text for more information. Graph reproduced with permission from Ref. [99], copyright (1994), Elsevier.

nZ4 echoes separated by 2tZ100 ms. The gradient strength was 50 T mK1 and the pulse length 10 ms, thus defining a pixel resolution of 78 mm. In Fig. 12, the sharp diffusion front can be seen at approximately 3 mm depth followed immediately by a slight slope (Fickian precursor) in the otherwise uniform PVC profile. The curve in the acetone softened PVC (0–3 mm) is due to relaxation time weighting. The peak at the right of the profile (11 mm) is the signal from the glue holding the PVC in place. This unusual precursor behaviour was explained in a later publication by McDonald et al. [100] as a second type of Case-II diffusion, where a low surface flux of solvent limits the ingress at the diffusion front. Similar complex diffusion dynamics were observed in STRAFI experiments studying water ingress into the starch polymer amylase [101] and into more complex starch and sucrose pellets [102]. STRAFI experiments have also been used to study the ingress of water into sodium polyacrylate [103]. Due to the presence of hydrophilic sites in the polymer, the uptake of water is rapid and prolonged. The STRAFI studies indicated that the water diffuses into the sodium polyacrylate with typical Case-I dynamics. These results were confirmed with conventional MRI measurements. The NMR-MOUSE has also been applied to the study of solvents in polymers [104]. The solvent self-diffusion coefficient of toluene in natural rubbers with varying degrees of polymer cross-linking was measured. These diffusion coefficients are correlated with the cross-link density and shear modulus of the rubber. This is another alternate method of determining cross-linking parameters in a rubber sample, rather than relying on the transverse relaxation time measurements (as discussed above) that can be influenced by the inhomogeneous magnetic fields of the NMR-MOUSE.

4.2.5. Stress and strain The transverse relaxation time (T2) of hard polymers is sensitive to the mechanical state of the material. This was observed with the NMR-MOUSE in stressed high-impact polystyrene sheets [14,97]. The stressed region was visually observed as a white line running across the sheet. The NMRMOUSE was moved across the sheet to determine the T2,eff relaxation time as a function of position relative to the stressed region. The measured T2,eff was seen to decrease by approximately 13% in the stressed polymer compared to the unaffected polystyrene. A recent application for observing the morphological condition of semi-crystalline polymers was the analysis of poly(ethylene) pipes [107]. The pipes were investigated new, after squeezing, and after annealing to well below the glass transition temperature. Relaxation analysis of CPMG data allowed the rigid (crystalline) and soft (amorphous) phases of the polymer to be distinguished from each other. The deformation (squeezing) of the pipe resulted in an increase of the short T2,eff component, corresponding to a reduction in mean crystallite size. The opposite effect was observed after annealing: the short T2,eff relaxation time component reduced corresponding to an increase in the mean crystallite size. Also in the same work changes in morphology induced by pressure applied internally to the pipe were identified by depth profiling using the NMR-MOUSE and an anomalous result was confirmed to be a shear-band by destructive conventional MRI imaging. Finally, the NMRMOUSE was used to determine the position of weld points in the pipes by moving the device down the pipe and observing a change in the amplitude ratio of the long and short T2,eff components. These measurement techniques have also been applied to soft natural rubber bands. A non-linear relationship between T2,eff and extension ratio, l, was observed by stretching a rubber band over the NMR-MOUSE [108] in agreement with earlier measurements [109] performed in homogeneous fields. The non-linear behaviour was attributed to stress-induced crystallisation. The angle between the direction of the applied stretching force and the B0 magnetic field was varied and a minimum in the transverse relaxation rate was observed periodically when the angle was close to the magic angle, 54.78. The segmental anisotropy of natural rubber was explored

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by stretching a large sheet of rubber and moving the NMRMOUSE across the surface of the material to obtain spatial information. A hole had been cut in the centre of the sheet and the greatest chain segmental anisotropy was detected on the elongated edges of this hole. The lowest chain segmental anisotropy was detected at the ends of the rubber sheet, where the stretching force was being applied. These measurements are all consistent with earlier work [110]. The segmental anisotropy in polymers has also been investigated using DQ measurements [77]. 4.3. Building materials 4.3.1. Cement hydration In situ analytical studies of surface layers of structures in the built environment are only possible using single-sided NMR techniques. The NMR-MOUSE has been applied in various situations requiring non-destructive analysis of sandstones and concretes, including the study of art frescoes. Prado [111] first used a hand-held device, similar to the NMR-MOUSE, to monitor the progressive hydration of rapid setting cement in a mortar mix. A single component decay was fitted to the CPMG echo train data and a decrease in both the signal intensity and the effective T2 relaxation time was observed as the cement hydrated. By adding a series of tuned circuits to the detector coil on the single-sided sensor, Prado was able to obtain NMR data as a function of depth (one-dimensional profiling) up to 40 mm below the sample surface [112]. The hydration of a cement paste layer 20 mm thick was observed by this method, and the free water content was seen to decrease almost uniformly throughout the sample, see Fig. 13. Significantly this measurement revealed an increase in the free water content at the surface of the sample (0–5 mm depth) that could not have

Fig. 13. Depth profiles obtained from hydrating cement paste as a function of time using a single-sided sensor, with jB0jw0.35 T close to the magnet surface. Profiles were obtained by adjusting the position of the sensitive volume within the magnetic field. The free water content decreases as the cement cures. Eventually, only a small water signal is observed near the surface of the sample. Graph reproduced with permission from [112].

