CORRECTION FOR THE EFFECT OF INDUCED B-0 SHIFTS IN LOCALIZED SPECTROSCOPY AND IMAGING BY DIRECT FREQUENCY-MODULATION

A new method is presented for the amelioration of eddy-current-induced B0 shifts based on direct FM modulation of the spectrometer transmitter and receiver reference frequencies. The waveform for correction is based on a dynamic spherical-harmonic expansion of the temporal field, requiring no functional fitting and accounting fully for all gradient transitions in a given sequence. The importance of fully sequence-specific correction of these B0 shifts during slice selection in imaging and localized experiments is demonstrated experimentally and modeled in part using rotation-matrix calculations. The results demonstrate the utility of the correction technique and indicate that it is advantageous to correct even the small shifts associated with shielded gradients in order to produce optimal slice refocusing. © 1994 Academic Press, Inc.