[MRI-Interactions with magnetically active and electrically conductive material]

Patients undergoing MRI examinations are exposed to a strong static magnetic field and powerful electromagnetic alternating fields. Undesired or even dangerous effects could be caused if implants or objects with magnetic or electrically conductive elements are accidentally brought into the examination area.

Relevant interactions in MRI between magnetic/electric fields and body tissue as well as foreign materials are systematically presented, based on proven physical principles.

Natural components of the human body are mainly diamagnetic leading to only hardly perceptible magnetic forces in MRI. In contrast, ferromagnetic items as iron show translational forces of more than hundred times their weight force when brought to the entry of the bore. Lengthy ferromagnetic items are additionally subjected to torque. Materials with high electrical conductivity as metals and carbon fibre-reinforced plastic are also safety relevant. Especially long conductive structures as often present in implants are prone to induced strong electrical currents and high voltages at their end portions. Maximum voltages occurring at the implants and current density in adjacent tissue which might cause significant heating are hardly predictable for individual cases. Implants providing extended conductive loops for ring currents often show strong vibrations due to gradient switching. Counter forces must be considered when tilting conductive plates or ring structures inside the magnetic field area.