Numerical and experimental study of RF-induced heating of passive implantable medical devices at 5T MRI

Purpose

To assess the RF-induced heating of orthopedic implants in a 5T whole-body MRI system through electromagnetic simulations and experimental validation, with the goal of ensuring patient safety in ultra-high field (UHF) MRI.

Methods

Numerical and experimental studies were conducted to evaluate RF-induced heating in five titanium screws (4–12 cm) inside a 60-cm wide 5T whole-body MRI scanner using the standard ASTM phantom. The temperature rise over 15 min was determined through full-wave electromagnetic simulations and direct measurements. The Finite Difference Time Domain (FDTD) method was used to quantify the 1 g mass-averaged specific absorption rate (pSAR_1g) in 10 clinically relevant plate-and-screw configurations implanted in the Duke and Ella human body models at three anatomical locations: humerus, femur, and tibia.

Results

In the phantom study, the 6 cm screw exhibited the highest SAR and temperature rise, demonstrating a resonance effect at 5T. However, in human body models, the worst-case implant lengths shifted to 7–11 cm, highlighting the influence of tissue heterogeneity on resonance conditions. SAR values were also affected by the implant's position within the RF coil. The strong agreement between simulations and measurements validates the computational approach.

Conclusion

This study systematically evaluates RF-induced heating in orthopedic implants within the newly approved 5T whole-body MRI, demonstrating that implant length, positioning, and surrounding media significantly impact heating risks. The findings highlight the necessity for updated MRI safety guidelines at UHF strengths, as implant safety conditions at 5T systems may differ from those at 1.5 and 3T MRI systems.