TEMPORAL SPHERICAL-HARMONIC EXPANSION AND COMPENSATION OF EDDY-CURRENT FIELDS PRODUCED BY GRADIENT PULSES

A semiautomated technique for the determination of the gradient preemphasis parameters required in NMR experiments is presented based on a temporal spherical-harmonic expansion of the magnetic fields produced following a gradient pulse. The expansion provides the strength of the harmonic which has the same symmetry as the applied gradient and additionally provides data on the more complex fields generated. Since each of these harmonics can be produced by a coil in the spectrometer shim set, the induced eddy-current fields can be nulled by applying an appropriate waveform to each shim coil. The shape of each waveform is determined by the gradient-magnet transfer function which can be calculated from the measured harmonic responses. © 1993 by Academic Press, Inc.