THE CANADIAN JOURNAL OF NEUROLOGICAL SCIENCES PET Scanning in Partial Epilepsy

Many biologically active tracers are available for positron emission tomography (PET) investigations, but most studies of epilepsy have utilized F-fluorodeoxyglucose (FDG) to measure local cerebral metabolic rate for glucose. Over 70% of patients with medically refractory partial seizures demonstrate an interictal zone of hypometabolism corresponding to the epileptogenic region. This metabolic defect commonly involves the temporal lobe in patients with complex partial seizures of mesial temporal origin, and is encountered less consistently with seizures of extratemporal neocortical origin. Although false localization is less likely with FDG-PET than with EEG, the hypometabolic zone merely reflects a focal functional deficit and its epileptogenicity must still be demonstrated electrophysiologically. When hemispherectomy or large multilobar resections are planned in small children, FDG-PET also provides useful supporting evidence that the contralateral hemisphere is functioning normally. It is difficult to obtain FDG-PET scans and to interpret results during spontaneous partial seizures. Ictal scans can be more easily obtained with single photon emission computed tomography (SPECT), which may provide information for planning surgical resections. RESUME: Pet scan dans l'epilepsie partielle. En depit de la disponibilite de plusieurs traceurs radio-actifs pour les 6tudes de tomographic par Amission de positons (PET), la plupart des etudes realisees ont utilise le 18F-fluorod6soxyglucose (FDG) pour la mesure de metabolisme local du glucose. Chez plus de 70% des patients presentant des crises partielles complexes r6fractaires a la medication, on retrouve une zone d'hypo-m6tabolisme correspondant a la region 6pileptogene. Cette zone d'hypo-m&abolisme implique souvent le lobe temporal chez les patients ayant des crises partielles complexes d'origine mesio-temporale; elle est rencontree de facon plus inconstante chez ceux ayant des crises provenant des structures neo-corticales extra-temporales. Malgre que la possibility d'une fausse localisation soit moins grande avec les etudes FDG-PET qu'avec les etudes electroencephalographiques, il reste a demontrer electrophysiologiquement que la zone d'hypoperfusion, qui represente tout au plus un deficit fonctionnel localise est 6pileptogene. Le FDG-PET peut fournir de pr6cieuses donn6es sur l'integrite fonctionnelle de l'hSmisphere contralateral lorsqu'une h6misph6rectomie ou une large resection multi-lobaire est necessaire chez de jeunes enfants. II est difficile d'obtenir des Studes FDG-PET durant les crises partielles spontanees; de plus, 1'interpretation des r6sultats est problematique. Les mesures du m&abolisme cerebral regional en phase ictale sont plus facilement obtenues avec la tomographic par Amission de photon unique (SPECT) qui peut fournir de precieux renseignements pour la planification de la resection chirurgicale. Can. J. Neurol. Sci. 1991; 18:588-592 Experience with the use of positron emission tomography (PET) to evaluate human local cerebral function extends now for well over a decade. The requirements for an on-site medical cyclotron, in particular, have limited the proliferation of PET facilities, and this technique remains largely a research tool. As a result, PET has not enjoyed the same extensive application for clinical diagnosis as has occurred with, for instance, magnetic resonance imaging (MRI). A major focus of clinical PET research, however, has been on partial epilepsy and perhaps the area of greatest demonstrated clinical value for PET at the present time is in the presurgical evaluation of patients with partial seizures who are candidates for resective surgical treatment. New PET systems are now being developed with self-contained, stand alone cyclotrons which require no shielding, organic chemistry modules for preparation of reagents, and the latest generation tomograph, which can all be operated by a single technician, at a price competitive with structural imaging systems. This will facilitate the establishment of clinical PET centers and should permit the more widespread use necessary to better understand the contributions PET could make to diagnosis of partial epilepsy. At the present time, however, our experience remains limited to patients with severe complex partial seizures who are candidates for surgery. Interictal Localized Abnormalities Many PET centers have now confirmed that approximately 70% of patients with severe partial seizures demonstrate an interictal zone of hypofunction corresponding to the epileptogenic region. This has been demonstrated for glucose metabolism, using F-fluorodeoxyglucose (FDG)," oxygen metabolism using O-oxygen,'' and blood flow using Nammonia and 0-water. -'" There is a high degree of correlation between the presence of a zone of hypofunction and the existence of a structural lesion demonstrated by pathological analysis of resected tissue; however, the PET identified hypofunction extends well beyond the area of morphological abnorFrom the Departments of Neurology and Anatomy, Laboratory of Nuclear Medicine, and Brain Research Institute, UCLA School of Medicine, Los Angeles Reprint requests to: Reed Neurological Research Center, UCLA School of Medicine, Los Angeles, California, U.S.A. 90024