Diffusion tensor imaging of cryptogenic and acquired partial epilepsies

Abstract

Current optimal MRI identifies a relevant structural abnormality in up to 80% of patients with refractory partial seizures. Identification of a structural lesion is fundamental to pre-surgical evaluation. We used diffusion tensor imaging (DTI) and statistical parametric mapping (SPM) to examine objectively the diffusion properties, and hence structural organization, of cerebral tissue in 10 patients with partial seizures and acquired lesions and 30 patients with partial seizures and normal MRI. Fractional anisotropy and mean diffusivity maps were calculated and, using SPM, individual patients were compared with a group of 30 control subjects. Diffusion tensor imaging and voxel-by-voxel statistical comparisons identified significant increases in diffusivity and significant reductions of anisotropy in all patients with acquired non-progressive cerebral lesions and partial seizures. In all of these patients, the areas of increased diffusivity, and in nine patients the areas of decreased anisotropy, concurred with abnormalities identified on visual inspection of conventional MRI. In addition, there were 10 areas which were normal on conventional imaging which exhibited abnormal anisotropy or diffusivity. Individual analyses of the 30 patients with partial seizures and normal optimal MRI identified a significant increase in diffusivity in eight of the subjects. In six of these, the areas of increased diffusivity concurred with the localization of epileptiform EEG abnormality. Analysis of anisotropy in the MRI-negative patients revealed significant differences in two patients, one of which concurred with electroclinical seizure localization. Group analysis of nine patients with normal conventional MRI and electroclinical seizure onset localizing to the left temporal region revealed a significant increase in diffusivity and a significant reduction in anisotropy within the white matter of the left temporal lobe. DTI analysed using SPM was sensitive in patients with acquired cerebral damage. Significant differences in the diffusion indices in individual MRI negative patients and the group effect in patients with left temporal lobe epilepsy suggest that minor structural disorganization exists in occult epileptogenic cerebral lesions. These techniques are promising, non-invasive imaging methods for identifying the cause of partial seizures, and can contribute to pre-surgical evaluation.

Spatial normalization of diffusion tensor MRI using multiple channels

“…Most researchers have used ROI methods that begin by identifying anatomical brain regions and by comparing the anisotropy or the extent of the region having thresholded high anisotropy (Peled et al, 1998;Kubicki et al, 2002;McGraw et al, 2002). For studies where the region of interest (ROI) of potential abnormality is difficult to define precisely, voxel-based methods tend to be used Rugg-Gunn et al, 2001). A voxel-based strategy is more exploratory and is suitable for identifying unanticipated or unpredicted/unhypothesized areas of abnormal white matter morphology.…”

Advances in neuroimaging in patients with epilepsy

“…108,131,132 Reduced FA and increased mean diffusivity have been demonstrated adjacent to malformations of cortical development visible on MR imaging. 35,129 Increased perpendicular diffusivities have also been found adjacent to malformations of cortical development, suggesting a dominant effect of abnormal myelin in the white matter adjacent to these malformations.…”

Functional magnetic resonance imaging: A review of methodological aspects and clinical applications

“…A new approach is the use of FA maps as the basis for morphometry. Alterations in the diffusion tensor have successfully been related to schizophrenia (112)(113)(114), epilepsy (115,116), stuttering (117), dyslexia (118), and demyelination disorders (119,120). In most of these studies, the anisotropy of diffusion is evaluated.…”

“…The FA of diffusion reflects the coherence of the direction of fibers within each voxel. Lower FA values can indicate decreased fiber coherence or myelination defects (115,116,121). Voxel-based morphometry techniques used to compare the diffusion tensor-derived tissue properties between patient and control groups encounter difficulties arising from normalization problems.…”