Ictal SPECT analysis in epilepsy: subtraction and statistical parametric mapping techniques.

Seizures are associated with an increase in regional cerebral blood flow (rCBF). In partial seizures the increased blood flow closely corresponds with the site of seizure origin. Using tracers that accumulate and remain "fixed" in different areas of the brain proportional to rCBF at the time of injection, ictal SPECT is now an important tool for localization of seizures in a presurgical evaluation. However, the best methods for interpretation of partial seizure-induced changes in rCBF remain unclear. Numerous computer-aided tools have been used to increase objectivity and accuracy of ictal SPECT analysis. This review examines the uses of ictal-interictal subtraction methods and statistical parametric mapping (SPM) to enhance interpretation and utility of ictal SPECT. The review covers the evolution of advanced ictal SPECT imaging analysis techniques and the authors' clinical experience with the use of subtraction and SPM methods. The authors discuss the impact of ictal SPECT subtraction or difference imaging methods and the initial evidence for proof-of-principle that SPM can be used to provide objective, accurate analysis of ictal SPECT scans in patients with temporal and extratemporal lobe epilepsy. The limitations of both methodologies are discussed, and suggestions for further study of validation, improvement, and routine clinical implementation of advanced analysis methods are provided.