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been observed in the earlier work [111] where only the cement surface was studied. A later study [113] used a conventional U-shaped NMRMOUSE [14,16] to observe the hydration of various cementitious materials. These data were compared with identical measurements performed using a commercial bench-top spectrometer (both magnets operated at approximately the same proton resonant frequency w20 MHz). Transverse relaxation decays were inverted to provide an indication of the pore size distribution in gypsum plaster, Portland cement, and a concrete mix. The results from both systems were similar, although the distributions of relaxation times obtained by the NMR-MOUSE were generally of lower resolution and discrete peaks could not be resolved in the cement paste and concrete samples. Stretched exponential fits indicated a better correlation in the two sets of measurements. The Surface-GARField [28] was specially developed to provide spatial resolution up to 50 mm into cement and concrete structures in the built environment. Although still in the development stage, the magnet has already been proven capable of providing two-dimensional relaxation correlation [30] information from 10 mm below the surface of a white cement block [29]. These measurements can potentially provide far more information on porosity and chemical exchange of water between pores than is available from conventional relaxation analysis [31]. The Surface-GARField magnet should be capable of providing non-destructive profiles of water through built structures, an investigation that normally requires the extraction of core samples. It may also be possible to observe the ingress and effectiveness of polymer surface treatments. 4.3.2. Surface treatments STRAFI was used to observe the ingress of a siloxane coating treatment into mortar samples [114]. The mortar (a mix of sand, Portland cement, and water in the weight ratio of 3:1:1, respectively) was made using D2O to allow the polymer to be observed. The profiles revealed that the polymer penetrated to a depth of 1.5 mm after 24 h. The NMR-MOUSE has also been applied to the analysis of historical building materials [12]. Samples of sandstone with and without silicon oxide stone strengthener and samples of historical brick material were studied to obtain distributions of T2 relaxation times. These samples were all cores previously removed from buildings. Measurements as a function of depth were performed on the water-saturated sandstones using a larger version of the NMR-MOUSE; see Fig. 14. It was clear that the largest pores, those with a corresponding T2 relaxation time O10 ms, had been partially blocked by the silicon oxide strengthener. The pores closest to the treated surface contained less water than those deeper in the sample. The brick samples proved more difficult to measure reliably. Mercury intrusion porosimetry (MIP) measurements were used to calibrate the distribution of relaxation times in terms of the pore sizes for terra cotta. However, varying concentrations of paramagnetic—and in some cases ferromagnetic—impurities in the bricks shifted the T2 relaxation time distributions making a

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prevented moisture exchanging across the air-paint interface. This was seen as a relatively fast T2 relaxation in the long time component. As the old treatment was removed, the long T2 component decayed more slowly, indicating the pores had been opened and had absorbed water vapour. The addition of a modern treatment shifted the relaxation time distribution back to shorter values, indicating that the large pores had been blocked whilst the small pores had been left open, as expected from the process of fresco consolidation. Fig. 14. Comparison of relaxation time distributions in untreated and treated Sander sandstone samples as a function of depth using a NMR-MOUSE with a proton resonance frequency of 9 MHz, corresponding to jB0jw0.23 T. See text for details. Graphs reproduced with permission from Ref. [12].

calibrated measurement nearly impossible. The successful measurement of sandstone led to a later in situ study of sandstone window frames at Paffendorf Castle in Germany [115]. Here, the application of a surface treatment was seen to reduce the number of large pores and shift the distribution to shorter relaxation times suggesting the treatment had partially filled the open porosity. 4.3.3. Cultural heritage The NMR-MOUSE has also been used to study pore size distributions in ancient art frescoes. Recent measurements on artificial microporous porcelain samples [116] have demonstrated the validity of the earlier work conducted on frescoes. The first of these measurements was performed in a cryptoporticus at Colle Oppio in Rome [117]. The NMRMOUSE was placed 1 mm away from the fresco surface to avoid damaging the structure. Strong signals were obtained from the wet fresco at short and long relaxation times, indicating the presence of both small and large pores, and that they contain water. The nearby bricks, like those supporting the Fresco, gave similar results, whereas dryer bricks elsewhere in the cryptoporticus had less signal at longer relaxation times, indicating that the larger pores were dry. A later study on frescoes in the house of Vasari in Florence [19] allowed areas where the paint film had detached from the plaster support to be remotely identified. A Bruker Biospin unilateral NMR ProFiler was used for these experiments. Surface measurements of the paint film revealed a decrease in the Hahn echo intensity where the paint had become detached from the plaster surface. Measurements 3 mm below the surface of the plaster were constant, indicating that the difference in the signal from the paint is not due to a difference of moisture content in the underlying support. A two component T2 relaxation decay was observed in the surface layers. A broadening of the T2 relaxation distribution was observed in regions where hygroscopic salts had collected. Although these salts eventually form outcroppings and are visible as a white discolouration on the surface, the NMR measurement may provide an early warning of salt deposition. It was also possible to identify, through the T2 relaxation times and relaxation time distributions, the surface treatment that been applied to the paint. Old proteic substances previously used to preserve the frescoes blocked the pore structure and

