B0 shimming device for mri

A magnetic resonance (MR) apparatus comprising:

• a) magnet means for generating a main magnetic field in a sample region;
• b) encoding means for generating encoding magnetic fields superimposed to the main magnetic field,
• c) RF transmitter means for generating MR radiofrequency fields;
• d) driver means for operating said encoding means and RF transmitter means to generate superimposed time dependent encoding fields and radiofrequency fields according to an MR sequence for forming images or spectra;
• and
• e) acquisition means for acquiring an MR signal from said object;
• wherein said magnet means comprise a primary magnetic field source providing a static magnetic field B0 and at least one secondary magnetic field source providing an adjustable magnetic field B′;
• wherein said secondary magnetic field source comprises at least two spatially distinct portions of a first magnetic material and of a second magnetic material, respectively, said first magnetic material having a first magnetic moment density m1 and said second magnetic material having a second magnetic moment density m2, and means for independently adjusting said second magnetic moment density m2 by variation of an external control parameter,
• the apparatus further comprising means for determining an instant spatial distribution of the static magnetic field resulting from said static magnetic field B0 and said adjustable magnetic field B′.