Resolution in NMR Imaging

The attainable resolution is limited by spectroscopic and hardware factors. Spectroscopic factors are the linewidth and the spread of the chemical shift of an NMR signal, diffusion processes and susceptibility gradients, both within the object and at its boundaries. Hardware factors may be the magnetic field inhomogeneity or instability, nonlinearity of the magnetic gradient field and the achievable signal-to-noise ratio. 

The difficulties of solid state imaging arise because the solid state linewidth is 1000 times its solution counterpart. Increasing the gradient by three or four orders of magnitude to maintain spatial resolution in solids imaging is a formidable task, and much effort has gone into finding alternatives to such a brute force approach [3]. 

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Many methods for improvements of the spatial resolution in NMR imaging of solid materials have been proposed with different impact on applications in polymer science [8, 9, 11-13, 21]. For the great variety of techniques only those are reviewed in Sections 5.1.3 and 5.1.4 below, which are being applied successfully to problems in polymer science. Depending on the dominating features, the techniques can be classified into frequency and phase encoding approaches. 

Resolution in NMR imaging of solids is primarily limited by the low sensitivity of NMR. Typical detection limits are of the order of 10 spins in the solid state, so that minimum voxels are approximately 104p,m (or a cube 20 pm on edge). Along any one dimension the resolution can be somewhat higher, but the volume remains approximately constant. The various approaches to solid state imaging present additional limitations to the resolution. 

The new avenues in NMR imaging may be focused on (a) improving spatial resolution, (b) imaging protons with short relaxation timeT2, and (c) imaging nuclei other than proton. 

Figure 7.2 Generation of spatial resolution in NMR (a) Conventional NMR imaging with magnetic field gradients. A magnetic field gradient Gx in x0 direction (top) converts the NMR spectrum (bottom) of an object (middle) into a projection of the object, (b) Localization of NMR signals from a large object by use of a surface coil...

In principle, the determination of molecular uptake may be based on any experimentally accessible quantity which is a function of the amount adsorbed. Being directly sensitive to a certain molecular species, in this respect the application of spectroscopic methods is particularly suitable. IR spectroscopy has been successfully applied to studying molecular uptake by beds of zeolite catalysts [26-28] as well as—in combination with IR microscopy [29, 30]—on individual crystallites. Similarly, NMR spectroscopy has also been used to monitor the time dependence of the sorbate concentration within porous media [31]. Moreover, recent progress in NMR imaging allows the observation of concentration profiles within porous media with spatial resolution below the mm region [32-34],... 

The approximation of f(t) being a constant defines the limitations in spatial resolution of NMR imaging by frequency encoding. Here, the pulse response is acquired in the presence of a time-independent gradient, say = dBJdx. After the rf excitation pulse ky grows linearly with time according to Equation... 

The principles of spatial resolution and contrast in NMR imaging have been presented in this chapter. An overview of selected applications of NMR to investigations of fluid systems, technical elastomers and rigid polymers has been given. The examples chosen demonstrate the potential of NMR for measurement of macroscopic properties of polymer materials. The importance of developments of NMR methods and equipment for materials science applications was underlined by example of the NMR MOUSE. 

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