Various types of implants incorporate magnets for a variety of reasons. For example, the magnet may be used to retain the implant in place (e.g., certain prosthetic devices), to guide a ferromagnetic object into a specific position, to permit the functional aspects of the implant, to change the operation of the implant (e.g., adjustable shunt valves), or to program the device (e.g., certain cardiac devices). Because there is a high likelihood of perturbing the function, demagnetizing, or displacing these implants, MR procedures typically should not be performed in patients with these implants or devices. However, in some cases, patients with magnetically-activated implants and devices may undergo MR procedures as long as certain precautions are followed. For example, see section entitled Cerebrospinal Fluid (CSF) Shunt Valves and Accessories.
Implants and devices that use magnets (e.g., certain types of dental implants, magnetic sphincters, magnetic stoma plugs, magnetic ocular implants, otologic implants, and other similar devices) may be damaged by exposure to MR systems which, in turn, may necessitate surgery to replace or reposition them. For example, Schneider, et al. (1995) reported that an MR examination is capable of demagnetizing the permanent magnet associated with an otologic implant (i.e. the Audiant magnet). Obviously, this has important implications for patients undergoing MR procedures. Notably, cochlear implants often incorporate a magnetic component which may pose problems for the patient referred for an MRI examination. Please see the section entitled, Cochlear Implants.
Whenever possible, and if this can be done without risk to the patient, a magnetically-activated implant or device (e.g., an externally applied prosthesis or magnetic stoma plug) should be removed prior to the MR procedure. This may permit the examination to be performed safely. Knowledge of the specific aspects of the magnetically-activated implant or device is essential to recognize potential problems and to guarantee that an MR examination may be performed without risk.
Extrusion of an eye socket magnetic implant in a patient imaged with a 0.5-Tesla MR system has been described by Yuh, et al. (1991). This type of magnetic prosthesis was used in the patient after enucleation. For this prosthesis, a removable eye prosthesis adheres with a magnet of opposite polarity to a permanent implant sutured to the rectus muscles and conjunctiva by magnetic attraction through the conjunctiva. This “magnetic linkage” is intended to permit the eye prosthesis to move in a coordinated fashion with that of eye movement. In the reported incident, the static magnetic field of the MR system produced sufficient attraction of the ferromagnetic portion of the prosthesis to cause serious injury to the patient.
Certain dental prosthetic appliances utilize magnetic forces to retain the implant in place. The magnet may be contained within the prosthesis and attached to a ferromagnetic post implanted in the mandible or the magnetic component may be the implanted component. Therefore, whether or not an MR procedure may be performed without problems depends on the configuration of the magnetically-active dental implant and other factors.
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