[Metallic middle-ear implants in magnetic resonance tomography: hearing impairment and other hazards]

Concurrent with the expanding clinical applications of nuclear magnetic resonance (NMR) imaging, patients with metallic middle-ear implants will certainly be exposed to this strong magnetic field in the future. To determine potential hazards, associated with movements of steel- or Platinum stapes-prostheses, several tests were performed in a 0.5 tesla NMR unit and the induced forces were calculated. Although the commonly used paramagnetic steel-wire or platinum-alloys will not dislodge in vivo, ferromagnetic prostheses may present a risk. Prior to exposure to the magnetic field, information about the implanted material should therefore be obtained. A side-effect of the induced current flow is the attenuation of the sound-vibrations of the stapes prosthesis. This, 5-10 dB impairment of transmission develops only at a certain position of the patient's head, when the prosthesis vibrates perpendicularly to the magnetic field's Z-axis. Patients with a metallic prosthesis should be informed about this purely physical, harmless phenomenon prior to entering the NMR-cylinder.