Assessing Implant Safety and RF Heating In an Elekta-Unity
Abstract
Purpose
This work aims to quantify RF-induced heating and local SAR distributions near passive metallic implants within the Elekta Unity. Given the system operates in First Level Mode (4W/kg wbSAR) and the prevalence of implants in the MR-guided adaptive radiotherapy (MRgART) population, assessing thermal safety under these elevated limits is critical.
Methods
Electromagnetic and thermal simulations were performed using a finite-difference time-domain solver (Sim4Life). RF pulses were tuned to 64MHz, approximating the Marlin 1.5T RF transmit-receive system. A virtual human phantom (IT’IS) was selected to match the average biometrics of the local MRgART patient population. Simulated implantation of an open-source hip implant model was performed in situ, with any void filled by water. Simulations were normalized to 4W/kg whole body SAR over 15m of RF exposure.
Results
Spatial distributions of local SAR peaked in the periphery as expected and at the head and stem of the implant. Under baseline operating conditions (500W input power), a ΔT of 0.64℃ was observed at the tip of the stem. However, when normalized to 4W/kg, a 15m MRI examination led to ΔT upwards of 6.3℃ at the tip of the stem. Temperature difference maps indicated increased heating in the leg and pelvic regions, up to 0.55℃. Temperature rises of about 0.1℃ were found at the left lateral edge of the prostate with the hip prosthesis present
Conclusion
Increases in local SAR under First Level Mode restrictions with implantable devices underscore the need for further investigation into local heating and gradient field induced heating in various implantable devices common in patients treated on MRgART systems. Furthermore, nearby heating may increase the TER a non-trivial amount in adjacent treatment areas, suggesting future work assessing CEM43 as a first step into assessing TER.