Inter-ictal brain SPET in frontal epilepsy: correlations with stereo-electroencephalography

Ictal Brain Hyperperfusion Contralateral to Seizure Onset: The SPECT Mirror Image

PURPOSE

Ictal single-photon emission computed tomography (SPECT) may help localize the seizure-onset zone (SOZ) by detecting changes in regional cerebral blood flow induced by epileptic discharges. This imaging method also reveals hyperperfusions in areas of seizure propagation, including the hemisphere contralateral to the SOZ. We have studied the occurrence, the topography, and the clinical value of such contralateral ictal hyperperfusion areas (HPAs).

METHODS

We examined data from presurgical evaluations of 36 consecutive patients with pharmacoresistant partial epilepsy of various localizations. Ictal and interictal SPECT examinations were made with 99mTc-ECD, and the scans were processed for coregistration, normalization, subtraction, and merging with MRI images.

RESULTS

Contralateral HPAs were observed in 72% of the patients: 50% of mesiotemporal epilepsy cases with hippocampal sclerosis, 85.7% of the other mesiotemporal epilepsies, 85.7% of neocortical lateral temporal epilepsies, and 87.5% of extratemporal epilepsies. Contralateral HPAs were usually symmetrical to the SOZ, forming a mirror image, observed in 57.1% of the patients. They could be slightly asymmetrical in mesiotemporal epilepsies, perhaps because of the particular anatomic pathways linking temporal lobes. In neocortical epilepsies, they were located in the cortex homotopic to the SOZ.

CONCLUSIONS

We show that the symmetrical nature of the mirror image usually does not disturb SPECT interpretation. It can confirm the location of the SOZ (11 patients) and even occasionally improve the precision of its definition (nine patients) by restraining several potential SOZ-related HPAs to a single one or by permitting a restricted localization of the SOZ in a large HPA.

Interictal diffusion MRI in partial epilepsies explored with intracerebral electrodes

Patients with refractory partial seizures may benefit from epilepsy surgery. However, invasive investigations are often needed to define the precise location and limits of the epileptogenic zone (EZ). In this study, we asked whether diffusion tensor imaging (DTI) might provide a non-invasive alternative to locate the EZ or at least provide insights to help place intracerebral electrodes for stereo-electroencephalography (SEEG). Whole brain DTI and voxel-based analysis (SPM99) was used to assess diffusion properties objectively in 16 epilepsy patients investigated with SEEG. Epilepsy was symptomatic in two patients and cryptogenic in the 14 remaining patients. The suspected onset of seizures was temporal in 10 patients, frontal in 2 and occipital in 4. Individual maps of apparent diffusion coefficient (ADC) and fractional anisotropy (FA) were calculated and compared to a database of 40 healthy volunteers. Thirteen of 16 patients exhibited diffusion abnormalities. ADC abnormalities were better correlated with SEEG data than FA abnormalities which were usually located at a distance or in the white matter. A significant increase in ADC (P < 0.01) was found in 11 patients and was located in the regions explored with depth electrodes in 7 of them. Surgery outcome was available in 3 of these 7 patients (2 were seizure free and 1 not). DTI specificity was better in extratemporal lobe epilepsy (83%) than in temporal lobe epilepsy (20%). When abnormalities concurred with the SEEG data, the concordance was optimal between the localization of the diffusion abnormalities and the irritative zone defined by SEEG. These encouraging, preliminary results, suggest that DTI examinations may provide accurate spatial data on the location and extent of the epileptogenic network in extratemporal lobe epilepsies.

Double-blind stereo-EEG and FDG PET study in severe partial epilepsies: are the electric and metabolic findings related?

The aim of this study was to evaluate, in 16 patients with drug-resistant partial epilepsy who were waiting to undergo surgical treatment, the relation between positron emission tomography (PET) findings with fluorine-18 fluorodeoxyglucose ([18F]FDG) in the interictal state and the different stereo-electroencephalography (SEEG) patterns that characterize: (a) the epileptogenic zone (low-voltage fast-activity discharge before or concurrent with ictal clinical symptoms), (b) the irritative zone (spikes, spikes and waves, isolated or grouped in short bursts) and (c) the lesional zone (continuous, sometimes polyrhythmic slow waves or continuous delta waves or very important voltage depression). SEEG was performed following an individually defined electrode implantation strategy. Whereas at least one area of hypometabolism was detected by visual interpretation of PET/[18F]FDG images in all the subjects in the study, there was poor agreement between PET/[18F]FDG quantitative measures of regional metabolism and SEEG findings. Normal metabolic rates were found in up to 62% of the areas with abnormal SEEG activity, independent of the type of electrical activity, i.e. epileptogenic, irritative, or lesional, while abnormal metabolic rates were found in up to 23% of the areas with normal SEEG activity. In conclusion, whereas the visual interpretation of interictal studies of glucose utilization in our series of drug-resistant epileptic patients consistently allowed the localization of an area of temporal hypometabolism, the quantitative and regional metabolic analysis demonstrated that such a finding is not specifically related to any of the three very different SEEG patterns (epileptogenic, irritative, lesional) or combinations thereof. These results complement those of previous interictal and ictal single-photon emission tomographic studies and of receptor studies in epileptics, suggesting functional and biochemical heterogeneity within the interictal hypoperfused/hypometabolic area in epileptic patients, and contribute to the debate on the use and interpretation of interictal PET/[18F]FDG studies in patients with medically refractory partial seizures.

Proportional anatomical stereotactic atlas for visual interpretation of brain SPET perfusion images

A semi-automatic method was developed to determine the anterior (AC) and posterior (PC) commissures on brain single-photon emission tomographic (SPET) perfusion images, and then to draw the proportional anatomical Talairach's grid on each axial SPET image. First, the AC-PC line was defined on SPET images from the linear regression of four internal landmarks (frontal pole of the brain, inferior limit of the anterior corpus callosum, sub-thalamic point and occipital pole). Second, the SPET position of AC and PC points on the AC-PC line was automatically determined from measurements made on hard copies of magnetic resonance (MR) images of the patients. Finally, a proportional Talairach's grid was automatically drawn on each axial SPET image. To assess the accuracy of localization of AC and PC points, co-registered technetium-99m hexamethylpropylene amine oxime SPET and MR images from 11 subjects were used. The mean displacements between estimated points on SPET and true points on MRI (Deltax=sagittal, Deltay=frontal and Deltaz=axial displacement) were calculated. The mean displacements (in mm) were Deltax=-1.4+/-1.8, Deltay=-1.7+/-3.3 and Deltaz=-1. 1+/-2.5 for AC, and Deltax=-1.8+/-1.8, Deltay=0.3+/-3.2 and Deltaz=-1.3+/-2.7 for PC. These displacements represented an error of less than 5 mm at the anterior or posterior pole of the brain or at the vertex. Intra- and inter-observer comparisons did not reveal significant differences in mean displacements. Thus, this semi-automatic method results in reproducible and accurate stereotactic localization of SPET perfusion abnormalities. This method can be used routinely for repeat follow-up studies in the same subject as well as in different individuals without requiring SPET-MRI co-registration.