Abstracts

When performing EEG-fMRI studies to measure the metabolic response of epileptic spikes, one must assume a shape for the model hemodynamic response function (HRF). The HRF to interictal spikes is unknown but most EEG-fMRI studies use the model from the response to auditory stimuli (Glover response). To account for uncertainty, multiple HRFs have been used, proving more sensitive to detect activations and deactivations missed with the standard model. Age-related changes in HRFs have been discussed controversially in adults, but no study has looked at this in children with epilepsy., We evaluated 25 patients with epilepsy from different age groups: 0-2, 2-5, 6-12 and 12-18 years. Patients underwent an EEG-fMRI session in a 3-T scanner, with each spike type determining one EEG-fMRI study, resulting in 45 studies. Timings of the spikes were used for an event-related analysis to generate maps of t-statistics. HRFs were calculated in activation and deactivation regions for the three clusters with the highest t-stat value for each study. HRFs with a low signal/noise ratio (SNR) were excluded from analysis. For each HRF, we analyzed the time to peak and amplitude, and data was correlated with the age groups., 1 study showed activation, 6 showed deactivation and 38 showed both. A total of 154 of 236 (65.3%) calculated HRFs passed the SNR criterion (60% for activations and 70% for deactivations). The number of spikes did not affect the SNR. The SNR for activations increased with age (p [lt] 0.05), as well as the t-stat (p [lt] 0.005). Age had no significant effect on any measure for deactivations. HRFs had a tendency to peak later in deactivations (mean = 7.4s, SD = 2.8, range 0 to 14s) than activations (mean = 6.2s, SD = 2.6, range 1 to 13 s). Children aged 0-2 had a later time to peak for the HRF for activations than older children (p [lt] 0.001), but not for deactivations. These children also had a large number of HRFs that did not pass the SNR criterion for activations (52.5% accepted) and deactivations (54%)., We were able to calculate a large number of HRFs with good SNR in children with epilepsy, and confirmed a high intersubject variability as in adults. This suggests that the use of multiple HRFs should be considered in children undergoing EEG-fMRI experiments, and may indicate similar metabolic mechanisms during epileptic discharges in children and adults. The lower SNR observed in HRFs for activations in the younger children might be correlated with the more rare activations (and more common deactivations) seen in pediatric as compared to adult patients. Very young children showed larger differences in the HRF than all other age groups suggesting that the underlying physiology of the HRF in these might be different from older children., (Supported by Canadian Institutes of Health Research.)