NMR microscopy

NMR microscopy has become a well-established method in many different areas of research. The scope of the disciplines involved is extremely broad and is still expanding, encompassing chemical, petrochemical, biological and medical research, plant physiology, aerospace engineering, process engineering, industrial food processing, materials and polymer sciences. 

The most difficult materials to study by NMR microscopy are those with short T2 or T2 relaxation times and/or with low concentrations of the nudear spins, which normally result in poor NMR signal intensities. One possibility for improving the image quality is to adapt the shape and size of the rf coils to the size of the objects in order to achieve the best possible filling factor and therefore the best sensitivity [1]. In addition, methods with short echo or detection times have been developed, such... 

NMR microscopy systems are built as add-ons to NMR spectrometers. The general technology of an NMR spectrometer is not the subject of this chapter, only the parts that are of particular importance in NMR microscopy applications, such as the magnets, the shim systems, the gradient systems, the NMR microscopy probes and the gradient amplifiers, will be described in some detail. 

The types of objects and the corresponding areas of research where NMR microscopy is involved are manifold [14] ... 

Some topics and the number of contributions per selected category vary from conference to conference as a result of changes in the major areas of research and by the number of conference participants per research area. Other conferences take place covering specialized topics where NMR microscopy plays an important role, e.g., the International Conference on Magnetic Resonance in Porous Media , the International Society for Magnetic Resonance in Medicine or the International Conference on Application of Magnetic Resonance in Food Science . 

NMR microscopy is ultimately an innovative method of research and it is not surprising that most of the commercially installed systems, approximately 80%, are installed in public scientific research centers, where new applications are continuously being developed. The method is not particularly widely distributed in industry, where standardized methods are more often used. However, NMR microscopy is mainly used in the pharmaceutical industry for the development of new drugs, in the food industry for the development of new types of food, in the chemical industry for creating and characterizing new materials and in the polymer industry, e.g., for creating new mixtures for tires. 

Many investigations are made using the commercially available standard NMR microscopy hardware and software, although in some cases this hardware is modified in order to fulfill specific requirements and to expand the number of possible applications. Such modifications and expansions then become part of new commercially available hardware and software if they are useful for a larger number of users, as was the case recently with the development of the Rheo-NMR [17]. 

Some applications are shown in the following sections, where the standard hardware, software and methods have been partially modified or connected to special experimental constructions. In the past, other applications have been performed by specific groups, who built their own dedicated rf probes and/or gradient systems. These originally specialized products have now found their way into the commercially available NMR microscopy products of today [18, 19]. 

Fig. 2.1.6 Unassembled NMR microscopy probe with dedicated 15 mm resonator for expanded temperature ranges between — 100 and +200 °C (probe base, glass dewar, rf resonator, temperature sensor and fixation parts).

NMR microscopy is appropriate to study the flow behavior of complex materials, the flow in complex geometric structures and processes such as extrusion, injection moulding, flow in nozzles, pipes, etc., because the velocity vectors can be directly... 

For a manufacturer the difficulty is to estimate future developments or trends in NMR microscopy. Based on dedicated laboratory-made hardware developed by the NMR microscopy users and on their requests for new commercial hardware and software, the following topics could become more important micro-coil applications, multiple receiver systems and multi-coil arrangements, NMR microscopy at very high magnetic fields, MAS imaging and localized 1H MAS spectroscopy and localized single-shot 2D spectroscopy. There are no clear-cut distinctions between most of the individual topics, as will be discussed in the following sections. 

Fig. 2.1.13 Planar spiral micro-coil of 1.2 mm outer diameter (left), mounted as an exchangeable rf insert on a standard NMR microscopy probe (right) (Bruker-Biospin GmbH, Rheinstetten, Germany). The micro-coil is embedded, together with a glass sample holder, in a slotted PTFE holder.

Multiple receive systems with multi-coil arrays have become widely distributed in medical MRI. The benefit is the enhancement of the signal-to-noise ratio per time or a reduction of the acquisition time. This technique is not used in NMR microscopy for objects of intermediate size in standard bore (52-mm id) and wide bore (89-mm id) magnets, which are the most widely distributed magnet types for NMR microscopy. The main reason is the restricted space in such magnets for the shim... 

Fig. 2.1.17 Conventional NMR microscopy gradient system (Micro2.5, 2.5 C cm-1 A-1, 40-mm inner diameter) (right) and a conventional MAS probe (left). The probe is mounted into the separate gradient system instead of the original imaging probe.

A quite different approach to rheo-NMR was taken by Xia and Callaghan [12], in an NMR microscopy measurement of the velocity profile of a high molecular weight polymer solution flowing through a capillary. In this study anomalous polymer diffusion was found at a radius within the pipe at which the local shear rate exceeded... 

P. T. Callaghan 1991, Principles of NMR Microscopy, Clarendon Press, Oxford. 

A. Klemm, H. P. Muller, R. Kimmich 1997, (NMR microscopy of pore-space backbones in rock, sponge, and sand in comparison with random percolation model objects), Phys. Rev. E 55, 4413. 

Y. Xia, P.T. Callaghan 1991, (Study of shear thinning in high polymer solution using dynamic NMR microscopy), Macro. Mol. 24 (17), 4777 1786. 

Hyperpolarized 129Xe NMR Spectroscopy, MRI and Dynamic NMR Microscopy for the In Situ Monitoring of Gas Dynamics in Opaque Media Including Combustion Processes... 

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