Several neuroimaging techniques can be used to evaluate seizure localization and plan surgical treatment of structural lesions.
() scans of the head are usually reserved for the acute evaluation of new-onset seizures, owing to their ready availability in settings such as the emergency room. They can detect etiologies such as acute hemorrhage or 50 but they are less sensitive than () in detecting structural lesions. Middle and posterior fossa lesions can be masked by bone artifact on CT. 49
MRI is the gold standard in evaluating epilepsy. It can identify lesions, such as a neoplasm, that may be missed on CT. New-onset nontraumatic seizures should be evaluated with gadolinium-enhanced T1-weighted, T2-weighted, and fluid-attenuated inversion recovery (FLAIR) images. To evaluate a chronic disorder, however, the use of contrast may be unnecessary unless there has been a recent change in seizure type, frequency, or intensity.51
MRI is free of radiation and is therefore very safe.52 It can be used to verify the placement of invasive electrodes in evaluations for epilepsy surgery, and to document the extent of cortical resection after surgery.
Some brain tumors have characteristic findings that can distinguish them from non-neoplastic lesions or even suggest a specific type of tumor. It may be difficult, however, to differentiate low-grade gliomas from other tumor types presenting with epilepsy:
Focal calcifications are seen in 25% of tumors, but CT scans show hyperintensity in more than 70%.55 To identify focal lesions, T2-weighted sequences are more sensitive to low-grade tumors, arteriovenous malformations, focal gliosis, and hamartomas.57–59
Other neuroimaging techniques, while not currently primary neurodiagnostic modalities in the evaluation of tumors and epilepsy, are helpful adjuncts. They can increase the level of certainty that the region of seizure activity and eloquent cortex have been correctly identified, provide prognostic tumor data, and guide intracranial placement.
Positron emission tomography (PET) scanning is a functional glucose metabolic imaging technique that assesses cerebral . Patients with low-grade tumors may demonstrate areas of hypometabolism interictally, and hypermetabolism may be seen during events. These areas tend to be larger than the anatomic abnormality itself.60 PET with fluoro-2-deoxy-D-glucose (FDG) uptake can have prognostic value in low-grade gliomas.61 Those tumors that display areas of increased FDG uptake are likely to recur or progress to higher grades.
Derlon et al. compared metabolic patterns between astrocytomas and oligodendrogliomas using PET FDG (glucose) and 11C-L-methylmethionine (amino acid) uptake.62 Although both tumors showed glucose hypometabolism, amino acid uptake was increased only in oligodendrogliomas. This suggests that specialized PET protocols may aid in distinguishing tumor types. Continued research using this technology may help improve diagnosis, therapy, and assessment of possible tumor progression.
Single photon emission computerized tomography () is a functional cerebral perfusion imaging technique used in the noninvasive physiologic evaluation of seizures to help define focal areas of abnormality. SPECT is less expensive than PET and is available in more hospitals. Electrographically, widespread blood flow changes may be seen during a seizure, but well-timed SPECT injections close to ictal onset may identify the origin of seizure onset. SPECT aids in localization by demonstrating cerebral areas that have decreased regional cerebral blood flow (rCBF) interictally and increased areas of rCBF ictally. There are several difficulties in accomplishing ictal SPECT, however. Injecting the radioisotope on time can be a challenge (particularly with nocturnal seizures), given the unpredictability and short duration of some seizures, the need to obtain the isotope, the limit of its , and the limited sensitivity and specificity of postictal data.63
More recently, co-registered subtraction SPECT imaging from MRI has been shown to improve reliability in localizing seizure foci postictally.64
() is a noninvasive technique that measures metabolic activity, allowing for the comparison of neural and tumoral elements containing protons. Relevant metabolites are creatine, N-acetylaspartate, lactate, and carbohydrate-containing phospholipids. Assessment of carbohydrate (CHO) peaks and lac tate levels helps to assess tumor aggressiveness and distinguish radiation damage from tumor recurrence.
In a recent study of 11 pediatric patients with low-grade gliomas, Lazareff et al. showed that MRS CHO values were a viable noninvasive prognostic tool to follow tumor progression, with higher ratios correlating with more rapid tumor growth.65
MRS has also been shown to be of prognostic value when treating recurrent malignant gliomas with radiosurgery.66
Functional MRI (fMRI) is another noninvasive imaging technique that maps changes in rCBF and concentrations of oxyhemoglobin or deoxyhemoglobin on task performance. It is a useful tool to identify regions of eloquent cortex that need to be spared in lesion resection.67,68
Adapted from: Mangano FT, McBride AE, and Schneider SJ. Brain tumors and epilepsy. In: Ettinger AB and Devinsky O, eds. Managing epilepsy and co-existing disorders. Boston: Butterworth-Heinemann; 2002;175–194.
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