| Radiofrequency (RF) electromagnetic radiation exposure during a magnetic resonance imaging (MRI) examination can cause unwanted heating of the patient. This heating can be localized near metallic objects in the body such as chronic implants or invasive metallic devices used during MRI-guided surgical interventions. Previously, metallic implants were generally considered a contraindication for MR imaging. However, with the recent advances in interventional MRI, which have enabled MRI guidance of surgical interventions without the harmful long-term effects of exposure to ionizing radiation as is the case with conventional x-ray fluoroscopy, there has been renewed interest in understanding the local RF heating problem from invasive and implanted metallic devices.; Previous studies have identified some of the variables that affect the relative magnitude of RF heating with metallic devices, but until now, no safety study has been able to establish safety thresholds for using metallic devices during MRI. This is primarily because there has been no comprehensive framework established for studying the problem of RF heating. In addition, there is difficulty in applying present regulatory guidelines for RF heating in practice to address the highly localized heating that can occur with internal metallic devices. Thus, the methods used to examine RF heating with metallic devices have been inconsistent and often deficient.; In this work, a model for local RF heating is presented and used to establish safety thresholds for a representative device in each of the two main classes of devices—internal RF transmitters and passive internal metallic devices. This is the first time in the published literature that safety thresholds have been developed for specific internal metallic devices.; The RF heating model also serves as a basis for (i) a rationale for updating regulatory guidelines; (ii) a new metric for reporting the safety of an internal metallic device (the safety index); and (iii) a simple, new method for measuring the RF safety of a metallic device. Finally, new strategies for improving the RF safety of MRI with metallic devices are presented, both by designing safer devices and by designing safer scanners. |
