Variability in RF-induced heating of a deep brain stimulation implant across MR systems

Purpose: To compare the MRI-related heating per unit of specific absorption rate (SAR) profile of a conductive implant between two 1.5-Tesla/64 MHz MR systems using a transmit/receive (t/r) head coil configuration. Materials and Methods: Deep brain stimulation (DBS) leads were configured within a gel-filled phantom of the human head and torso. Temperature variation at each of four contacts of the bilaterally-placed leads was monitored using fluoroptic thermometry. MRI was performed using the t/r head coils of two different-generation 1.5-Tesla MR systems from the same manufacturer. Temperature changes were normalized to SAR values for the head (Delta T/SAR-H), and the slope of this Delta T/SAR-H by time relationship was compared between the two scanners. Results: The Delta T/SAR-H for the implant ranged from 3.5 to 5.5 times higher on one MR system as compared to the other (P < 0.01) depending on the measurement site. Conclusion: The findings support previous observations that console-reported SAR does not constitute a reliable index of heating for elongated, conductive implants, such as the DBS hardware system tested. In contrast to our previous findings using a t/r body coil, the data presented here reveal marked differences between two MR systems using t/r head coils (the coil configuration was consistent with the implant manufacturer's imaging guidelines).