Safety of metallic implants in magnetic resonance imaging

Magnetic resonance (MR) imaging has become a commonly accepted medical procedure. Manufacturers of medical implant devices are submitting claims that their devices are safe and effective in a MR environment. This paper concentrates on the issue of heating of patients due to the interaction of metallic implants with the strong radiofrequency (RF) magnetic field produced by the MR scanner. The commercially available program XFDTD was used to calculate the specific absorption rate (SAR) distribution in a realistic model of the human body. The body contained a metallic implant and was exposed to RF magnetic fields at 64 MHz from a model of a MR birdcage body coil. The results of the calculation showed that the magnitude of the increased heating of tissues due to the presence of the metallic implant depended on the dimensions, orientation, shape, and location of the metallic implant in the patient. This increased heating of surrounding tissues primarily concentrates in a small volume near the tip of the metallic wire. When the whole-body SAR was normalized to 1 W/kg, a calculated value of 41 W/kg was obtained at this location if the absorption was averaged over 1 g of tissue. However, a maximum value of 310 W/kg was calculated when the absorption was averaged over 1/8 g of tissue.