Fast fMRI Sequence Allows for Analysis of BOLD Responses to Single Interictal Epileptic Spikes

Aims: Classical EEG fMRI is a noninvasive method to identify epileptic networks. Because of the limited temporal resolution, it is not sensitive enough to detect the blood oxygenation level dependent (BOLD) responses to single interictal epileptic spikes (IES).¹ Averaging of multiple spikes requires a sufficient number of IES within a recording session as well as low trial-by-trial variability, which is rare. The new ultrafast MR encephalography (MREG) sequence² offers a higher temporal resolution and may provide the possibility to detect single-trial BOLD responses to IES.

Methods: MREG–fMRI with simultaneous EEG with 64-channel cap (Brain Products) was recorded for a 20-minute period for each subject. fMRI (MREG) was performed with the following parameters: 3T, TR = 100 ms, TE = 36 ms, 64 × 64 × 64 matrix, voxel size = 3 mm isotropic. EEG artifacts were corrected with average subtraction methods. IES were marked by an epileptologist and categorized into “spike types” depending on topography and morphology. Data are analyzed using general linear models with a separate model for each individual spike, respiratory, and cardiac confounds to model unaliased physiological noise and a fifth-order autoregressive noise model.

Results: Of the 38 epilepsy patients recorded, we included data with optical motion correction and > 4 spikes/20 minutes, yielding 14 sets. BOLD maps of 299 single IES were analyzed in the first five patients. The individual spikes of the same spike class elicit variable BOLD responses in all the five patients. Positive BOLD effects were seen over the spike onset zone in at least one single spike per patient. In one patient, no BOLD effects were seen in the map following all IES, but some effects were observed in the individual spike BOLD maps. We will show examples in which BOLD effects in the single IED showed better congruence with the EEG than the average BOLD map.

Conclusion: With EEG–MREG, BOLD maps of individual IES can be analyzed. Individual maps show a high variability and might provide important information about the epileptic network.

Keywords: EEG, fMRI, MREG, epilepsy, single spike analysis, BOLD effect.

References

References

1 Krakow K, Messina D, Lemieux L, Duncan JS, Fish DR. Functional MRI activation of individual interictal epileptiform spikes. Neuroimage 2001;13(3):502–505

2 Zahneisen B, Hugger T, Lee KJ, et al. Single shot concentric shells trajectories for ultra fast fMRI. Magn Reson Med 2012;68(2):484–494