Method and apparatus for compensation of field distortion in a magnetic structure using spatial filter

In an apparatus for nuclear magnetic resonance scanning and imaging (MRI) having a magnetic structure, said magnetic structure comprising:

• a cavity disposed therein and an opening exposing said cavity to receive a body part;
• first and second spaced apart ferromagnetic elements having surfaces which define a portion of said cavity;
• means for producing a magnetic field within said cavity, said opening causing said field to distort;
• means for compensating for said field distortion, said compensating means including a third ferromagnetic element disposed adjacent said first ferromagnetic element having at a surface interfacing with said cavity a magnetostatic potential different from said magnetostatic potential of said first ferromagnetic element.

A method for compensating for field distortion in a magnetic structure for MRI including first and second ferromagnetic elements which interface with and define a portion of an open cavity disposed therein, comprising:

• generating a magnetic field within said cavity, said open cavity resulting in field distortions in a first imaging region within the cavity;
• computing the field on a surface of a second region which encompasses said first imaging region;
• and
• reducing the distortion within said first imaging region including the step of controlling the magnetostatic potential of the surfaces of the first and second ferromagnetic elements which interface with the cavity but which lie outside said first imaging region.