When a metallic implant is exposed to the high strength magnetic fields of MRI systems, it can experience rotational and translational forces. Studies have shown that these forces can be great enough to dislodge or move the implant when surrounded by soft tissues within the body. Implants must be able to withstand exposure to MRI field strength and the resulting forces to assure patient safety during MR procedures.
Device magnetic resonance compatibility and testing criteria should be evaluated using measurements of actual magnetic field strength in the bore of the MRI. Four MRI systems were studied to document maximum field strength exposure. Maximum magnetic field strength, directly affecting the rotational force, was located 70cm from the opening of the magnet bores. Maximum gradient, directly affecting the translational force, was located over a range of 30 - 50cm from the opening of the magnet bores, or where the measured field was 60 - 75% of the rated magnetic field strength. All maximal locations were inside the magnet bore, off the center axis, and near the walls of the magnet bore. These measurements can be used to identify optimal testing criteria for evaluation of the MR compatibility of metallic implants.