Power deposition inside a phantom for testing of MRI heating

The radio frequency (RF) magnetic field in magnetic resonance imaging (MRI) induces currents in the human body. The resulting power deposition is quantified in W/kg as the specific absorption rate (SAR). Medical implants may concentrate the induced RF currents, resulting in potentially large temperature rise near an implant. The ASTM standard describes methods and a phantom for in-vitro testing of heating near an implant in MRI and these methods have been used for testing of neural electrodes and other implants. In this work, calculations were made of the heating pattern in a phantom in order to better predict the in vivo temperature rise from the measured in vitro rise. Results on the coronal and axial cross sections at the torso landmark were obtained. The coronal plot shows that maximal SAR occurs near the edges of the phantom at the landmark. The axial plot shows left to right and top to bottom asymmetries in the heating due to elliptical polarization of magnetic field. Results also show that the SAR is less for the eye and neck and increases with increasing conductivity. In conclusion, the results of the methods developed here can enhance the efficacy of using in vitro measurements of RF-induced heating near implants to assess patient safety in MRI.