Radiofrequency heating induced by 7T head MRI: Thermal assessment using discrete vasculature or pennes' bioheat equation

Purpose: To evaluate and compare the maximum temperature (Tmax) in the head after exposure to a 300 MHz radiofrequency (RF) field induced by a magnetic resonance imaging (MRI) coil using two thermal simulation methods: Pennes' bioheat equation (PBHE) and discrete vasculature (DIVA). Materials and Methods: The electromagnetic field induced in the head by a 7T birdcage coil was simulated using finite-difference time-domain (FDTD) and validated by MRI. The specific absorption rate (SAR) distributions normalized to the 10-gram maximum or the whole-head average were used for PBHE and DIVA simulations. Results: For all cases, the T-max in PBHE was slightly higher than in DIVA. The T-max was 37.9-38.4 C, depending on the simulation method or perfusion rate. Conclusion: In some situations, RF exposure limited to SAR(max), 10g led to a T-max higher than allowed by International Electrotechnical Commission (IEC) regulations. Therefore, it is advisable to use thermal simulations to evaluate RF safety of MRI. The simulation method used only slightly influenced the observed maximum temperature; the observed temperature with PBHE was higher in all situations. So PBHE is an appropriate method for RF safety assessment of MRI in the head. Using DIVA simulations, it was found unlikely that the body temperature increases significantly due to energy deposited by a head coil under normal circumstances.