Abstracts

Atrophy of the hippocampus and the amygdala as determined by volumetric MRI is a well-known sign for lateralizing the seizure focus in patients with temporal lobe epilepsy (TLE). However, in about 10-20% of TLE patients volumetric MRI is normal.
Texture analysis based on gray-level co-occurrence matrices quantifies the spatial pattern of gray level intensity pairs. Changes in texture values within brain tissue may be indicative of structural and organizational changes therein. Texture analysis can quantify information that may not be readily accessible through conventional MRI analysis.
The purpose of this paper was to lateralize the seizure focus in TLE with normal volumetric MRI using texture analysis of the hippocampus and amygdala.
Preoperative 3D MR images were acquired on a 1.5T scanner using a fast field echo sequence (slice thickness=1mm; approximately 170 slices with an isotropic voxel size of 1 mm³).
1. [italic]Volumetric MRI[/italic]: Absolute volumes and left-right asymmetry scores were computed based on manual segmentation of the left- and right hippocampi and amygdalae, and transformed into z-scores via comparison to an age- and sex-matched group of 30 neurologically normal control subjects. In our database of 230 consecutive TLE patients, 26 had bilaterally symmetrically normal hippocampal and amygdalar volumes using a cut-off of 2 standard deviations below the mean of the control group.
2. [italic]Texture analysis[/italic]: For each of the 26 TLE patients with normal MRI, four gray-level co-occurrence matrices (GLCMs) were constructed, one each for the left and right hippocampus and amygdala. GLCMs were calculated by tallying the number of occurrences of each gray level intensity pair separated by a distance of 3 voxels collapsed across directions. From each matrix, a set of six texture features was calculated. A left-right asymmetry index was then calculated for each structure/feature combination. Left, right, and asymmetry values were converted to z-scores via comparison to a similarly analyzed group of 30 control subjects.
3. [italic]Data analysis[/italic]: We performed leave-one-out linear discriminant analysis (with stepwise feature selection) trained on the MRI texture data to lateralize the seizure focus. The gold standard was lateralization based on EEG data, and the possible outputs of the classifier were left-lateralized or right-lateralized patients.
The classifier correctly lateralized the seizure focus in 26/26 patients (100%).
MRI texture analysis of hippocampus and amygdala is a powerful tool for lateralizing seizure focus in TLE patients with normal volumetric MRI. Texture analysis provides evidence of structural and organizational changes within the mesial temporal lobe that are not evident through volumetric analysis.
[Supported by: The Scottish Rite Charitable Foundation of Canada and Canadian Institutes fo Health research (CIHR)]