4.3.4. Water ingress Various other building materials have also been studied using STRAFI and mobile devices. The ingress of water into ceramic wall and floor tile segments was monitored using STRAFI [118]. Unglazed tiles were saturated by water within 4 min of exposure, whereas glazed tiles were seen to be waterproof for 23 h. In the case of glazed tiles where the surface glaze was damaged, the water was seen to penetrate slowly and a dynamic equilibrium was established between water ingress at the upper surface and evaporation at the lower surface. 4.3.5. Wood as a building material The NMR-MOUSE has been applied to the study of wood used in construction. The wood was typically seen to exhibit two relaxation time components [119] where the short component is associated with protons in macromolecules (including cellulose and lignin) and the long component is associated with water. A direct correlation between the water content and the spin-echo signal intensity was determined in test samples, and the moisture content measurements made by the NMR-MOUSE on other woods agree with those from gravimetric measurements. The long component of the relaxation time analysis, T2,eff, also appears to correspond to the type and condition of the wood [115], see Table 1. These studies offer the possibility of in situ measurements in construction and conservation industries. 4.4. Paper The NMR-MOUSE has found something of a niche application in the non-destructive study of paper, particularly historical documents. The short transverse relaxation times present in dry paper could only be measured with Table 1 Relaxation time components recorded in different wood samples using the NMR-MOUSE Type of wood

T2,eff,short (ms)

T2,eff,long (ms)

Along/(AshortCAlong)

Pine Pine weakly degraded Pine strongly degraded Birch Weathered oak Hardwood floor

0.11G0.02 0.13G0.03

0.81G0.08 0.77G0.09

0.30 0.35

0.10G0.01

0.55G0.12

0.24

0.13G0.01 0.17G0.01 0.11G0.01

1.18G0.10 0.85G0.02 0.54G0.08

0.27 0.37 0.30

Table reproduced with permission from Ref. [115].

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improvements to the original NMR-MOUSE design [14] that reduced the dead-time of the system [16]. In the same publication, initial measurements of paper using the NMRMOUSE revealed a two component transverse decay as expected [120–122], see Fig. 15. The short T2 component, typically around 27 ms was attributed to the cellulose and was reported to be virtually independent of the paper sample. The long T2 component, associated with the bound water in the paper, was used as an indication of the condition of the material. This relaxation time component varies significantly depending on the age of the paper. Later studies examined a wider range of historical papers all in different states of degradation [18]. As the paper degraded, the quantity of water decreased and this was clearly observed in the relaxation measurements as a reduction in both intensity and T2. Measurements of the most severely degraded and damaged paper revealed no bound water component. The relaxation time of the short T2 component from the cellulose was also seen to decrease with a reduction in water content. This anomaly was attributed to chemical exchange between the protons in the water and cellulose. In a continuation of this work [123], filter papers—considered to be pure cellulose— were artificially aged by oxidation using sodium metaperiodate. The concentration of the sodium metaperiodate solution and the exposure times were varied. The damage induced by the oxidation reactions was monitored using solid-state 13C CP-MAS NMR analysis. The paper was then measured destructively (i.e. being cut and placed in a test-tube) using a conventional bench-top proton NMR spectrometer, and nondestructively using the NMR-MOUSE. Both systems operated at approximately the same 1H resonant frequency (w20 MHz) and due to the absence of diffusion in the samples, the measured relaxation times were independent of the strength of the magnetic field gradient. Despite having a characteristic

Fig. 15. Two-component transverse relaxation measured in paper obtained using a NMR-MOUSE with jB0jZ0.5 T close to the magnet surface. Insert shows echo amplitude on a natural log scale to emphasise the two distinct components. Graphs reproduced with permission from Ref. [18], copyright (2003), Elsevier.

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strong gradient, the unilateral NMR-MOUSE provided the same relaxation times as the homogeneous bench-top system. In all cases, increasing exposure to the oxidising agent resulted in a reduction in the net T2 relaxation time. Even very low concentrations of sodium metaperiodate and short exposure times provided a noticeably reduced relaxation time. This damage was not visible, nor could it be detected by the 13C analysis, suggesting that proton relaxation time measurements are extremely sensitive to the degradation of paper. A separate study used a NMR-MOUSE to monitor the degradation of the Codex Major, a 17th Century manuscript belonging to the library of Palazzo Altaemps in Rome [108,124,125]. In these studies, T1 and T2 analysis both yielded two relaxation time components, from which the ratios of cellulose to water and crystalline to amorphous cellulose could be deduced. Most of the paper studied in the Codex Major appeared to have similar relaxation properties to ‘normal’ good quality paper, and so was assumed to be in reasonable condition. The presence of paramagnetic impurities in the ink complicated the measurements, although evidence for degradation arising from the acidic ink was observed. However, stains on the paper had not damaged the underlying structure in any way. With further studies to characterise the relaxation properties of paper in different degrees of degradation, the NMR-MOUSE could provide an excellent method of non-destructively analysing ancient manuscripts and other historical documents. 4.5. Bio-systems 4.5.1. Tendons (in vivo) Human tendons exhibit different transverse (T2) relaxation times depending on their orientation in a magnetic field, in a similar manner to stretched rubbers [108] (see above). A consequence of this phenomenon is that conventional MRI measurements of tendon can provide erroneous results if the tendon is in an unfavourable orientation and the patient is unable to move within the confines of a whole-body scanner system. The NMR-MOUSE has been used to perform in vivo investigations of the orientational dependence of the T2 relaxation times in the Achilles tendon—the so-called magicangle phenomenon [126]. Initially, measurements were recorded using the NMR-MOUSE on tendons from pig and cow carcasses. These confirmed that the measured T2 was at a minimum when the tendon and magnetic field, B0, were parallel, and maximum when the angle between the tendon and the magnetic field was close to 558, see Fig. 16. This is because molecular mobility is restricted by the tightly packed tendon fibres and so the dipole–dipole interactions are averaged only partially and in an anisotropic fashion, leading to an angular dependence in 1/T2 proportional to 1=2ð3 cos2 qK1Þ2 . Further measurements conducted on human volunteers confirmed that the recorded T2 values varied by approximately a factor 2 depending on whether the Achilles tendon was aligned parallel, or at the magic-angle, to the B0 field. The single sided nature of the NMR-MOUSE allowed the anisotropic relaxation behaviour of the tendons to be easily

