MRI-induced SAR on Pacemaker Leads - Numerical Simulations on Three Human Phantoms

Numerical simulations were performed to evaluate the Specific Absorption Rate (SAR) induced at the tip of a pacemaker (PM) implant by the 64 MHz radiofrequency (RF) field used in 1.5T Magnetic Resonance Imaging (MRI) procedures. The analysis was performed by using a commercial FDTD software (SEMCAD X, SPEAG, Switzerland) and aimed at the evaluation of the impact that the patient ‘s morphology has on the induced local SAR at the implant tip. In particular three human phantoms were studied: a 34-year old man model, a 26-year old woman, and a 6-year old boy. The three phantoms reproduce more than 70 tissues of the human body with a resolution of 1 mm. Inside each phantoms, realistic implant configurations were modelled, considering both left and right pectoral implants, and atrial and ventricular stimulations. The local SAR values at the lead tip was compared for the three phantoms and sensible differences were observed: with a RF excitation set to produce an whole-body average SAR of 2 W/kg without any implants, local SAR values ranged from 641W/kg (woman model – right ventricular implant) to 3 W/kg (boy model – left atrium implant). We also observed that, in general, ventricular implants showed a higher SAR compared to atrial ones, as well as right pectoral implants compared to left ones. However, not always a higher implant area or a longer lead path implied higher SAR at the tip, indicating the coupling mechanisms between the implant and the RF field are likely to be more complex that the only area-dependent induction law.