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Structural cerebral abnormalities underlying epilepsy identified with MRI

Hippocampal sclerosis
Hippocampal sclerosis (HS) is the single most common pathology underlying refractory focal
epilepsy, and is amenable to surgical treatment. The hippocampus is best visualised in two
planes: along its long axis and orthogonal to this. These imaging planes may be readily
determined on a sagittal scout image: the axial plane being in the line joining the base of the
splenium of the corpus callosum to the inferior, posterior border of the frontal lobe and the
coronal plane being perpendicular to this, parallel to the anterior border of the brainstem.

The features of HS identified by MRI are hippocampal atrophy, demonstrated with coronal
T1-weighted images, and increased signal intensity within the hippocampus on T2-weighted
spin-echo images9, decreased T1-weighted signal intensity and disruption of the internal
structure of the hippocampus10. Atrophy of temporal lobe white matter and cortex, dilatation
of the temporal horn and a blurring of the grey-white matter margin in the temporal neocortex
variably accompany HS1114. Entorhinal cortex atrophy may also occur in TLE with normal
hippocampi15.

Sophisticated and computationally expensive analyses of three-dimensional hippocampal
surface shape, and specifically deformation, have shown distinct regional changes, for
example, in the CA1 region in hippocampal sclerosis and in the medial aspect of the head of
the hippocampus in patients with temporal lobe epilepsy and normal conventional MRI.
Moreover, diffuse atrophy or contralateral hippocampal abnormalities suggested a poor post-
operative outcome16.

Quantitative MRI assessment of the hippocampus
Assessment of hippocampal atrophy can be improved by measuring hippocampal volumes.
The use of contiguous thin slices enhances the reliability of measurements and permits
localisation of atrophy along the length of the hippocampus17. Hippocampal volumetry is
demanding and time-consuming, requiring a skilled operator and a post-processing computer.
In clinical practice, hippocampal asymmetry of 20% or more is reliably visually apparent to
skilled neuroimaging specialists, but lesser degrees of asymmetry require quantification18.
Attempts have been made to automate hippocampal volume estimations and although voxel-
based approaches have given promising results, they remain inferior to expert manual
assessment19,179,180.

The T2-weighted signal intensity may be quantified by measurement of hippocampal T2
relaxation time (HT2) and this is a useful identifier of hippocampal pathology. HS may be of
varying severity along the length of the hippocampus, and may be confined to the anterior
part of the head20. A T2 relaxometry technique incorporating a FLAIR sequence obviates
possible contamination from high T2 in cerebrospinal fluid (CSF)21. Hippocampal volume
corrected for intracranial volume and HT2 are useful for identifying contralateral
hippocampal abnormality22. The same technique is useful for identifying amygdala
pathology23.

Malformations of cortical development
Malformations of cortical development (MCD) are increasingly being recognised in patients
with seizure disorders previously regarded as cryptogenic. The range of MCD identified with
MRI include schizencephaly, agyria, diffuse and focal macrogyria, focal polymicrogyria,
minor gyral abnormalities, subependymal grey matter heterotopias, bilateral subcortical
laminar heterotopia, tuberous sclerosis, focal cortical dysplasia and dysembryoplastic
neuroepithelial tumours (DNTs). DNTs are benign developmental tumours and commonly
underlie refractory partial seizures. The features are of a focal, circumscribed cortical mass