Simulated and experimental approaches to the development of novel test phantoms for radiofrequency heating of implanted medical devices

Magnetic resonance imaging (MRI) has cemented itself as the gold standard for imaging of soft tissues and is only increasing in popularity. Given the rising number of MRI scanners and medical device being implanted into patients, it is becoming increasingly likely that patients undergoing MRI will have an implanted medical device (IMD). The presence of an elongated metallic IMD inside a scanner could result in dangerous interactions with the radiofrequency fields during MRI, thus some of these IMDs preclude the patients from being scanned. Orthopedic devices typically fall into this category due to their high potential for RF induced heating, and typically perform poorly in the current standard test method for RF heating. That said, there exists a subset of orthopedic IMDs that still ‘fail’ the current safety testing by heating slightly above the current acceptance criterion. It is hypothesized that such IMDs are not truly a hazard to the patient but are likely failing due to the conservative nature of the current RF heating test (ASTM F2182-19a). In this thesis, novel test platforms are presented for more realistic evaluation of RF heating in orthopedic IMDs, which were used to experimentally challenge the behavior of their simulated counterparts. These test platforms were designed to address the simplifications in the current ASTM test standard that led to exaggerated heating compared to what is expected in patients, namely geometry/material mimicking and perfusion cooling. Heating of a sample implant was simulated (Sim4Life) in these novel test platforms, along with experimental verification of two phantoms to determine agreement with simulation. Simulations (and experimental work) indicated that IMD heating in these realistic phantoms could be anywhere from 20-50% lower than the current ASTM phantom, which is a reasonable estimate of the magnitude of the safety margin involved. It appears perfusion cooling is most effective at reducing IMD heating (compared to geometry/tissue mimicking differences), though improved experimental verification is required before these simulations can influence regulatory change. Introducing empirical evidence of perfusion cooling to regulatory conversations around implant safety would improve access to MRI for the millions living with such marginally unacceptable orthopedics