Interactions of medical implants with the magnetic fields in MRI

Interactions of the static, gradient and radio frequency (RF) magnetic fields in a magnetic resonance (MR) imager with implanted medical devices are qualitatively and quantitatively reviewed. The static field has typical intensity of 1.5 T, but may be as large as 8 T. Spatial variations in the static field induce potentially large forces on magnetic components. Torque is exerted on magnetic objects, tending to align the long axis along the field. Moving conducting objects will experience force and torque due to induced eddy currents. The gradient fields are of audio frequency and peak intensity of the time-varying magnetic field dB/dt may exceed 100 T/s. Metallic implants tend to locally focus the gradient currents, enhancing the potential for peripheral nerve stimulation. The frequency of the RF magnetic field is proportional to the intensity of the static field and is 64 MHz for 1.5 T. The principal bioeffect of the RF field is tissue heating. Elongated metallic implants will locally focus the RF currents, resulting in the potential for dangerous temperature rises. From calculation and measurement of the interactions of an implant with the fields in an MR imager, the safe operating conditions for a patient with an implant may be determined.