Objective detection of epileptic foci by 18F-FDG PET in children undergoing epilepsy surgery

Abstract

PET has been used for the presurgical localization of epileptic foci for more than 20 y; still, its clinical role in children with intractable epilepsy remains unclear, largely because of variable analytic approaches and different outcome measures. The purpose of the present study was to evaluate and optimize the performance (lateralization and lobar localization value of epileptic foci) of objective voxel-based analysis of (18)F-FDG PET scans in a pediatric epilepsy population.

Methods: Twenty children with intractable focal epilepsy (mean age ± SD, 11 ± 4 y; age range, 6-18 y) who underwent interictal (18)F-FDG PET, followed by 2-stage epilepsy surgery with chronic subdural electrocorticographic monitoring, and were seizure-free after surgery were included in this study. PET images were analyzed using both a visual-analysis and a statistical parametric mapping (SPM) method. Lateralization value and performance of lobar localization (in lateral and medial surfaces of all lobes, total of 8 regions in each epileptic hemisphere), calculated for 3 different statistical thresholds, were determined against intracranial electrocorticography-determined seizure-onset region and surgical resection site.

Results: SPM using a statistical threshold of P less than 0.001 provided 100% correct lateralization, which was better than visual assessment (90%). Although visual and SPM analyses (with both P < 0.001 and P < 0.0001 thresholds) performed similarly well (with a sensitivity and specificity of 74% or above) in the localization of seizure-onset regions, SPM detected 7 of 9 seizure-onset regions, mostly in medial cortices, that were missed by visual assessment. Also, SPM performed equally well in both hemispheres, compared with visual analysis, which performed better in the left hemisphere. No statistical difference in performance was observed between visual and SPM analyses of children with abnormal versus normal MRI findings or of children with gliosis versus developmental pathology. Clinical variables, such as age, duration of epilepsy, age of seizure onset, and time between PET and last seizure, showed no correlation with sensitivity or specificity of either visual analysis or SPM analysis.

Conclusion: SPM analysis, using a young adult control group, can be used as a complementary objective analytic method in identifying epileptogenic lobar regions by (18)F-FDG PET in children older than 6 y.

FIGURE 1

Role of SPM in lateralizing epileptic focus in case of bilateral visual abnormality on ¹⁸F-FDG PET scan for 14.5-y-old boy with intractable seizures and normal MRI results. (A) Visual analysis showed bilateral parietal hypometabolism (arrows), with no clear lateralization. (B) SPM analysis, both with threshold of P < 0.001 (upper panel) and with threshold of P < 0.0001 (lower panel), however, could lateralize epileptic focus to right hemisphere by showing single area of hypometabolism in right parietal region. Area of hypometabolism was much smaller with higher threshold (P < 0.0001). Subsequent subdural electroencephalographic monitoring also showed seizure onset from same region. Patient underwent right parietal lobe resection and has been seizure-free for 2 y.

FIGURE 2

Role of SPM in identifying medially located epileptic foci. (Upper) Axial and coronal slices of ¹⁸F-FDG PET scan. (Lower) Results of SPM analysis, with threshold of P < 0.001, on axial and coronal slices of T1-weighted template MR image. On visual assessment, ¹⁸F-FDG PET scan showed area of hypometabolism in left frontal cortex (solid arrow) in 9-y-old boy with intractable epilepsy and normal MRI results. SPM analysis detected hypometabolism in same area (solid arrow) but also revealed additional area of hypometabolism in left medial frontal cortex including cingulum (broken arrow), missed by initial visual analysis and only suspected later on reanalysis (broken arrow). Subdural electroencephalographic monitoring showed independent seizure onset from both lateral and medial frontal cortices (captured by interhemispheric electrodes). Patient underwent left frontal lobe resection, including medial frontal lobe, and has been seizure-free for 6 y.