Abstracts

Rationale: Determination of seizure onset zone (SOZ) in refractory epilepsy is essential for epilepsy surgery. PET hypometabolism and ictal SPECT hyperperfusion may correspond to SOZ. PET and SPECT have lower spatial resolution than MRI, which limits areas identified as possible SOZ to larger volumes of the brain that include both gray and white matter. This report describes the usefulness of gray matter segmentation of functional neuroimaging to localize SOZ in patients with difficult to localize epilepsy.Methods: As part of an IRB-approved study of post-processing techniques in epilepsy imaging, 10 adult patients aged 19-48 were selected based on the criteria that their epilepsy work-up included MRI, PET, ictal and interictal SPECT, followed by intracranial EEG (iEEG) monitoring. These patients had seizure foci which were difficult to localize by PET or SPECT alone. The gray matter segmentation of the MRI was used to identify the cortical regions in the coregistered PET and ictal-interictal SPECT subtraction perfusion blobs with SPM (ISAS) as possible SOZ. The locations of the most hypometabolic PET regions and the most hyperperfused blobs were compared to each other and to the iEEG localization. The level of correspondence of volumes of interest (VOI) between two modalities was recorded as being in the same subgyrus (highest level of correspondence), gyrus, sublobe, lobe, hemisphere, or no correspondence.Results: Of 10 patients, 6 had segmented PET localizations concordant with iEEG with subgyral specificity, 2 with lobar specificity, and 2 with hemispheric specificity. 3 of 10 patients had segmented SPECT hyperperfusion blobs concordant with iEEG with subgyral specificity, 2 with sublobar specificity, 3 with lobar specificity, and 2 with no specificity. Hypometabolic PET regions and perfusion blobs indicated matching volumes of interest in subgyral region in 2 patients, sublobar in 4, lobar in 2, and hemispheric in 2.Conclusions: Segmented PET scans frequently resulted in high correspondence to iEEG onset zone. Segmented SPECT results were less likely to have such correspondence between perfusion blobs and iEEG localization. This may be due to other factors such as bias towards PET at the institution and variation in performing SPECT scans. Segmentation adds significant specificity in identifying SOZ in PET scans; more investigation is needed to determine the same for ictal-interictal SPECT subtraction perfusion blobs.