Abstract
A “solid” in this context means that the sample is far away from complete motional averaging of secular spin interactions. This situation certainly arises in rigid materials like crystalline or glassy substances. However, it also occurs in non-glassy macromolecular systems, elastomers or liquid crystals, for instance.
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Notes
U Haeberlen once proclaimed in his book [179] “narrow is beautiful!” In this context we may counter: sure, however, broad is meaningful!
To a certain degree the conventional imaging pulse sequences can be adapted to the requirements of very viscous systems. Typical modifications of this sort are “oscillating gradient” methods [146, 150, 317], the “back-projection low-angle shot” (BLAST) technique [184], and constant-time imaging procedures of the simplest form [164]. In the following, techniques are envisaged which, at least in principle, are suitable for imaging of materials of any degree of rigidity or softness.
The highest — although stationary — field gradients in use are those readily available in the stray field of superconducting magnets. On this basis a special scanning technique, the stray-field imaging (Strafi) method, has been suggested [429, 533]. With gradients in the order of 50 T/m, even extremely short RF pulses take the character of “soft” pulses. Therefore they act slice-selectively. Instead of evaluating frequency-encoded k-space signals, the object is scanned slice by slice. The signals are solid echoes or accumulated echoes of a multiple-solid-echo train (see Sect. 4.1). The echo amplitudes are acquired as a function of the position on the gradient axis by shifting the object step by step along this axis. In this way a data set representing the projection of the object on the gradient direction is obtained. Gradually turning the sample and repeating the scans along the gradient direction provides a complete set of projections from which 2D or 3D image data can be reconstructed using the backprojection procedure [295]. Note that STRAFI is a scanning rather than an FT imaging method.
Note that spectral parameters such as line moments can also be evaluated directly from the curvature of solid echoes (compare Eq. 4.24).
The dots indicate the intervals where the signal is sampled.
The switching times of the phase-encoding gradient pulses should be much shorter than the encoding intervals in order to maintain the full encoding efficiency in the free-evolution periods. If this turns out to be a technical problem the gradients may also be left on during the whole pulse sequence including the magic sandwich. The artifacts arising on these grounds are minor [117]. Merely a sort of blurred inherent LOSY slice selection mechanism is produced by spin-locking in the presence of gradients.
MRFM is to be distinguished from experiments where isolated electron spins are detected by scanning tunnelling microscopy (STM) [30, 160] via the modulated tunnelling current [319].
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© 1997 Springer-Verlag Berlin Heidelberg
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Kimmich, R. (1997). Imaging of Solid Samples. In: NMR. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-60582-6_35
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DOI: https://doi.org/10.1007/978-3-642-60582-6_35
Publisher Name: Springer, Berlin, Heidelberg
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