MRI method for determining a magnetic field map from a B0 reference scan and a WASSR scan

A method of operating a medical imaging system, the method comprising:

• receiving magnetic resonance data, wherein the magnetic resonance data comprises B0 field data of a reference scan for a plurality of voxels and water saturation data of a WASSR scan for a subset of voxels of the plurality of voxels, the subset of voxels having fewer voxels than the plurality of voxels, the water saturation data comprising data of a limited number of sample points per voxel of the subset of voxels;
• determining a local absolute water saturation frequency for each voxel of the subset of voxels using the water saturation data of the WASSR scan;
• and
• reconstructing a field map comprising a local absolute water saturation frequency for each voxel of the plurality of voxels, wherein the reconstruction of the field map comprises determining relative frequency differences between the voxels using the B0 field data of the reference scan and adding a frequency offset to the relative frequency differences based on the determined local absolute water saturation frequencies of the subset of voxels.

A non-transitory computer readable medium storing machine executable instructions for execution by a processor controlling a medical imaging system, wherein execution of the machine executable instructions causes the processor to control the medical imaging system to:

• receive magnetic resonance data, wherein the magnetic resonance data comprises B0 field data of a reference scan for a plurality of voxels and water saturation data of a WASSR scan for a subset of voxels of the plurality of voxels, the subset of voxels having fewer voxels than the plurality of voxels, the water saturation data comprising data of a limited number of sample points per voxel of the subset of voxels;
• determine a local absolute water saturation frequency for each voxel of the subset of voxels using the water saturation data of the WASSR scan;
• and
• reconstruct a field map comprising a local absolute water saturation frequency for each voxel of the plurality of voxels, wherein the reconstruction of the field map comprises determining relative frequency differences between the voxels using the B0 field data of the reference scan and adding a frequency offset to the relative frequency differences based on the determined local absolute water saturation frequencies of the subset of voxels.