Considerations in the design of MRI magnets with reduced stray fields

One of the major considerations in siting choice for an MRI system within a hospital is its interactions with its environment. This interaction places restrictions on the proximity of equipment sensitive to magnetic field and limits areas of general public access. In addition special account must be taken of the possible impact of environmental iron on magnet homogeneity. To date, the approach adopted by the MRI system scanner manufacturers to these problems has been to employ either YOKE or MIRROR iron (PASSIVE) shielding frequently requiring significant structural modifications with associated costs. An alternative approach to the shielding problem has been investigated at OXFORD using a geometry utilising superconducting counter running coils alone and prototypes with Central fields of 0.5T, 1.0T and 1.5T have been tested.