Reducing the Risk of Gradient Induced EEG Hardware Heating During Simultaneous EEG-fMRI

Abstract

Simultaneous electroencephalography (EEG) and functional MRI (fMRI) has the potential to provide a more complete characterisation of brain function. Acquiring these data simultaneously requires the EEG hardware to be MR safe, since the EEG cap, amplifiers and powerpacks must be located within the MR environment. Placing them in this environment exposes them to both static and varying (radiofrequency (RF) and gradient) magnetic fields, presenting a potential heating risk. Previous studies have typically positioned the amplifiers and powerpacks outside of the bore, meaning that this risk is largely limited to the EEG cap. However, current guidelines recommend positioning the amplifiers within the bore. This reduces the distance between the EEG electrodes and amplifier, improving EEG signal quality and reducing risks of RF heating at the electrodes, but presents a potential heating risk for the amplifiers and powerpacks from the varying magnetic field gradients. In this study, we demonstrate that placing the amplifiers and powerpacks in the bore presents a significant heating risk, both at 7T and 3T. At 7T, we show that EPI sequences with a slew rate of at least 88 T/m/s resulted in significant heating of the EEG amplifiers. This heating effect could be negated by reducing the slew rate to 61 T/m/s. At 3T, we replicate the EEG amplifier heating and show that an alternative approach to reducing the gradient slew rates experience by the hardware is to increase the distance between the EEG amplifier/powerpacks and the isocentre of the magnet. Increasing the distance between the EEG hardware and MR isocentre by 30 cm mitigates the heating risk to the amplifiers and powerpacks, regardless of sequence parameters (e.g., slew rate). Therefore, whilst reducing the distance between the electrodes and amplifiers may improve EEG signal quality and reduce the potential for electrode RF heating, we show that risk of heating of hardware by gradient switching is greatly increased. This is particularly important to consider with the faster gradient slew rates available on modern MRI scanners, desired to increase spatial resolution. Experimenters must make careful decisions about hardware placement to ensure that the EEG-fMRI setup they use is safe, taking account of the varying risks that are posed by different approaches.