The invention relates to a method of acquiring MRI image data comprising the following steps: performing a 3 -dimensional Bl mapping of a first volume using a first voxel size, selecting an MRI protocol, performing the Bl -shim in accordance with the MRI protocol, performing the MRI protocol to acquire MRI imaging data of a second volume using a second voxel size, wherein the first voxel size is larger than the second voxel size, wherein the first volume is larger than the second volume, and wherein the second volume is contained within the first volume.
