Dependence of RF Heating on SAR and Implant Position in a 1.5T MR System

Purpose: We evaluated radiofrequency (RF) heating of a humerus implant embedded in a gel phantom during magnetic resonance (MR) imaging for the specific absorption rate (SAR), angle between the implant and static magnetic field (B0), and position of the implant in the irradiation coil. Methods: We embedded a stainless steel humerus implant 2 cm deep in tissue-equivalent loop and mass phantoms, placed it parallel to the static magnetic field of a 1.5T MR scanner, and recorded the temperatures of the implant surface with RF-transparent fiberoptic sensors. We measured rises in temperature at the tips of the implant by varying the SAR from 0.2 to 4.0 W/kg and evaluated RF heating of the implant for its angle to B0 and its displacement along B0 from the center of the RF irradiation coil. Results: RF heating was similar for the loop and mass phantoms because the eddy current flows through the periphery of both. As the SAR increased, the temperature at the implant tip increased, and there was a linear relationship between the SAR and temperature rise. The values were 6.4°C at 2.0 W/kg and 12.7°C at 4.0 W/kg. Rise in temperature decreased steeply as the angle between the implant and B0 surpassed 45°. In addition, as the implant was displaced from the center of the RF coil to both ends, the rise in temperature decreased. Conclusion: The rise in temperature in deep tissue was estimated to be higher than 1.0°C for SAR above 0.4 W/kg. RF heating was greatest when the implant was set parallel to B0. In MR imaging of patients with implants, there is a risk of RF heating when the loop of the eddy current is formed inside the body.