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Fig. 16. Variation of transverse relaxation rate as a function of angle between tendon and magnetic field obtained using a NMR-MOUSE with jB0jw0.4 T close to the magnet surface. A minimum in the relaxation rate is observed near the magic angle (ca. 54.78). Graph reproduced with permission from Ref. [126], copyright (2000), Elsevier.

assessed and this technique could provide an indication of the condition of the tendon. The NMR-MOUSE has also been used to study the self-diffusion anisotropy of free water in Achilles tendons [104]. Since the water is confined to the nanoscopic regions between collagen fibres aligned with the tendon, the measured self-diffusion coefficient was seen to vary with the angle of orientation of the tendon relative to the B0 magnetic field. The maximum self-diffusion coefficient was recorded when the tendon and field were parallel, and the minimum selfdiffusion coefficient recorded when they were perpendicular. This agreed with theoretical simulations as discussed in Ref. [104]. 4.5.2. Skin (in vitro and in vivo) The NMR-MOUSE has also been used to provide in vitro profiles of pig skin [97]. In these measurements, analysis of the T2 relaxation times at different depths allowed the epidermis and subcutis to be distinguished. Later, GARField profiles were used to study the hydration of human skin both in vitro [7,127] and in vivo [9,127]. The in vitro measurements were conducted on segments of abdominal epidermis extracted during cosmetic

surgery procedures. Significantly, in the early experiments [127], it was demonstrated that nearly identical profiles could be obtained from multiple skin samples from the same donor. The proton signal intensity in the profiles was also shown to scale with water content. Measurements of a hydrated skin sample drying in air showed the water preferentially evaporates from the stratum corneum (upper) layer. The profiles also indicated the skin reduced in thickness as it dried. Later measurements of human skin [7] demonstrated that it is possible to differentiate the stratum corneum from the viable epidermis by careful selection of the parameters used to record the profiles, see Fig. 17(a). Experiments also demonstrated the feasibility of observing the ingress of skin-care products into the skin using a number of model systems. The application of deuterated decanol appeared to increase the mobility of protons in the stratum corneum, implying an increase in skinmoisturisation rather than skin-hydration. Likewise glycerine appeared to improve the skin-moisturisation, see Fig. 18(a) where in vitro profiles were recorded from a skin sample after the application of glycerine. The profiles were recorded using the original GARField magnet with nZ16 quadrature echoes and an echo spacing of 2tZ1000 ms. By measuring the position of the half-maximum of the diffusion front as a function of time, see Fig. 18(b), it was possible to infer a Fickian diffusion mechanism for the ingress of the glyercine into the skin with a diffusion coefficient of 1.3!10K9 cm2 sK1. In vivo skin profiles were first conducted by measuring the tip of a volunteer’s index finger [127] since it was the only portion of skin that could be placed in the original GARField magnet. Profiles were recorded from a volunteer with normal skin and a volunteer with a dry skin complaint. Considerably lower signal intensity was observed from the finger with dry skin, although after the application of a moisturising cream, both fingers exhibited approximately the same signal intensity. This moisturising effect had almost completely disappeared approximately 30 min after initial application, indicating that the cream has little, if any, long term benefits. The development of the Open-GARField [8] allowed in vivo studies to be performed on human forearm and sideof-hand skin [9]. Whilst the resolution of these in vivo measurements was not as good as the in vitro measurements

Fig. 17. Differentiation of the stratum corneum (SC) and viable epidermis (VE) layers in human skin measured (a) in vitro and (b) in vivo using two different GARField magnets. In both cases, jB0jZ0.7 T in the sensitive volume. Graphs reproduced with permission from Refs. [7] and [9], respectively.

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developed a miniature single-sided NMR magnet and probe system that can be used to make in vivo measurements of T1, T2 and the self-diffusion coefficient within the walls of human coronary arteries. The magnet/probe is placed on the end of a standard catheter and inserted into the patient. This system has been specifically designed to diagnose the presence of lipidrich vulnerable plaques—a weakened segment of the artery wall—that are too small to be accurately identified with external imaging techniques. The direct measurement of NMR parameters within the arterial wall can help classify suspect regions identified from a conventional MRI scan without the need for invasive surgery.

