In one embodiment, a ferromagnetic frame for use in an MRI system comprises first and second opposing ferromagnetic elements magnetically coupled by a flux return path. First and second poles are provided, separated by a distance to define an imaging volume therebetween. The first pole is movably supported...http://www.google.com.hk/patents/US7215231?utm_source=gb-gplus-share專利 US7215231 - MRI system
