Methodological Issues on the Estimation of the MRI-Induced SAR in Tissues in Contact with Implanted Thin Metallic Structures

Abstract

AbstractA numerical study on the effects of the exposure to the radiofrequency (RF) field of 1.5T magnetic resonance imaging (MRI) scanners on biological tissues in contact with implanted thin metallic structures has been carried out. The results of the model show that the presence of a metallic implant yields to a significant increase in the local specific absorption rate (SAR). Present standards or guidelines do not specifically address the problem and use methods to define safe exposure conditions and limits that cannot reveal high SAR gradients, such as the ones that rise close to thin metallic implanted objects. Such standards suggest a mass of 10 g as averaging volume for SAR calculation. We computed the SAR distribution inside a “human visible dataset” (HVD) torso model, implanted with a pacemaker and its lead, considering both the left and right pectoral positioning. The RF induced SAR was calculated over a 10g, 1g and 0.1g volume: the underestimation related to the 10g and 1g masses, compared to the 0.1g mass, was greater than 90% and 60%, respectively. In addition, the deposited power at the implant tip inside the HVD model was compared to the one of simplified models, where the same physical properties were assign to different tissues, or only particular body district were modeled: a variation ranging ±30% was observed.KeywordsMRIRF fieldImplantable devicesSAR