Effects of Hip Prostheses In Situ Exposed to 64 and 128 MHz RF Fields

The interaction between the RF field and metallic prosthetic devices may lead to an increase in specific absorption rate (SAR) with consequent tissue heating in areas surrounding the prostheses and to distortions in the B1 field, thus changing the local image contrast. Numerical simulations of the electric field, B1 field, and local SAR averaged over 10 g of tissue (SAR10g) around bilateral CoCrMo alloy hip prostheses in situ in anatomically realistic voxel models of an adult male and female due to RF fields from a birdcage coil driven at 64 or 128 MHz are described. Results suggest that significant perturbation of the B1 field occurs close to a prosthesis and that the electric field is enhanced in the vicinity of the ends of a prosthesis leading to enhancement of local SAR10g in these regions. Caution is required when the whole-body-averaged SAR is 2 W kg−1 since the SAR10g close to an extremity of a prosthesis is likely to exceed standard/safety guideline limits under these conditions. Compliance could conflict with metal artifact reduction sequences using shorter B1 RF pulses with increased amplitude and consequently higher SAR. Keywords: RF safety; hip prostheses; local SAR; numerical modelling; metallic implants; MRI safety