5. Conclusion

Fig. 18. (a) In vitro profiles of skin with glycerine added obtained using a GARField magnet with jB0jZ0.7 T in the sensitive volume. The lower heavy solid line shows the initial skin profile. Profiles were recorded (from bottom, diamonds to top, circles) 0.2, 3.5, 5.7, 7.4, 9.5, 11.7, 13.8, and 15.9 h after application. (b) Displacement of the glycerine front (measured as the halfmaximum of the profiles) plotted with respect to the square root of time, from which a Fickian ingress mechanism (linear fit) is inferred. Graphs reproduced with permission from Ref. [7].

(due to the difficulty of aligning the skin relative to the magnetic field) the stratum corneum and viable epidermis could still be clearly distinguished; see Fig. 17(b). Measurements of the T1, T2 and self-diffusivity of water in the skin layers were possible. It was noted that these differed significantly to the in vitro measurements and these were attributed to the effectively infinite reservoir of water present behind the in vivo skin. These differences actually improved the profile contrast between the two skin layers. As with the earlier experiments, it was possible to observe the effects of applying moisturising skin-care products to the skin. However, whilst reproducible profiles were obtained from each volunteer, further studies would be required to identify the NMR parameters associated with normal and abnormal skin. In the future, it should be possible to determine the ingress rates of topical applications in vivo and diffusivity studies could provide increased understanding of the barrier properties of skin. Skin profiles have also been reported by Casanova et al. using the profiling NMR-MOUSE [25]. 4.5.3. Vessel walls (in vivo) It is now also possible to apply these measurement techniques inside the human body. Blank et al. [128] have

The technology to conduct MRI of planar samples and planes within samples has expanded rapidly in the last 10 years. The technique of STRAFI began by using the large magnetic field gradient in the fringe field of conventional superconducting magnets. However, it was not widely used for materials science applications, possibly because of the cost of the equipment, and even fewer planar sample studies were conducted with this technique. STRAFI was made available to a wider range of materials scientists applications with the introduction of the low-cost GARField magnets. Since the original GARField was constructed, the magnet design has been used for many different applications, including the study of paints, glues, polymer coatings and films, and (with the Open-GARField) human skin in vivo. GARField has been shown to be particularly useful for observing the ingress of fluids into materials, and through the analysis of planar sample profiles has shed light on the curing of thin films and coatings. The list of applications for GARField is still growing, as is the technique’s international recognition as a valuable tool in the field of materials science. At the same time the NMR-MOUSE, and now the SurfaceGARField, has taken stray-field measurements and profiling beyond the laboratory, allowing samples that could never be studied with conventional NMR/MRI to be examined. The NMR-MOUSE began as a tool for studying soft polymer materials, particularly rubber products, and has since been successfully applied in situ to the study of building materials, cultural heritage restorations, analysis of plastic products, historical documents, and human skin. A number of measurement techniques have been developed and expanded for use in the grossly inhomogeneous magnetic fields of the NMR-MOUSE, including quantitatively accurate relaxation time and self-diffusion measurements. Now with the introduction of spectroscopic measurements from a unilateral magnet, even portable chemical analysis seems a possibility. The Surface-GARField offers to take the near-surface profiling of large samples a stage further by potentially providing a profile depth of up to 50 mm. Undoubtedly these unilateral magnet systems will continue to provide further analysis to an ever increasing range of applications.

