A comparison of phantom materials used in evaluation of radiofrequency heating of implanted medical devices during MRI

The safety of implanted medical devices within the magnetic resonance imaging (MRI) environment is an issue that must be continually addressed. All magnetic fields developed during MRI are potentially hazardous to medical implants, specifically attraction to the static field, improper device operation due to the gradient and/or radiofrequency (RF) fields, and induced heating by the RF field. Experiments must be performed to evaluate how a device will perform when exposed to these fields. In lieu of performing human or animal experiments, various phantom materials are often used. Presented here is a comparison of the local temperature rise that occurs in a saline phantom, saline phantoms with 1 and 3% thickener added, and saline phantoms with 1.1 and 3% gelling agent added, using current applied to a resistor as a heat source, to simulate the RF heating that occurs in the presence of elongated medical implants. Temperatures were measured at the resistor and at 1, 2, and 3 cm from the resistor using a fluoroptic thermometer. The maximum temperature increase was approximately 10°C, for the saline phantom with low gelling agent concentration, and the saline phantom with high thickener concentration. All other phantom compositions exhibited smaller temperature deviations during experimentation. Additionally, only the saline phantom with 1.1% gelling agent concentration showed a measurable increase in temperature at distances greater than 1 cm from the heat source. A theoretical model of the heating that occurs during these experiments, assuming the resistor acts as a point source, was also developed. This model was in reasonable agreement with the heating seen in phantoms having only conduction present. The experimental results presented herein show that a gelled phantom is the most appropriate phantom material for studies evaluating implanted medical devices under a worst case RF heating scenario, i.e. maximal temperature rise.