Numerical study of SAR for multi-component orthopaedic hip replacement system during MRI

In this study we present numerical simulations of the Specific Absorption Rate (SAR) for multi-component orthopaedic hip replacement systems. The SAR is used to evaluate the radio frequency (RF)-induced heating of the devices during magnetic resonance imaging (MRI). Because multi-component orthopaedic hip replacement systems have many combinations of components with various designs and sizes, it is computationally intensive, and almost impossible, to evaluate the SAR and the corresponding temperature rise for each possible combination and configuration. In this study, an effective searching strategy and a computational simulation model are developed to evaluate the factors associated with induced SAR in the tissue near an orthopaedic hip replacement system, and to find the “worst case” peak SAR for all possible combinations. The finite-difference time-domain (FDTD) was used to calculate the peak SAR for a typical hip replacement system inside the American Society for Testing and Materials (ASTM) phantom for both 1.5 Tesla (T) and 3T MRI systems. The results indicate that the stem and screw lengths are the most important factors influencing the peak SAR for both field strengths, 1.5T/64 MHz and 3T/128 MHz, respectively. The peak 1 gram averaged SAR reaches 216 W/kg and 103 W/kg for 64 MHz and 128 MHz, respectively. We also found that shortest stems, and the longest screws, typically induce higher peak SAR.