Epilepsy Figure 5: Magnetoencephalography Left MEG channels EKG R L R L A PP A
Fp1–F3 F3–C3 C3–P3 P3–O1 Fp1–F7 F7–T3 T3–T5 T5–O1 Fz–Cz Cz–Pz Fp2–F4 F4–C4 C4–P4 C4–O2 Fp2–F8 F8–T4 T4–T6 T6–O2
R LL R
3.00 pT–75.00 u 0.3000 s
3.00 pT–75.00 u 0.3000 s
3.00 pT–75.00 u 0.3000 s
A 35-year-old man presented with long-standing generalised tonic–clonic seizures, with magnetic resonance imaging revealing a subtle cortical dysplasia in the right frontal lobe. Left: A magnetoencephalography (MEG) study with simultaneous electroencephalography recording demonstrated frequent high-amplitude spike and slow-wave discharges that were maximal in the right frontal region (right panels); manual single equivalent current dipole fitting was performed on these spikes (shown in orange) and dipoles (shown as yellow triangles on all MEG images) clustered in the right frontal regions. The patient underwent a lesionectomy and was seizure-free after surgery. EKG = electrocardiogram; pT = pico-teslas; s = seconds; u = units. Adapted from Chang et al., 2009,56
with permission.
Single Photon Emission Computed Tomography SPECT uses photon-emitting radioisotopes attached to molecules to label brain regions of interest, mainly areas of hypermetabolic activity during ictal activity. The images are captured using relatively fast and low-resolution gamma cameras. The majority of studies in epilepsy use technetium-99m hexamethylpropylene amine oxime (99mTc-HMPAO) or technetium-99m ethyl cysteinate diethylester (99mTc-ECD), both of which are lipophilic compounds that get irreversibly trapped in the epileptogenic hyperemic region at the time of the seizure.21,31
Because of the rapid initial tracer uptake
and distribution, the radiotracer can be injected at the time of seizure onset. The patient can then be sedated and transferred to SPECT scanners to acquire the brain scan. SPECT imaging has the advantage of capturing the dynamic ictal process and being less subject to movement artifacts (see Figure 4B).21
Many studies have assessed the sensitivity and specificity of SPECT imaging in revealing epilepsy and have reported favourable results. A meta-analysis showed that the highest sensitivity reported in ictal SPECT is 97–100 %, post-ictal SPECT 75–77 %, and interictal SPECT 43–44 %.40,41
the basal ganglia, which may reflect more of the ictal dystonia caused by seizure propagation rather than area of seizure onset.45
Right MEG channels
In addition,
other brain regions have been noted to show SPECT changes in TLE, including hyperperfusion in the posterior frontal lobes, parietal lobes, and cerebellum and hypoperfusion in the bilateral frontal lobes.46 These differences may indicate inter-subject differences in functional network connectivity or may reflect different temporal evolution of an ictal event based on when the tracer was injected. A large TLE series attempted to examine this very question of evolving dynamics of the seizure network in 37 TLE patients with varying delays in tracer injection. The authors found significant ictal hyperperfusion in the ipsilateral temporal lobe and hypoperfusion in the ipsilateral orbitofrontal, bilateral superior frontal, contralateral cerebellar, and ipsilateral striatum regions.47
Future controlled studies with improved
temporal resolution with SPECT injection and scanning are needed to elucidate the dynamics of the seizure network.
The different radiotracers may confer different detection rates in different types of epilepsy. For instance, in TLE, HMPAO and ECD have shown similar sensitivities in detecting the epileptogenic region, but HMPAO is superior to ECD in terms of both sensitivity and degree of hyperperfusion in neocortical epilepsy.42
Despite the
high sensitivity of ictal SPECT, several studies have questioned the specificity of SPECT imaging, especially when SPECT does not correlate with EEG data.43,44
In one study, SPECT erroneously localised
supplementary motor area seizures to the bilateral cingulate.43 SPECT should be used for partial seizure evaluation but may not be useful for generalised seizures.
In another study, patients with mesial TLE were found to have localised hyperperfusion not limited to the temporal lobe but also in
260
One disadvantage of SPECT is the qualitative nature of image interpretation, which is subject to interpreter bias. One way to circumvent this problem is to create automated comparison of the baseline interictal SPECT with subtraction ictal SPECT co-registered to MRI (SISCOM). Implementation of this analysis method showed high predictive value when the hyperperfused area was completely resected.48–52
This result has been confirmed in a large cohort of patients with non-localising MRI and EEGs, where the presence of focal SPECT hypermetabolism has the highest odds ratio prediction of seizure-free outcome.35
The clinical value and unique ability to capture
dynamic network processes during ictal activity make SPECT an attractive imaging modality in the detection and study of epilepsy.
Magnetoencephalography
MEG was first introduced in 1968 and works by detecting magnetic fields generated by neuronal activity using arrays of superconducting quantum interference devices (SQUIDs). MEG is a non-invasive
EUROPEAN NEUROLOGICAL REVIEW
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