An experimental evaluation of the relationship between the induced radiofrequency heating near an implanted conductive medical device during MRI, scanner reported B1+rms, and scanner reported average transmit power

Abstract Background Time-varying radiofrequency (RF) fields necessary to perform magnetic resonance imaging (MRI) may induce excessive heating near implanted conductive medical devices during MRI. Both time and space-averaged root mean square of the effective magnetic field (B1+rms) and whole-body average specific absorption rate (SAR) (average RF power per unit body weight) have been proposed as metrics to control the induced heating and avoid unintended thermal injury. Purpose To evaluate the relationship between the induced RF heating near an implanted conductive medical device, scanner-reported B1+rms, and scanner-reported RF power. Methods RF heating was measured near the electrodes of deep brain stimulation (DBS) lead placed in a gel phantom using fluoroptic temperature probes in a commercial 3T scanner during MRI. Four transmit and receive RF coil combinations were used, a circularly polarized head transmit and receive coil, a 20-channel head and neck, a 32-channel head, or a 64-channel head and neck receive-only coil with a whole-body transmit coil. RF heating was induced by running a 2D GRE sequence with two RF pulse types (fast and normal) with varying flip angles of 30°, 60°, and 90° and by turning the receive-only coils off and on. The scanner-reported B1+rms and RF power were recorded. Results Measurements show that the induced temperature change correlated linearly with both the scanner-reported B1+rms and RF power for each coil combination. However, the variation in the induced heating for various RF coil combinations appeared to be much larger for the scanner-reported B1+rms compared to the scanner-reported RF power. Conclusion Additional studies across other MR scanners are needed to better understand the full extent of the variation in the induced heating near implanted conductive devices as a function of the scanner-reported B1+rms and RF power to develop conservative and reliable patient labeling.