Automatic shimming for localized spectroscopy.

Localized in vivo nuclear magnetic resonance studies often require a high spectral resolution not achievable with the basic shim of a whole-body magnetic resonance magnet. Therefore, the magnetic field homogeneity needs to be optimized in the selected volume of interest within a reasonable time. For this purpose, a method of automatic shimming was developed and tested on phantoms and volunteers. The volume selection is performed by means of a surface coil or by using a localization method which generates a stimulated echo from the volume of interest. The optimization procedure uses the time integral over the magnitude of the free induction decay or echo signal as homogeneity criterion. A complete shimming process generally requires only 80 transients. Test experiments were conducted on various volume sizes ranging from 2 X 2 X 2 cm3 to 15 X 15 X 15 cm3 inside a large phantom. The resulting linewidth in small volumes at the magnet center compared well with the natural linewidth determined by means of the Carr-Purcell-Meiboom-Gill sequence. As expected, shimming in selected volumes at off center positions led to somewhat broader lines. Results obtained on volunteers demonstrate the practical value of this rapid, automatic shimming method for in vivo studies.