DYNAMIC SHIM UPDATING - A NEW APPROACH TOWARDS OPTIMIZED WHOLE-BRAIN SHIMMING

The static magnetic field within two widely spaced axial slices of the human brain was mapped in five subjects following global shimming. Th is revealed a first order field shift in the anterior-posterior direct ion between the cerebellum and cerebrum, which has implications for fu nctional and spectroscopic magnetic resonance imaging, A new method is described called dynamic shim updating (DSU) to compensate for these field differences whereby the shim correction fields are updated in re al time during multislice data acquisition to match the current imagin g or spectroscopy slice, A hardware unit is presented to demonstrate t he method using the first order shim corrections, which can be updated virtually instantaneously between slice acquisitions to give optimal shimming of each slice. The efficiency of the approach is demonstrated using field mapping and high speed MR imaging (echo-planar imaging), which are sensitive to field inhomogeneity.