Shimming is a major requirement for in vivo NMR spectroscopy and is becoming increasingly important also for imaging methods, such as those used for fMRI. The importance of shimming to the quality of the MR experiment increases with field strength due to the induced macroscopic susceptibility changes, which can be on the order of several ppm. A major limitation for shimming in vivo is the time requirement and the need to shim higher-order coils at locations distant from the isocenter, where intrinsic shim coil orthogonality is lost (1). Several methods have been proposed, most of which rely on mapping the field in image planes (e.g., 2,3). The purpose of this study was to investigate a method (FASTMAP) which is based on the measurement of the magnetic field along selected bars in space, with greatly reduced computational and measurement requirements, whether shimming at 4T using FASTMAP is feasible and to explore the potential improvements possible with this technique. The present experience suggests that second-order shim coils can improve spectral resolution substantially and provide reproducible line shapes for volumes localized throughout the entire brain.
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