Experimental Evaluation of Relationship Between Radiofrequency Heating Near Implanted Conductive Devices, Scanner-Reported B1+rms, and Transmit Power

Abstract

Time-varying radiofrequency (RF) fields necessary to perform magnetic resonance imaging (MRI) may induce excessive heating near implanted conductive medical devices during MRI. The time and space-averaged root-mean-square effective magnetic field (B_1+rms) and specific absorption rate (SAR) have been proposed as metrics to control the RF-induced heating and avoid unintended thermal injury. We experimentally evaluate the relationship between the RF-induced heating near an implanted conductive medical device, scanner-reported B_1+rms, and RF power. RF heating was measured near the electrodes of a commercial deep brain stimulation (DBS) lead placed in a tissue-equivalent gel phantom using fluoroptic temperature probes in a commercial 3T scanner during MRI. Four RF transmit/receive coil combinations were used: a circularly polarized head transmit/receive coil, a 20-channel head/neck, a 32-channel head, or a 64-channel head/neck receive-only coil with a whole-body transmit coil. RF heating was induced by a 2D GRE sequence using two RF pulse types (fast and normal), three flip angles (30°, 60°, and 90°), and turning the receive-only coils off/on. The scanner-reported B_1+rms and RF power were recorded. Measurements show that temperature change correlates linearly with both RF power and square of B_1+rms for each coil and combination. However, the variation in heating for various RF coils and combinations was much larger for B_1+rms compared to RF power. Additional studies across other MR scanners are needed to better understand the extent of variation in RF-induced heating near implanted conductive devices as a function of scanner-reported B_1+rms and RF power to develop conservative and reliable patient labeling.