The BOLD Response to Interictal Epileptiform Discharges
We studied single-event and average BOLD responses to EEG Interictal Epileptic Discharges (IEDs) in four patients with focal epilepsy, using continuous EEG-fMRI during 80 minute sessions. The detection of activated areas was performed by comparing the BOLD signal at each voxel to a model of the expected signal. Since little is known about the BOLD response to IEDs, we modeled it with the response to brief auditory events (Glover 1999). For each activated area, we then obtained the time course
... d the time course of the BOLD signal for the complete session and computed the actual average hemodynamic response (HR) to IEDs. In two patients, we observed clear BOLD responses to single IEDs. The average response was composed of a positive lobe peaking between 6s and 7s in all patients and a negative undershoot in three patients. There were important variations in amplitude and shape between average HRs across patients. The average HR presented a wider positive lobe than the Glover model in three patients and a longer undershoot in two. There was a remarkable similarity in the shape of the HR across areas for patients presenting multiple activation sites. There was also a tendency towards a linear correlation between the amplitude of individual BOLD responses and the amplitude of the corresponding EEG spike. The possibility of a longer HR and the predictability of its amplitude could be used to improve statistical detection of activation in simultaneous EEG-fMRI. The variability in average HRs across patients could reflect in part different pathophysiological mechanisms. We propose to study in a group of selected patients the relationships between IEDs and the BOLD signal, using continuous EEG-fMRI. MATERIALS AND METHODS Recording: We recorded within a 1.5T MR scanner (Siemens Vision, Siemens, Erlangen, Germany) 21 channels of EEG on patients presenting focal epileptiform discharges during their clinical scalp EEG evaluation. EEG recordings were made with an EMR32 amplifier (Schwarzer, Munich, Germany). We acquired 7 to 10 runs of 120 frames, each frame consisting of 25 BOLD 64*64 images, with a TR of 50ms, a voxel size of 5x5x5mm, and a repetition time of 3s. Each run therefore lasted approximately 6 minutes. Given the one to two minute inter-run gap, the complete session lasted up to 80 minutes. 18:217-228 Glover G.H. 1999. Deconvolution of impulse response in event-related BOLD fMRI. Neuroimage 9(4):416-29. Gotman J. 1983. Measurement of small time differences between EEG channels: method and application to epileptic seizure propagation. Electroenceph. Clin. Neurophysiol. 56(5):501-14. Hoffmann A., Jager L., Werhahn K.J., Jaschke M., Noachtar S., Reiser M. 2000. Electroencephalography during functional echo-planar imaging: detection of epileptic spikes using post-processing methods. Magn. Reson. Med. 44(5):791-8. Hoge R.D., Atkinson J., Gill B., Crelier G.R., Marrett S., Pike G.B. 1999. Investigation of BOLD signal dependence on cerebral blood flow and oxygen consumption: the deoxyhemoglobin dilution model. Magn. Reson. Med. 42(5):849-63.