gms | German Medical Science

Objective: The number of clinically oriented 7 Tesla MRI studies is increasing in the last years. Most 7 T research sites still conservatively exclude all subjects with metallic implants, regardless of type or location. The purpose of this study was to investigate potential RF-induced heating of titanium miniplates during a 7 T MRI scan using a custom-built transmit/receive RF coil. Signal distortion and signal loss around the implanted miniplates after craniotomy should be assed in vivo for several MRI pulse sequences.

Method: The safety assessment consisted of three main investigations using (1) numerical simulations in simplified models, (2) electric and magnetic field measurements and validation procedures in homogeneous phantoms, and (3) analysis of exposure scenarios in a heterogeneous human body model including thermal simulations. Signal distortion and signal loss around the implants were assessed in vivo in five patients after craniotomy with implanted miniplates.

Results: Numerical calculations showed, that field distortions remain localized within the direct vicinity of the implants. Parallel field polarization was found to be the worst case, resulting in a 10% increase in 10-g-averaged SAR and 30% in point-wise SAR. Using a heterogeneous human head model, the implants caused field distortions and SAR elevations in the numerical simulations that were distinctly lower than the maximum local SAR value caused by the RF coil alone. Also, the position of the maximum 10-g-averaged SAR remained unchanged by the presence of the implants with maximum local temperature increase remaining below 2 °C in 2 thermal simulations. Implants caused only minor artifacts in vivo at 7 T that would not likely affect the diagnostic image quality.

Conclusions: In vivo RF-related heating is negligible after bone flap re-fixation with titanium miniplates using our custom-built RF coil at 7 Tesla. Identical transmit power restrictions apply with or without the implants. Diagnostic image quality is not likely to be affected by implant related artifacts.