Environmental considerations

Mapping Intimacies ◽

10.1093/oso/9780199565092.003.0012 ◽

2017 ◽

Author(s):

A. G. Wright

Keyword(s):

Magnetic Fields ◽

Collection Efficiency ◽

Liquid Argon ◽

Harsh Environments ◽

Oil Well ◽

High Permeability ◽

Electric Field Gradients ◽

Field Gradients ◽

High Shock ◽

Power Spectral

Magnetic fields, with a magnitude comparable with that of the earth (10−4 tesla), affect trajectories of electrons and hence gain and collection efficiency. The inclusion of a high-permeability shield usually offers sufficient protection. Photomultiplier (PMT) performance is affected by electric field gradients generated by the proximity of a metal housing. The design criteria of such housings are discussed. Strong magnetic fields of the order of a tesla require special devices. Operation in harsh environments such as those encountered in oil well logging requires performance at high temperature (200 °C) and in situations of high shock and vibration expressed in terms of power spectral density. Rugged PMTs can meet all these requirements. Applications at cryogenic temperatures, such as liquid argon, can also be met with special PMTs.

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Nuclear magnetic resonance microscopy

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100156110 ◽

1989 ◽

Vol 47 ◽

pp. 828-829

Author(s):

Paul C. Lauterbur

Keyword(s):

Magnetic Field ◽

Nuclear Magnetic Resonance ◽

Magnetic Resonance ◽

Magnetic Fields ◽

Signal To Noise ◽

Field Gradients ◽

Useful Signal ◽

Magnetic Field Gradients ◽

Observation Times ◽

Nuclear Magnetic

Nuclear magnetic resonance imaging can reach microscopic resolution, as was noted many years ago, but the first serious attempt to explore the limits of the possibilities was made by Hedges. Resolution is ultimately limited under most circumstances by the signal-to-noise ratio, which is greater for small radio receiver coils, high magnetic fields and long observation times. The strongest signals in biological applications are obtained from water protons; for the usual magnetic fields used in NMR experiments (2-14 tesla), receiver coils of one to several millimeters in diameter, and observation times of a number of minutes, the volume resolution will be limited to a few hundred or thousand cubic micrometers. The proportions of voxels may be freely chosen within wide limits by varying the details of the imaging procedure. For isotropic resolution, therefore, objects of the order of (10μm) may be distinguished.Because the spatial coordinates are encoded by magnetic field gradients, the NMR resonance frequency differences, which determine the potential spatial resolution, may be made very large. As noted above, however, the corresponding volumes may become too small to give useful signal-to-noise ratios. In the presence of magnetic field gradients there will also be a loss of signal strength and resolution because molecular diffusion causes the coherence of the NMR signal to decay more rapidly than it otherwise would. This phenomenon is especially important in microscopic imaging.

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