MRI interactions with medical implants

Interactions of medical implants with the static, pulsed gradient, and radio frequency (RF) magnetic fields in MRI are described. Force and torque are exerted on the implant by the static field, which has a representative intensity of 1.5 T. The pulsed gradient fields are of audio frequency and induce currents in the patient. The intensity of the gradient currents may be close to the nerve stimulation threshold. A metallic implant will cause concentration of these currents, possibly resulting in nerve stimulation. The RF field has a frequency of 64 MHz for a 1.5 T static field. The principal RF bioeffect is tissue heating and an elongated implant will concentrate the induced RF currents. Temperature rises in excess of 10°C in the tissue near the tip of a lead wire may occur. The amount of RF-induced temperature rise depends on the implant geometry, its location in the body, and the type of MRI pulse sequence used. Functionality of medical implants may be affected by the MRI magnetic fields.