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References [1] P.C. Lauterbur, Nature 242 (1973) 190. [2] P. Mansfield, P.K. Grannell, J. Phys. C Solid State 6 (1973) L422. [3] A.A. Samoilenko, D.Y. Artemov, L.A. Sibeldina, Jetp Lett. 47 (1988) 417. [4] P.J. McDonald, Prog. Nucl. Mag. Res. Spec. 30 (1997) 69. [5] P.J. McDonald, B. Newling, Rep. Prog. Phys. 61 (1998) 1441. [6] P.M. Glover, P.S. Aptaker, J.R. Bowler, E. Ciampi, P.J. McDonald, J. Magn. Reson. 139 (1999) 90. [7] L. Backhouse, M. Dias, J.P. Gorce, J. Hadgraft, P.J. McDonald, J.W. Wiechers, J. Pharm. Sci. 93 (2004) 2274. [8] G. Bennett, J.P. Gorce, J.L. Keddie, P.J. McDonald, H. Berglind, Magn. Reson. Imaging 21 (2003) 235. [9] P.J. McDonald, A. Akhmerov, L.J. Backhouse, S. Pitts, J. Pharm. Sci. 94 (2005) 1850. [10] S.J.F. Erich, J. Laven, L. Pel, H.P. Huinink, K. Kopinga, Appl. Phys. Lett. 86 (2005) 134105. [11] P.J. Doughty, P.J. McDonald, Drying of coatings and other applications with GARField in: S. Siegfried, S. Han (Eds.), NMR Imaging in Chemical Engineering, Wiley-VCH, Weinham, 2005, pp. 89–106. [12] S. Sharma, F. Casanova, W. Wache, A. Segre, B. Blumich, Magn. Reson. Imaging 21 (2003) 249. [13] G. Zimmer, A. Guthausen, U. Schmitz, K. Saito, B. Blumich, Adv. Mater. 9 (1997) 987. [14] G. Eidmann, R. Savelsberg, P. Blumler, B. Blumich, J. Magn. Reson. Ser. A 122 (1996) 104. [15] B. Blu¨mich, V. Anferov, S. Anferova, M. Klein, R. Fechete, M. Adams, F. Casanova, Concept. Magn. Reson. B 15 (2002) 255. [16] S. Anferova, V. Anferov, M. Adams, P. Blumler, N. Routley, K. Hailu, K. Kupferschlager, M.J.D. Mallett, G. Schroeder, S. Sharma, B. Blumich, Concept. Magn. Reson. 15 (2002) 15. [17] B. Blu¨mich, S. Anferova, F. Casanova, K. Kremer, J. Perio, S. Sharma, Kaut. Gummi Kunstst. 57 (2004) 346. [18] B. Blu¨mich, S. Anferova, S. Sharma, A.L. Segre, C. Federici, J. Magn. Reson. 161 (2003) 204. [19] N. Proietti, D. Capitani, R. Lamanna, F. Presciutti, E. Rossi, A.L. Segre, J. Magn. Reson. 177 (2005) 111. [20] P.J. Prado, B. Blumich, U. Schmitz, J. Magn. Reson. 144 (2000) 200. [21] M.C.A. Brown, D.A. Verganelakis, M.J.D. Mallett, J. Mitchell, P. Blumler, J. Magn. Reson. 169 (2004) 308. [22] F. Casanova, B. Blumich, J. Magn. Reson. 163 (2003) 38. [23] J. Perlo, F. Casanova, B. Blumich, J. Magn. Reson. 166 (2004) 228. [24] J. Perlo, F. Casanova, B. Blumich, J. Magn. Reson. 176 (2005) 64. [25] J. Perlo (Eds.), Fifth Colloquium on Mobile NMR, Perugia, 2005, p. L08. [26] J. Perlo, V. Demas, F. Casanova, C.A. Meriles, J. Reimer, A. Pines, B. Blumich, Science 308 (2005) 1279. [27] A.E. Marble, I.V. Mastikhin, B.G. Colpitts, B.J. Balcom, J. Magn. Reson. 174 (2005) 78. [28] P.J. McDonald, P.S. Aptaker, UK GB pat. application no. 0426957.7, 2004. [29] P.J. McDonald, J. Mitchell, M. Mulheron, P.S. Aptaker, J.-P. Korb, L. Monteilhet, Cement Concrete Res. (2006) DOI: 10.1016. [30] Y.-Q. Song, L. Venkataramanan, M.D. Hu¨rlimann, M. Flaum, P. Frulla, C. Straley, J. Magn. Reson. 154 (2002) 261. [31] P.J. McDonald, J.-P. Korb, J. Mitchell, L. Monteilhet, Phys. Rev. E 72 (2005) 011409. [32] R.J.S. Brown, R. Chandler, J.A. Jackson, R.L. Kleinberg, M.N. Miller, Z. Paltiel, M.G. Prammer, Concept. Magn. Reson. 13 (2001) 340. [33] T.B. Benson, P.J. McDonald, J. Magn. Reson. Ser. A 112 (1995) 17. [34] K. Zick, STRAFI solids imaging Technical Report NMR/B353/393 BRUKER Anlytische Messtechnik GmbH, Silberstreifen, D-7512 Rheinstetten 4/Karlsruhe, 1993. [35] A.N. Garroway, J.B. Miller, US pat. no. 5,126,674, 1992. [36] P. Glover, P.J. McDonald, B. Newling, J. Magn. Reson. 162 (1997) 207. [37] J.A. Jackson, W.H. Langham, Rev. Sci. Instrum. 39 (1968) 510.

