Evolution and Localization of Postictal Blood Flow Changes in Partial Seizures Demonstrated by SPECT: Use of Quantitative Difference Images

Ictal single photon emission computed tomography (SPECT) is a sensitive means of seizure localization in partial epilepsy. The purpose of this study is to enhance the diagnostic yield of postictal SPECT for epilepsy and to overcome the difficulties in its conventional interpretation. We developed a method for coregistration, normalization, and subtraction of interictal from ictal SPECT to reveal positive difference images (and ictal from interictal to reveal negative difference images) of cerebral blood flow (CBF) during seizures. This method also allows quantification of regions of greatest percent increase and decrease, normalized to seizure duration. We studied 12 patients who had confirmed epileptogenic regions by surgery and pathology (seven temporal, five extratemporal). In all 12 patients, with either early (during ictus) or late injection (after seizure termination), the difference images showed increases or decreases respectively in blood flow consistent with the epileptogenic region. We highlight the localizing value of the negative difference change in perfusion (state of hypoperfusion) that persists for up to 100 seconds after seizure termination and corresponds to the epileptogenic region. Application of this technique for the quantification of perfusion changes during seizure or immediately after seizure cessation enhances the diagnostic yield of SPECT in both temporal and extratemporal epilepsy.