Improvements on an in vivo automatic shimming method (FASTERMAP)

Improvements on a localized, automatic shimming method described by Gruetter and Boesch (J. Magn. Reson. 96, 323-334 (1992)) and Gruetter (Magn. Reson. Med. 29, 804-811 (1993)) are presented. A spin-echo sequence employing a double sech refocusing scheme is used to acquire a field map along linear projections that improves the signal-to-noise ratio by at least a factor of two over the stimulated echo sequence previously used. To further improve the reliability of shim adjustments, a variance-weighted polynomial regression analysis is performed. This also extends the scope of application of this technique to global shimming. Localized H-1 spectra of human brain shimmed by this method are presented.