Functional Brain Imaging

There are two well validated techniques of functional brain imaging in use: PET and SPECT. Functional techniques using MR and MEG for cerebral localization are still in the development stage.

There are a number of different PET techniques but the most useful in pre-surgical assessment uses fluoro-deoxy-glucose (FDG) as the ligand. In temporal lobe epilepsy, in the interictal state, there may be an area of hypome-tabolism on the same side as the epileptic focus and this corresponds well with lateralization from other investigations, including depth electrode exploration. Patients with bilateral hypo-metabolism on FDG-PET have a worse outcome from temporal lobe surgery than those with unilateral hypometabolism. Hypometabolism with flumazenil C11 PET detects mesial temporal sclerosis and is more precise than FDG-PET, although not necessarily more efficient. It has been shown to give additional useful information in certain situations, as described by Ryvthin et al. [8]. In patients with some forms of cortical neuronal migration disorder, FDG-PET abnormalities may be seen when the MRI appears normal. Such a focal PET abnormality was first described in neonates who appeared to have non-focal epilepsy and it has been suggested that it may be useful in hemimegalen-cephaly. It has also been shown that in patients with tuberose sclerosis, those tubers which are likely to be epileptiform can be identified using C11, labelled tryptophan.

The use of SPECT for imaging in epilepsy patients is more complex. The usual ligand is HMPAO and Bonte first described abnormalities in epilepsy in 1983. Studies indicate that interictal SPECT demonstrating hypometabolic activity is unreliable, except when it correlates well with other tests. Ictal SPECT, obtained after the ligand had been administered during or close to a seizure, demonstrates hypometabo-lisms more reliably. Further studies showed that if the time course of the changes in relation to the seizure was taken into account, ictal SPECT was very useful in non-invasive assessment of temporal lobe epilepsies. There has been a lot of interest in the use of SPECT in frontal-lobe epilepsy. The necessity to mix chemicals to produce the ligand was a time-limiting factor, but a pre-mixed version is now available. However, the brief duration and rapid spread of frontal-lobe seizures still make capture with SPECT difficult, since some may be of briefer duration than the circulation time of the tracer. However, co-registration of SPECT on MRI has been described as useful in extratemporal epilepsies by O'Brien et al. [9].

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