[38] S. Rahmatallah, Y. Li, H.C. Seton, I.S. Mackenzie, J.S. Gregory, R.M. Aspden, J. Magn. Reson. 173 (2005) 23. [39] G. Zimmer, A. Guthausen, B. Blumich, Solid State Nucl. Magn. Reson. 12 (1998) 183. [40] J.D. Edwards, B.V. Jayawant, W.R.C. Dawson, D.T. Wright, IEE P-Elect. Pow. Appl. 146 (1999) 627. [41] L.J. Burnett, J.A. Jackson, J. Magn. Reson. 41 (1980) 406. [42] J. Felder, B. Rembold, Frequenz 57 (2003) 221. [43] R.L. Kleinberg, Well logging in: D.M. Grant, R.K. Harris (Eds.), Encyclopedia of NMR, Wiley, New York, 1996, pp. 4960–4969. [44] F. Ba˜libanu, K. Hailu, R. Eymael, D.E. Demco, B. Blu¨mich, J. Magn. Reson. 145 (2000) 246. [45] A.R. Rath, S.B.W. Roeder, E. Fukushima, Rev. Sci. Instrum. 56 (1985) 402. [46] E. Fukushima, J.A. Jackson, US Pat 6,489,872, 1999. [47] B. Luong, J.C. Goswami, A. Sezginer, D. Davies, IEEE T. Magn. 37 (2001) 1015. [48] R.K. Cooper, J.A. Jackson, J. Magn. Reson. 41 (1980) 400. [49] R.L. Kleinberg, A. Sezginer, D.D. Griffin, M. Fukuhara, J. Magn. Reson. 97 (1992) 466. [50] Y.M. Pulyer, IEEE T. Magn. 38 (2002) 1553. [51] Y. Yao, Y. Fang, C.S. Koh, IEEE T. Magn. 41 (2005) 1504. [52] D. Sakellariou, C.A. Meriles, A. Moule, A. Pines, Chem. Phys. Lett. 363 (2002) 25. [53] D. Sakellariou, C.A. Meriles, A. Pines, CR. Phys. 5 (2004) 337. [54] M.D. Hu¨rlimann, J. Magn. Reson. 152 (2001) 109. [55] M.D. Hu¨rlimann, Magn. Reson. Imaging 19 (2001) 375. [56] L.J. Zielinski, P.N. Sen, J. Magn. Reson. 147 (2000) 95. [57] L.J. Zielinski, P.N. Sen, J. Magn. Reson. 164 (2003) 145. [58] L.J. Zielinski, M.D. Hu¨rlimann, J. Magn. Reson. 172 (2005) 161. [59] M.D. Hu¨rlimann, D.D. Griffin, J. Magn. Reson. 143 (2000) 120. [60] M.D. Hu¨rlimann, J. Magn. Reson. 148 (2001) 367. [61] E.L. Hahn, Phys. Today 4 (1953) 4. [62] E.L. Hahn, Phys. Rev. 80 (1950) 580. [63] J.G. Powles, J.H. Strange, Proc. Phys. Soc. 82 (1963) 6. [64] E.D. Ostroff, J.S. Waugh, Phys. Rev. Lett. 16 (1966) 1097. [65] H.Y. Carr, E.M. Purcell, Phys. Rev. 94 (1954) 630. [66] S. Meiboom, D. Gill, Rev. Sci. Instrum. 29 (1958) 668. [67] J.E. Tanner, E.O. Stejskal, J. Chem. Phys. 49 (1968) 1768. [68] D.E. Demco, A. Johansson, J. Tegenfeldt, J. Magn. Reson. Ser. A 110 (1994) 183. [69] R. Kimmich, NMR-Tomography, Diffusometry, Relaxometry, Springer, Berlin, 1997. [70] D.H. Wu, J. Magn. Reson. Ser. A 116 (1995) 135. [71] P. Blumler (Eds.), Spatially resolved magnetic resonance, Proceedings of the Fourth International Conference on Magnetic Resonance Microscopy, 1997, pp. 95–102. [72] Y.-Q. Song, M.D. Hu¨rlimann, C. Flaum, J. Magn. Reson. 161 (2003) 222. [73] P.R. Laity, P.M. Glover, J. Godward, P.J. McDonald, J.N. Hay, Cellulose 7 (2000) 227. [74] A. Wiesmath, C. Filip, D.E. Demco, B. Blumich, J. Magn. Reson. 149 (2001) 258. [75] R. Graf, D.E. Demco, S. Hafner, H.W. Spiess, Solid State Nucl. Magn. Reson. 12 (1998) 139. [76] M. Schneider, L. Gasper, D.E. Demco, B. Blu¨mich, J. Chem. Phys. 111 (1999) 402. [77] A. Wiesmath, C. Filip, D.E. Demco, B. Blumich, J. Magn. Reson. 154 (2002) 60. [78] M. Wallin, P.M. Glover, A.-C. Hellgren, J.L. Keddie, P.J. McDonald, Macromolecules 33 (2000) 8443. [79] A.-C. Hellgren, M. Wallin, P.K. Weissenborn, P.J. McDonald, P.M. Glover, J.L. Keddie, Prog. Org. Coat. 43 (2001) 85. [80] A.V. Ouriadov, R.P. MacGregor, B.J. Balcom, J. Magn. Reson. 169 (2004) 174. [81] S.J.F. Erich, J. Laven, L. Pel, H.P. Huinink, K. Kopinga, Prog. Org. Coat. 52 (2005) 210.

