Effect of injection time on postictal SPET perfusion changes in medically refractory epilepsy

Single-photon emission tomography (SPET) brain imaging in epilepsy has become an increasingly important noninvasive tool in localizing the epileptogenic site. Ictal SPET demonstrates the highest localization sensitivity as compared with postictal and interictal SPET. While ictal SPET consistently reveals hyperperfusion at the epileptogenic site, postictal SPET reveals either hyper- or hypoperfusion depending on the timing of radiopharmaceutical injection. Much discussion in the literature exists about exactly when the transition from hyper- to hypoperfusion occurs at the epileptogenic site in postictal SPET. The systematic examination of two clinical variables - time of injection from seizure onset and offset - was useful in understanding postictal perfusion changes. Twenty-seven patients with medically refractory epilepsy receiving postictal and interictal SPET scans were studied. Quantitative SPET difference imaging was used to evaluate perfusion changes in relationship to injection time. Perfusion changes were found to reflect the time of injection in relation to seizure onset, but to be somewhat independent of seizure offset. Thus, the majority of patients (8/12, 67%) receiving postictal injections within 100 s after seizure onset demonstrated hyperperfusion, while all patients (15/15, 100%) receiving postictal injections more than 100 s after seizure onset showed hypoperfusion. The explanation of this phenomenon is unknown but the findings appear to parallel known changes in cerebral lactate levels.