Abstracts

Rationale: Combining intracranial EEG (ICE) with functional MRI (fMRI) is of particular interest in the study of epilepsy as it would allow the detection of much smaller interictal epileptiform discharges than scalp EEG-fMRI, and may help further investigate the spatiotemporal mechanisms of seizures. To our knowledge, ICE-fMRI never been performed at 3 Tesla. After verifying the safety of performing ICE-fMRI at 3 Tesla using a phantom model, we developed an implementation protocol to slowly build up to a full ICE-fMRI study in humans. We report our findings relating to spike-associated Blood Oxygen Level-Dependant (BOLD) signal changes in two subjects studied using this protocol to date. Methods: Functional MRI at 3 Tesla with concurrent intracranial EEG was performed. The scan session was broken up into short (5-10 minute) blocks of MR scanning, in between which a neurological examination of the subject was completed. Prior to collection of useful MR images, a series of short, low-energy scans (with radiofrequency pulses disabled) were performed in order to ensure subject comfort and safety. Subject 1: Twelve channels of intracranial EEG were recorded from subdural strips implanted on the left posterior temporal and middle frontal lobes in a 20-year-old female with bilateral periventricular gray matter heterotopia. Subject 2: Twenty channels of intracranial EEG were recorded bilaterally from two subdural strips laid anterior-posterior along mesial temporal surfaces in a 29 year-old female with bilateral temporal seizures and mild left amygdalar enlargement seen on MRI. In Subject 1, interictal discharges (n = 105) were maximal over the posterior middle temporal gyrus. In Subject 2, discharges originating from the each mesial temporal surface (n = 194 left, 284 right) were modeled separately as they did not always occur in unison. Functional MRI images were processed using FSL and statistical maps were generated and thresholded at P = 0.01. Results: Subject 1 showed BOLD signal increases in both superior temporal gyri associated with epileptiform discharges recorded in the left temporal lobe. This subject also demonstrated bifrontal and biparietal spike-associated BOLD signal decreases. Subject 2 showed a maximal positive BOLD change in both temporal lobes, which was greater on the side of discharge when left and right spikes were modeled independently. It was interesting to note that no BOLD signal increases were seen in subcortical areas in either study. Conclusions: Intracranial EEG-fMRI can be performed safely at 3 Tesla. Both BOLD increases and decreases were observed in this study, and positive BOLD changes appear to be generally concordant with location of discharges recorded by ICE. Of note, runs of only 5 or 10 min of EEG-fMRI were performed as part of our implementation protocol, yet a significant number of epileptiform discharges and meaningful analyses were obtained from each run. This highlights the ability of ICE-fMRI to record and analyze many epileptiform discharges over brief periods of time.