Functional Imaging

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Single photon emission computed tomography (SPECT) in epilepsy has mainly been confined to the

Figure 3 Imaging findings in temporal lobe epilepsy with hippocampal sclerosis. Left: MRI coronal (top) and horizontal (bottom), inversion recovery sequence. Hippocampal sclerosis (arrow). Right: Interictal HMPAO SPECT shows extensive ipsilat-eral hypoperfusion.

imaging of cerebral blood flow in focal epilepsy. The tracer most widely used is 99Tc-HMPAO. Interictally, there is localized hypoperfusion in an area extending beyond the epileptogenic region. Initially, there was considerable skepticism about the clinical value of the technique because the early studies were of low resolution and the correlations with electroencephalo-graphic findings were imprecise. The sensitivity of interictal focus detection of SPECT in the literature ranges from 40 to 80%. In recent years there have been major technical developments in instrumentation. Using brain-dedicated multiheaded camera systems, the sensitivity of SPECT is similar to that of [18F] fluorodeoxyglucose (FDG) positron emission tomography (PET).

99Tc-HMPAO is distributed within a few minutes after injection in the brain, where it remains fixed for about 2 hr. If the radioisotope is injected during or shortly after an epileptic seizure, scanning can be carried out postictally without problems due to involuntary movements. Postictal and ictal SPECT is more sensitive than interictal SPECT and typically shows hyperperfusion ipsilateral to the epileptogenic focus. Another ligand used for SPECT is 123I-iomaze-nile, which is used to demonstrate benzodiazepine receptor binding, which is decreased in the epilepto-genic region.

Compared to SPECT, PET is superior with respect to spatial and contrast resolution. PET, however, is expensive and requires an on-site medical cyclotron. Ictal studies are difficult with PET because of the short half-life of positron emitting radioisotopes. PET has mainly been used to study interictal blood flow with nitrogen-13-labeled ammonia and oxygen-15 and also glucose metabolism with FDG in focal epilepsy. Localized hypoperfusion and hypometabolism in the epileptogenic area as shown by PET are seen as a reliable confirmatory finding in the presurgical assessment of temporal lobe epilepsy. PET has also been used for imaging of benzodiazepine receptor binding and opiate receptor binding in epilepsy.

The major application of functional MRI in epilepsy is the noninvasive mapping of eloquent cortex in the presurgical evaluation of patients based on activation studies. The clear advantage over PET is the superior spatial resolution, which is on the order of 3 mm. The coregistration of a high-resolution MRI allows excellent localization of activated regions.

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