Abstracts

Rationale: Simultaneous acquisition of intracranial EEG (iEEG) and functional MRI (fMRI) data could provide the temporal and spatial resolution necessary to reveal mechanisms underlying seizure generation. However, the introduction of iEEG electrodes in the MR environment has inherent risk and data quality implications that require consideration prior to clinical use. We aimed to assess three health risks associated with the presence of iEEG electrodes during MRI scanning using a 3T scanner: RF-induced heating, gradient switching-induced voltage on the electrode circuit, and magnetic field-induced torque on the electrodes. Additionally, we evaluated the feasibility of running a continuous iEEG-fMRI scanning protocol. Methods: Three head phantoms were constructed to mimic shape, size and conductivity of the human head. The first phantom contained a depth electrode, the second a grid electrode and the third a combination of depth, grid and strip electrodes that are commonly used for intracranial monitoring (Ad-Tech) in our institution. A body phantom was designed to model the human torso. MR scans were performed using a GE Discovery MR750 3T scanner. Temperature data were collected from MR sequences typically performed during an EEG-fMRI experiment using a fiber-optic thermometry system (Fiso Technologies), including alternative anatomical sequences. Voltage data were collected using an MR-compatible EEG system (Compumedics Neuroscan). EEG cable locations and electrode orientations were varied to mimic typical intracranial electrode implantations in patients. Torque induced on electrodes was assessed using the Deflection Angle Method developed by the American Society for Testing Materials. A 70-min iEEG-fMRI protocol including anatomical and functional scans was employed to assess heat accumulation. Results: No significant temperature increases were detected (< 1.0 C) when using the individual MR sequences common to an iEEG-fMRI protocol (Table 1). Additionally, repeated measures of a continuous 70 minute iEEG-fMRI protocol did not induce temperature changes > 1.0 C (Fig. 1). These results were consistent across different cable locations and electrode orientations. MR sequences with high Specific Absorption Rate (SAR), such as FLAIR and Fast-Spin Echo (FSE), generated significant temperature increase (> 1.5 C and > 4.9 C). Harmful voltages that could induce neuronal stimulation (< 10 KHz and > 100 mV) were not observed during any test condition. Deflections induced on electrodes were not significant (< 1 degree). Conclusions: Simultaneous iEEG-fMRI poses low risk and is feasible at 3T for the conditions reported. High SAR sequences (typically FSE sequences) can potentially result in clinically significant tissue heating and should be avoided. The settings described in our study produced no inadvertent induced voltages, which could result in neuronal stimulation. Simultaneous iEEG-fMRI at 3T is a promising tool for epilepsy research that may help elucidate the mechanisms of seizure generation and ultimately improve patient outcomes.