Role of FDG PET scan in seizure focus localization in patients with unremarkable brain MRI

Objectives: Selected cases of partial epilepsy patients whom are non-responsive to pharmacotherapy may benefit from surgery; therefore identification of structural lesions is vital in the pre-surgical evaluation. However, Brain MRI still fails to reveal apparent abnormality in approximately 20% of these patients. FDG PET is thought to be highly sensitive in localizing epileptogenic foci and can provide information complementary to brain MRI imaging in pre-surgical evaluation.

Material and Methods: This is a cohort study evaluating patients with refractory epilepsy. All patients have been evaluated clinically with EEG to localize epileptogenic focus in the brain cortex. All patients must have an unremarkable brain MRI to be included in this study. The PET images were evaluated by two nuclear medicine physicians and two radiologists simultaneously for possible hypometabolic (decreased metabolic activity) focus which could be suggestive for epileptogenic region in the brain cortex. We used expert visual analysis method to evaluate brain cortex metabolic activity. Finally, the clinical and EEG data together with PET imaging results were compared to determine degree of data congruency. The findings were stratified as exactly congruent, partially congruent and incongruent. Also we stratified patients with regard to clinical epileptogenic focus as localized in temporal lobe, frontal lobe or partially localized and tried to find out in which group of patients PET is most congruent with clinical and EEG findings.Results and discussion:In this study we included 110 patients (60 male and 50 female) with mean age of 27.95 years (ranging between 5 and 60 years). 69 patients had their seizure focus localized in temporal lobe by means of clinical and EEG evaluation in which 35 patients demonstrated exactly congruent PET results (51%). Remainder of these patients demonstrated either partially congruent PET results (8 patients, 11%) or totally incongruent PET findings (26 patients, 38%). 27 patients had their seizure focus localized in frontal lobe by means of clinical and EEG evaluation in which only 4 (15%) patients demonstrated partial congruency with PET results. The remainder of 23 (85%) patients demonstrated incongruent PET results and there was no case in frontal lobe focal seizures with congruent PET results. 14 patients had their seizure focus only partially localized in one hemisphere in which 7 (50%) patients demonstrated partial congruency with PET results. Evaluation of our PET scans demonstrated 36 negative brain PET in which 18 patients (50%) had their seizure focus localized in temporal lobe, 11 patients (30.5%) in frontal lobe and 7 patients (19.5%) only partially localized on clinical and EEG evaluation. All exact congruent PET results had their seizure focus localized in temporal lobe. Among partially congruent cases; 8 patients (39%) had their seizure focus localized in temporal lobe, 5 patients (22%) in frontal lobe and 8 other patients (39%) only partially localized. Although 50% of temporal lobe seizure patients demonstrated exactly congruent PET results, but these results may be further enhanced by means of more precise localization of seizure focus on clinical and EEG evaluations, in addition to using more advanced PET imaging software which enables the diagnostician to quantify image findings. Over all PET results in frontal lobe or partially localized patients are not promising in our study and further studies with larger sample size are required.Conclusion:FDG PET is useful tool to evaluate patients with refractory seizure who had localized seizure in temporal lobe on clinical evaluation. Utility of FDG PET scan in extratemporal seizure foci is much more limited. The more precise is the localization of seizure focus on clinical and EEG evaluation; the more helpful would be the subsequent PET scan to confirm that finding which would be to the patient’s best interest to be candidate for possible ablation surgery.