Structural and functional imaging is employed to help identify candidates for surgery, to plan surgical approaches, and to inform likelihood of favorable outcome. The primary role of imaging is to help identify and to confirm the location of the seizures focus. This is the place of structural MRI, FDG-PET (fluoro-deoxy glucose positron emission tomography), and SPECT (single-photon emission computerized tomography). Surgical outcome is high when focal abnormalities are found with these imaging techniques. Newer techniques used in planning epilepsy surgery, fMRI, and DTI (diffusion tensor imaging) are used to identify areas to spare or to avoid during resection.
Outcomes from surgery are related to the certainty of successfully identifying the seizure focus. When MRI identifies a clear focal lesion—mesial temporal sclerosis (MTS), focal cortical dysplasia (FCD), tumor, vascular malformation—then excellent outcomes for surgery (Engel I–II) are high, in the order of 75%–90%, presuming that the entire lesion is removed. When motor, sensory, or eloquent areas known to be in the epileptogenic zone are not resected, then outcomes are less successful. In contrast, when MRI is normal and other functional imaging findings are also normal, then the likelihood for excellent outcomes is, at best, 40%–50%, and closer to 30%. Video EEG (vEEG) and high-resolution MRI are a prerequisite before pursuing functional imaging that is performed to confirm or identify the seizure focus when structural imaging is normal or ambiguous. Functional imaging is also used to help target and reduce invasive monitoring (Fig. 6–1). Finally, if imaging findings are discordant with surface vEEG, then caution is warranted; if surgery is to be pursued, invasive monitoring is essential and outcome success is reduced.
Imaging evaluation flow diagram.
In the setting of a focal lesion, the seizure origin often arises from the margin of an identified tumor, FCD, stroke, or vascular malformation. In this circumstance, surgical planning may include functional imaging to target invasive monitoring to identify the adjacent epileptogenic cortex. Different approaches advocated by different epilepsy centers include: (1) removing the lesion, (2) perform intraoperative corticiography, or (3) conduct subdural grid monitoring.
POSITRON EMISSION TOMOGRAPHY (PET)
There is extensive experience with FDG-PET in children and adults with childhood-onset epilepsy. FDG-PET is performed in the interictal state with EEG monitoring to assure an interictal study. Younger children, those less than 5 years, or significantly cognitively impaired, may require sedation before image acquisition, but after the period of ligand uptake, because sedating medications may reduce cerebral glucose metabolism. Abnormal findings, defined as greater than two standard deviations using quantitative measures, are usually focal. They are typically decreases in regional glucose uptake, representing decreased regional cerebral metabolism for ...