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SINGLE PHOTON EMISSION COMPUTERISED TOMOGRAPHY

Single photon emission computerised tomography (SPECT) is principally used in the
investigation of the epilepsies to image the distribution of cerebral blood flow (CBF). The
most commonly used SPECT tracers for imaging CBF are 99mTc-hexamethyl-propylenamine
oxime (99mTc-HMPAO) and 99mTc-ethyl cysteinate dimer (ECD, bicisate). 99mTc-HMPAO is
given by vein and 70% brain uptake occurs in one minute. The subsequent image is stable for
six hours, as after crossing the blood-brain barrier 99mTc-HMPAO reacts with intracellular
glutathione, becoming hydrophilic and so is much less able to recross the blood-brain barrier.
Radio-labelled ECD is stable for six hours, easing study of brief ictal events120.

Inter-ictal SPECT studies

It was established in the 1980s that the marker of an epileptic focus studied inter-ictally in
adults and children with SPECT is a region of reduced CBF, but it was soon noted that the
results were not reliable. Lobar localisation (e.g. frontal versus temporal) has been more
difficult with, in one large representative series, correct localisation in 38% in inter-ictal
studies of patients with unilateral temporal lobe EEG focus121. Localisation with inter-ictal
SPECT is more difficult in patients with extratemporal epilepsy122,123. In a blinded
comparative study, inter-ictal SPECT was less effective at lateralising the focus of TLE than
MRI, with correct lateralisation in 45% compared to 86%. In consequence, inter-ictal SPECT
has little place in the routine investigation of patients with epilepsy.

Ictal and post-ictal SPECT studies

The increase in CBF associated with a seizure may be detected using SPECT and may provide
useful localising information in patients with partial seizures. An injection of 99mTc-HMPAO
at the time of a seizure results in an image of the distribution of CBF 12 minutes after tracer
administration. The general pattern is of localised ictal hyperperfusion, with surrounding
hypoperfusion, that is followed by accentuated hypoperfusion in the region of the focus,
which gradually returns to the inter-ictal state. Combined data from inter-ictal and ictal
SPECT scans give a lot more data than inter-ictal scans alone and may be useful in the
evaluation of both temporal and extratemporal epilepsy. In complex partial seizure disorders,
the epileptic focus has been identified in 6993% of ictal SPECT studies. A meta-analysis of
published data showed that in patients with TLE, the sensitivities of SPECT relative to
diagnostic evaluation were 0.44 (inter-ictal), 0.75 (post-ictal) and 0.97 (ictal)124.

In temporal lobe seizures, the occurrence of contralateral dystonic posturing was associated
with an ictal increase in CBF in the basal ganglia ipsilateral to the focus125. A characteristic
feature of temporal lobe seizures is an initial hyperperfusion of the temporal lobe, followed
by medial temporal hyperperfusion and lateral temporal hypoperfusion126.

Ictal 99mTc-HMPAO scans may be useful in the evaluation of patients with extratemporal
seizures and unremarkable MRI127. Asymmetric tonic posturing, contralateral head and eye
deviation and unilateral clonic jerking were associated with an ictal increase in CBF in the
frontocentral, medial frontal or dorsolateral areas127. Varying patterns have been seen in
patients with autosomal dominant frontal lobe epilepsy128.

The coregistration of post-ictal SPECT images with a patient’s MRI improves anatomical
determination of abnormalities of CBF129. A greater advance, however, has been the
coregistration of inter-ictal with ictal or post-ictal SPECT images, to result in an ‘ictal
difference image’ that may be coregistered with an individual’s MRI. This technique
enhances objectivity and the accuracy of data interpretation130,131. More recently, it has been