Application of PET and SPECT in epilepsy

PET and SPECT are valuable clinical tools in the management of patients with medically resistant, partial epilepsy who are under evaluation for surgical treatment. The value of PET for localization of seizure activity has been fi rmly established for patients with temporal lobe epilepsy and extratemporal lobe epilepsy. It is a very useful test partly because it is non-invasive. The localizing value of ictal SPECT is based on cerebral metabolic and perfusion coupling. Ictal hyperperfusion is used to localize the epileptogenic zone noninvasively, and is particularly useful in MRInegative partial epilepsy and focal cortical dysplasia. Use of subtraction ictal SPECT coregistered with MRI (SISCOM) improves localization area of hyperperfusion. However, ictal SPECT should be interpreted in the context of full presurgical evaluation Neurology Asia 2013; 18 (Supplement 1) : 23 – 25 Address correspondence to: Dr Yotin Chinvarun, Neurology unit, Pramongkutklao hospital and Medical College, Thailand PET IMAGING PET has been applied for presurgical evaluation since 1970. PET-FDG imaging especially when performed concurrently with surface EEG, may be useful for localizing the seizure focus for surgical resection. FDP-PET imaging remains important tools in localization of ictal onset zone, seizure propagation pathways, and functional defi cit zone in the presurgical evaluation of patients with refractory partial epilepsy. Interictal PET demonstrates areas of reduced glucose metabolism that correspond to epileptogenic zone in partial seizures. The value of PET for localization has been fi rmly established for patients with temporal lobe epilepsy and extratemporal lobe epilepsy. The sensitivity of FDG-PET to identify foci of seizure onset in temporal lobe epilepsy (TLE) is high at up to 80-90%.1 In TLE with hippocampal sclerosis, interictal FDG-PET shows up to 100% sensitivity. Mechanism of interictal hypometabolism on FDG-PET remains unclear in TLE. Possibly in TLE with MTS, the hypometabolism is caused by neuronal loss, deafferentation or partial volume effect. Hypometabolism is usually diffuse, involving entire temporal lobe of epileptogenic side, including polar and lateral regions as well as mesial structures. However, the localization of seizure foci in extratemporal lobe epilepsy is more diffi cult than with TLE. The hypometabolism of interictal PET is concordant with EEG focus in only 32% in frontal lobe seizure as compared to 77% in temporal lobe seizure.2 Also, the hypometabolic region is not necessarily congruent with the epileptogenic zone and may extend beyond the epileptogenic lesion.3 The hypometabolic region usually includes structural lesion, but often extends to involve more than one lobe or a whole hemisphere (Figure 1). Bilateral temporal hypometabolism is also commonly found.4 PET has been proposed as to refl ect more the functional deficit zone rather than the epileptogenic zone. In spite of this, there is still a good correlation with epileptogenic and a good lateralization. As a result, in cases where the lateralization by PET study is congruent with the other imaging modalities, there is better surgical outcome.5 It is very useful particularly in cases that are MRI negative or discordant in the various modalities for localization. In extratemporal lobe epilepsy, success rate of surgery is only 50% despite an extensive usage of PET and MRI imaging modalities.2 The area of abnormal glucose metabolism is commonly larger than epileptogenic zone, therefore the FDG-PET may regionalizes the epileptic focus, but is unable to indicate the exact localization of the epilepsy. In frontal lobe epilepsy, PET provides a correct localization in only 50% of cases with a normal MRI. However, PET together with Ictal SPECT is usually useful as a guide for intracranial grid placement. Ictal PET is usually only obtained by chance. Because the 18FDG uptake occurs in >40 minute period after injection, therefore data usually refl ects an amalgam of ictal, postictal and interictal conditions.6 Neurology Asia 2013; 18 (Supplement 1) 24 PET can also be used to evaluate the progression of the disease.7 In the longitudinal PET study, sequential PET can be used to evaluate the progress of disease particularly in pediatric epilepsy patients. There has been reports of good correlation of cortical hypometabolism change with improvement in seizure frequency, which may be a marker for disease progress. There may also be promising development using a multimodal approach in the near future. Fusion PET-MRI system with high resolution research tomography PET (HRRT-PET), using PET coregistered with high resolution MRI (such as 7T MRI) may be a useful tool to study the molecular-genetic and functioning of brain in epilepsy patients. PET can also be used to evaluate the specifi c receptor system. Receptors PET scanning that have been developed include GABAA receptor (Flumazenil: FMZ), alpha methyl tryptophan receptors and others. They aim to identify the specifi c receptor in refractory epileptic patients. They are likely to become important in the future and may provide insights and understandings of the complexes mechanisms of refractory epilepsy.8 It can be used to study development and termination of seizures. It has an invaluable role in the epileptogenesis research and to identify the underlying cause of epilepsy.