Positron Emission Tomography (PET)

Positron-emission tomography (PET) is a nuclear medicine imaging modality which detects gamma rays emitted by a positron-emitting radioactive tracer. The most common tracer used for neuroimaging is 2-deoxy-2 (18F) fluoro-d-glucose (FDG). It approximates for the metabolic processes in the brain providing a broad range of functional and metabolic information to help understand mechanisms of neurologic diseases and guide therapeutic approaches. Most settings have used 2-deoxy-2 (18F) fluoro-d-glucose (FDG) in the interictal state [35,47]. Ictal PET is not of much help as the half-life of radiotracer used extremely short. For patients with temporal lobe epilepsy interictal studies shows hypometabolism in epileptogenic areas. Interictal FDG-PET provides more information about epileptogenic foci compared to

structural neuroimaging, and also detects abnormality in epilepsy patients with normal MRI. Sensitivity of interictal FDG-PET is high for temporal lobe epilepsy, but is unclear for extratemporal lobe epilepsy.When a single region of metabolic abnormality corresponding to the EEG abnormality is detected on PET, further surgical treatment can be planned for controlling seizures and improving developmental outcome. Coregistration of FDG-PET/MRI helps for localizing seizure foci noninvasively and successful surgical treatment of patients with cortical dysplasia (CD), especially for patients with nonconcordant findings and normal MRI scan [48,49]. Ligand/neuroreceptor PET studies improves sensitivity and specificity for both temporal and extratemporal lobe epilepsy [50].

Neuroreceptor tracers show increased or decreased uptake in epileptogenic brain regions in the interictal state, unlike FDG-PET which shows decreased uptake in interictal period. Hence this type of neuroimaging is of more importance while detecting epileptogenic area in presence of multiple structural lesions, which confounds epileptogenic foci. AMT ([11C] methyl-l-tryptophan) PET effectively differentiates between epileptogenic and nonepileptogenic lesions in the interictal state in children with numerous structural lesions tuberous sclerosis. AMT PET [51] can also detect epileptic cortex in nontuberous sclerosis patient with normal MRI. FMZ PET [52,50] involves benzodiazepine labeling. 11C-flumazenil (FMZ), benzodiazepine receptor antagonist, shows reduced binding in the epileptogenic focus. Flumazenil-labeled PET studies are more specific than FDG studies, FMZ-PET image shows more restricted localization compared to FDG PET. Similarly, opiate, histamine, and N-methyl-d-aspartate receptor studies for epilepsy are under development

[9,50].

Magnetic Resonance Spectroscopy:
Magnetic resonance spectroscopy is a relatively new technique, where high strength magnetic field are used which provides both structural and functional details of the brain. It provides biochemical measurement of various brain metabolite, noninvasively [36]. MRS demonstrates reduction in N-acetyl aspartate (NAA) signal and increases in creatinine and choline signal. MRS is sensitive in showing biochemical abnormalities in specific epileptogenic foci, in seizure patient where structural neuroimaging failed to demonstrate lesion [36]. MRS, has been used in evaluation of both focal and generalized epilepsy. Magnetic resonance spectroscopy is indicated to screen pediatric epilepsy patients with metabolic derangements (inborn errors of metabolism) and for characterization of masses detected on MRI [53]. MRS yields additional details about the lesions detected on MRI and it also detects the lesion which were missed on MRI [54]. MRS has shown to be of importance for patients with temporal lobe epilepsy and is of prognostic importance for patients undergoing surgery for epilepsy, however its sensitivity for neocortical and extratemporal lobe epilepsy is not well established