Shim system

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 whole NMR imaging sensor system usually consists of a magnet, a shim system mounted inside the room-temperature bore of the magnet, a gradient system mounted inside the shim system and the rf coil mounted inside the gradient system. In the case of a saddle coil or a birdcage resonator, open access can be realized from the bottom to the top of the entire system with the coil diameter. 

If the spectrum involves only one resonance (or if linewidths do not allow for the separation of several resonances), a single experiment can be run with acquisition of the amplitude of each echo along the pulse train (for sensitivity enhancement, accumulations can be carried out). This experiment is especially valuable for determining the relative proportions of two species which differ by their transverse relaxation time, for instance the two types of water (free and bound), if exchange between these two states is sufficiently slow. For this type of measurement, a low resolution spectrometer (without any shim system) proves to be quite sufficient. 

Bruker Smart Magnet control System (BSMS) including the digital deuterium lock system and the BOSSl shim system... 

As chemists performing spectroscopy, we are usually looking for the highest spectral resolution possible. Generally, one of the factors that limit the spectral resolution is the uniformity of B0 (the homogeneity of the static field) over the volume of the sample. If the field strength differs from point to point within the sample, there will be a similar point-to-point variation of Larmor frequency, and the width of the resonance line will reflect that distribution in frequency. The shim system (Section 3.2), which is used in a spectrometer to improve B0 homogeneity, consists of a set of electrical coils within the... 

Lineshape and resolution tests on other nuclei follow a similar procedure to that above. Not all nuclei available with a given probe need be tested and typically only tests for inner and outer coil observations on multinuclear probes are required. This means the second test will often involve carbon-13 for which two samples are in widespread use the ASTM (American Society for Testing and Materials) test sample (40% p-dioxane in deuterobenzene also used for the sensitivity test) or 80% benzene in deuteroacetone. In either case on-resonance continuous-wave (CW) decoupling of protons should be used as this provides improved results for a single resonance relative to broadband decoupling. Rather long (30-40 s) acquisition times will be required for a well shimmed system. 

Figure 9.20 A diagrammatic representation of the main components of an NMR spectrometer (1) magnet (2) probe (sample holder + radiofrequency coil) (3) radiofrequency amplifier and pulse generator (4) computer (5) recorder (6) main console (7) lock and shimming systems (8) magnet power source.

The chemical shim system uses the soluble neutron absorber boron (in the form of boric acid), which is inserted in the reactor coolant during cold shutdown, partially removed at startup, and adjusted in concentration during core lifetime to compensate for such effects as fuel consumption and accumulation of fission products which tend to slow the nuclear chain reaction. The control system allows the plant to accept step... 

Control rods perform the reactivity control a soluble boron chemical shim system is not used in the IMR except for the backup shutdown system. 

---