Dispersion versus absorption (DISPA) plots as an index of static magnetic field inhomogeneity. Use for adjustment of spinning shims in a superconducting magnet

Abstract

A new method for evaluating static magnetic field inhomogeneity in an NMR sample is described. Deviation from circularity of a plot of dispersion vs absorption (DISPA) provides a measure of the deviation of an experimental NMR lineshape from the ideal Lorentz shape. From DISPA plots constructed from spectra simulated from specified z-gradient (“spinning shim”) inhomogeneity terms, it is clear that the magnitude and direction of displacement of an experimental DISPA curve from its reference circle can serve to discriminate and quantitate the inhomogeneity in each of z, z 2, z 3, z 4, and z 5. As an index for monitoring field inhomogeneity, DISPA is more sensitive than monitoring the absolute area under a free induction decay (particularly for higher order spinning shims), and much faster and more discriminating (between different z n terms) than iterative fitting of the frequency-domain absorption spectrum. Although each of the experimental spinning shim coils does not produce a z gradient corresponding exactly to its labeled z n function, we find that experimental DISPA plots nevertheless correlate well with their theoretical counterparts for z 1 and z 2. DISPA has already proved useful for automated phasing of FT NMR spectra; the present work suggests that DISPA may offer a simple, rapid, sensitive method for testing shim coil designs and ultimately for manual or automated shimming of the static magnetic field as well.