Abstracts

The integration of intracranial electroencephalography (EEG) with high field functional magnetic resonance imaging (fMRI) may help elucidate mechanisms underlying the generation of seizures. However, introducing intracranial EEG electrodes into the MR environment has inherent safety implications that require consideration. To date, safety testing of intracranial electrodes has not been performed at high field strengths. We therefore addressed this issue using a phantom model as a necessary prerequisite for future intracranial EEG-fMRI studies in humans. A phantom was constructed to model the shape, size, and conductivity of the human head. An intracranial electrode grid was secured on the phantom surface in an orientation similar to actual implantation. In vitro measurements of 1) electrode displacement, 2) temperature increase along the electrode lead and surrounding phantom tissue, and 3) induced current/voltage in the electrodes and leads were taken within the static 3 T MR environment (General Electric, Waukesha, WI). These measurements were also repeated during diffusion-weighted, fluid attenuation inversion recovery, fMRI, and T1-weighted anatomical MR imaging sequences. The measurements were repeated three times on different days. The intracranial electrodes showed no measurable movement or torque in the MR environment. Temperature change along the electrodes and surrounding tissue was [lt]1[infin]C over a one hour scanning period. Induced voltage levels ranged from 100 to 650 mV at a frequency of 127MHz. These preliminary results indicate that deflection and temperature effects are within safety limits ([lt]1mm and [lt]4[infin]C, respectively). Although the amplitude of induced voltage is within the physiologically effective range, the frequency is significantly higher than the [lt]10kHz range required to produce neuronal activation. These results suggest that intracranial EEG-fMRI should be safe and feasible at 3 Tesla. (Supported by Canadian Institutes of Health Research and Alberta Heritage Foundation for Medical Research.)