J. Mitchell et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 48 (2006) 161–181 [82] E. Ciampi, U. Goerke, P.J. McDonald, J.G. Chambers, B. Newling, J. Phys. D Appl. Phys. 35 (2002) 1271. [83] P.D.M. Hughes, P.J. McDonald, N.P. Rhodes, J.W. Rockliffe, E.G. Smith, J. Wills, J. Colloid Interf. Sci. 177 (1996) 208. [84] E. Ciampi, U. Goerke, J.L. Keddie, P.J. McDonald, Langmuir 16 (2000) 1057. [85] A.F. Routh, W.B. Russel, AIChE J. 44 (1998) 2088. [86] J.M. Salamanca, E. Ciampi, D.A. Faux, P.M. Glover, P.J. McDonald, A.F. Routh, A.C.I.A. Peters, R. Satguru, J.L. Keddie, Langmuir 17 (2001) 3202. [87] J.P. Gorce, D. Bovey, P.J. McDonald, P. Palasz, D. Taylor, J.L. Keddie, Eur. Phys. J. E 8 (2002) 421. [88] A.F. Routh, W.B. Russel, Langmuir 15 (1999) 7762. [89] E. Ciampi, P.J. McDonald, Macromolecules 36 (2003) 8398. [90] J. Mallegol, J.P. Gorce, O. Dupont, C. Jeynes, P.J. McDonald, J.L. Keddie, Langmuir (2002) 18. [91] B. Blu¨mich, V. Anferov, S. Anferova, M. Klein, R. Fechete, Macromol. Mat. Eng. 288 (2003) 312. [92] A. Guthausen, G. Zimmer, P. Blumler, B. Blumich, J. Magn. Reson. 130 (1998) 1. [93] S. Anferova, V. Anferov, M. Adams, R. Fechete, G. Schroeder, B. Blumich, Appl. Magn. Reson. 27 (2005) 361. [94] H. Kuhn, M. Klein, A. Wiesmath, D.E. Demco, B. Blumich, J. Kelm, P.W. Gold, Magn. Reson. Imaging 19 (2001) 497. [95] V. Herrmann, K. Unseld, H.B. Fuchs, B. Blumich, Colloid. Polym. Sci. 280 (2002) 758. [96] G. Guthausen, A. Guthausen, F. Balibanu, R. Eymael, K. Hailu, U. Schmitz, B. Blumich, Macromol. Mat. Eng. 276 (2000) 25. [97] B. Blu¨mich, P. Blu¨mler, G. Eidmann, A. Guthausen, R. Haken, U. Schmitz, K. Saito, G. Zimmer, Magn. Reson. Imaging 16 (1998) 479. [98] E.D. van Meerwall, Adv. Polym. Sci. 54 (1983) 1. [99] K.L. Perry, P.J. McDonald, E.W. Randall, K. Zick, Polymer 35 (1994) 2744. [100] P.J. McDonald, J. Godward, R. Sackin, R.P. Sear, Macromolecules 34 (2001) 1048. [101] I. Hopkinson, R.A.L. Jones, S. Black, D.M. Lane, P.J. McDonald, Carbohyd. Polym. 34 (1997) 39. [102] I. Hopkinson, R.A.L. Jones, P.J. McDonald, B. Newling, A. Lecat, S. Livings, Polymer 42 (2001) 4947. [103] T.G. Nunes, G. Guillot, J.M. Bordado, Polymer 41 (2000) 4643. [104] M. Klein, R. Fechete, D.E. Demco, B. Blumich, J. Magn. Reson. 164 (2003) 310. [105] A.E. Marble, I.V. Mastikhin, R.P. MacGregor, M. Akl, G. LaPlante, B.G. Colpitts, P. Lee-Sullivan, B.J. Balcom, J. Magn. Reson. 168 (2004) 164.

181

[106] S.K. Brady, M.S. Conradi, C.M. Vaccaro, J. Magn. Reson. 172 (2005) 342. [107] B. Blu¨mich, F. Casanova, A. Buda, K. Kremer, T. Wegener, Acta Phys. Pol. A 108 (2005) 13. [108] K. Hailu, R. Fechete, D.E. Demco, B. Blumich, Solid State Nucl. Magn. Reson. 22 (2002) 327. [109] P.T. Callaghan, E.T. Samulski, Macromolecules 30 (1997) 113. [110] M. Schneider, D.E. Demco, B. Blumich, Macromolecules 34 (2001) 4019. [111] P.J. Prado, Magn. Reson. Imaging 19 (2001) 505. [112] P.J. Prado, Magn. Reson. Imaging 21 (2003) 397. [113] J. Boguszynska, M.C.A. Brown, P.J. McDonald, J. Mitchell, M. Mulheron, J. Tritt-Goc, D.A. Verganelakis, Cement Concrete Res. 35 (2005) 2033. [114] S. Black, D.M. Lane, P.J. McDonald, D.J. Hannant, M. Mulheron, G. Hunter, M.R. Jones, J. Mater. Sci. Lett. 14 (1995) 1175. [115] B. Blu¨mich, F. Casanova, J. Perlo, S. Anferova, V. Anferov, K. Kremer, N. Goga, K. Kupferschla¨ger, M. Adams, Magn. Reson. Imaging 23 (2005) 197. [116] C. Casieri, F. De Luca, P. Fantazzini, J. Appl. Phys. (2005) 97. [117] B. Blu¨mich, S. Anferova, K. Kremer, S. Sharma, V. Herrmann, A. Segre, Spectroscopy 18 (2003) 18. [118] B. Newling, D.R. Ward, V. Vijayakrishnan, J. Porous Mater. 8 (2001) 193. [119] C. Casieri, L. Senni, M. Romagnoli, U. Santamaria, F. De Luca, J. Magn. Reson. 171 (2004) 364. [120] D. Capitani, A.L. Segre, D. Attanasio, B. Blicharska, B. Focher, G. Capretti, Tappi J. 79 (1996) 113. [121] D. Capitani, M.C. Emanuele, J. Bella, A.L. Segre, D. Attanasio, B. Focher, G. Capretti, Tappi J. 82 (1999) 117. [122] D. Capitani, A.L. Segre, C. Casieri, F. Sebastiani, F. De Luca, Magn. Reson. Imaging 19 (2001) 573. [123] N. Proietti, D. Capitani, E. Pedemonte, B. Blumich, A.L. Segre, J. Magn. Reson. 170 (2004) 113. [124] C. Casieri, S. Bubici, I. Viola, F. De Luca, Solid State Nucl. Magn. Reson. 26 (2004) 65. [125] I. Viola, S. Bubici, C. Casieri, F. De Luca, J. Cult. Herit. 5 (2004) 257. [126] R. Haken, B. Blumich, J. Magn. Reson. 144 (2000) 195. [127] M. Dias, J. Hadgraft, P.M. Glover, P.J. McDonald, J. Phys. D Appl. Phys. 36 (2003) 364. [128] A. Blank, G. Alexandrowicz, L. Muchnik, G. Tidhar, J. Schneiderman, R. Virmani, E. Golan, Magn. Reson. Med. 54 (2005) 105.