Pole piece for MR imager

A magnet for use in magnetic resonance imaging, the magnet comprising:

• a magnetically permeable core;
• two pole piece assemblies, each pole piece assembly having a face, said pole piece assemblies disposed on said core such that the pole faces are in opposed relationship forming first gap therebetween, at least one of said pole piece assemblies comprising
• a first ferromagnetic pole piece connected to the core and forming a cup having an inner face facing the first gap;
• and
• a second ferromagnetic pole piece movably mounted in the cup and covering substantially all of the inner face, the second pole piece forming at least a portion of the face of the pole piece;
• means for producing a main magnetic field in the gap;
• and
• means for translating the second pole piece within the cup in a direction along the principal plane of the inner pole piece.

A magnet for use in magnetic resonance imaging, the magnet comprising:

• a magnetically permeable core;
• two pole piece assemblies, each pole piece assembly having a face, said pole piece assemblies disposed on said core such that the pole faces are in opposed relationship forming first gap therebetween, at least one of said pole piece assemblies comprising
• a first ferromagnetic pole piece connected to the core and forming a cup having an inner face facing the first gap;
• and
• a second ferromagnetic pole piece movably mounted in the cup and having a weight approximately one-half the weight of the first pole piece, the second pole piece forming at least a portion of the face of the pole piece;
• means for producing a main magnetic field in the gap;
• and
• means for translating the second pole piece within the cup in a direction along the principal plane of the inner pole piece.