Simultaneous electroencephalography-functional MRI at 3 T: An analysis of safety risks imposed by performing anatomical reference scans with the EEG equipment in place

Purpose: To describe heating effects to be expected in simultaneous electroencephalography (EEG) and magnetic resonance imaging (MRI) when deviating from the EEG manufacturer's instructions; to test which anatomical MRI sequences have a sufficiently low specific absorption rate (SAR) to be performed with the EEG equipment in place; and to suggest precautions to reduce the risk of heating. Materials and Methods: Heating was determined in vivo below eight EEG electrodes, using both head and body coil transmission and sequences covering the whole range of SAR values. Results: Head transmit coil: temperature increases were below 2.2 degrees C for low SAR sequences, but reached 4.6 degrees C (one subject, clavicle) for high SAR sequences; the equilibrium temperature Teq remained below 39 degrees C. Body transmit coil: temperature increases were higher and more frequent over subjects and electrodes, with values below 2.6 degrees C for low SAR sequences, reaching 6.9 degrees C for high SAR sequences (T8 electrode) with Teq exceeding a critical level of 40 degrees C. Conclusion: Anatomical imaging should be based on T1weighted sequences (FLASH, MPRAGE, MDEFT) with an SAR below values for functional MRI sequences based on gradient echo planar imaging. Anatomical sequences with a high SAR can pose a significant risk, which is reduced by using head coil transmission.