Radiofrequency Dosimetry in Subjects Implanted with Metallic Structures Undergoing MRI: a Numerical Study

A numerical study to investigate the effects of the exposure to radiofrequency (RF) field on biological tissues implanted with thin metallic structures has been carried out, using the finite difference time domain (FDTD) solution technique. The particular case of the RF used in the clinical 1.5 T magnetic resonance imaging (MRI) scanners has been studied. The results of the model show that the presence of a metallic wire yields to a significant increase in the local specific absorption rate (SAR) and in temperature around the implant. Present standards or guidelines do not specifically address this problem and use threshold levels and methods to define safe exposure conditions that cannot apply to reveal high SAR gradients, such as the ones generated by thin metallic implanted objects. In particular, an averaging mass of 10 g, typically used for SAR calculation to evaluate the effect of RF exposure of biological tissues, is too big if compared to the SAR gradient generated by thin metallic structures. The SAR estimation according to this method cannot be considered a reliable indicator to evaluate the potential damage due to the RF induced heating at the interface between the implant and biological tissues. Five wires of different lengths and thicknesses were simulated and the SAR at the wire tip was evaluated over 10g, 1g, 0.1g averaging mass: the SAR underestimation related to the 10g and 1g masses, compared to 0.1g mass, was greater than 90 % and 60 %, respectively. In conclusion, present standards cannot be applied to thin metallic implants. The amount of averaging mass should be chosen as a function of the dimension of the implanted object and of the SAR gradient that is generated around it. At the same time, the averaging volume has to be not too small to be just a single “hot spot”, not relevant for the evaluation of potential tissue damage due to RF exposure.