Mesial Temporal Sclerosis MRI
The incidence of temporal lobe epilepsy (TLE) due to mesial temporal sclerosis (MTS) can be high in developing countries. Current diagnosis of MTS relies on structural MRI, which is generally unavailable in developing world settings.
The study of Mandell et al.shows strong evidence that temporal lobe and brain volume can be predictive of seizure outcome following temporal lobe resection, and that volumetric CT analysis of the temporal lobe may be feasible in lieu of structural MRI when the latter is unavailable. Furthermore, since the authors' methods are modality independent, these findings suggest that temporal lobe and normative brain volumes may further be useful in the selection of patients for temporal lobe resection when structural MRI is available 1).
MRI is the modality of choice to evaluate the hippocampus, however dedicated TLE protocol needs to be performed if good sensitivity and specificity is to be achieved.
Thin section angled coronal sequences at right angles to the longitudinal axis of the hippocampus are required, to minimize volume averaging.
Coronal volume and coronal high resolution T2 weighted image/FLAIR are best to diagnose MTS.
Findings include:
reduced hippocampal volume: hippocampal atrophy
increased T2 signal
abnormal morphology: loss of internal architecture (interdigitations of hippocampus)
Although comparing left to right side is easiest, it must be remembered that up to 10% of cases are bilateral, and thus if symmetry is the only feature being evaluated, many cases may be misinterpreted as normal.
Often mentioned, but probably one of the least specific findings, is enlargement of the temporal horn of the lateral ventricle.
If anything, care must be taken to not allow an enlarged horn to trick you into thinking the hippocampus is reduced in size.
When severe and long standing, additional associated findings include:
atrophy of the ipsilateral fornix and mamillary body
increased signal and or atrophy of the anterior thalamic nucleus
atrophy of the cingulate gyrus
increased signal and/or reduction in volume of the amygdala
reduction in volume of the subiculum
dilatation of temporal horn and temporal lobe atrophy
collateral white matter and entorhinal cortex atrophy
thalamic and caudate atrophy
ipsilateral cerebral hypertrophy
contralateral cerebellar hemiatrophy
loss of grey-white matter interface in the anterior temporal lobe
reduced white matter volume in the parahippocampal gyrus
Additional 3D volumetric studies can be performed, and although time consuming to post-process may be more sensitive to subtle hippocampal volume loss. Gadolinium is not required.
T2 relaxometry
T2 relaxometry may also be useful in detecting cases of hippocampal sclerosis.
Diffusion MRI
MR spectroscopy
MR perfusion
MR perfusion demonstrates similar changes to SPECT (see below) with blood perfusion depending on when the scan is obtained.
During the peri-ictal phases, perfusion is increased, not only in the mesial temporal lobe but often in large parts of temporal lobe and hemisphere. In interictal periods, in contrast, perfusion is reduced 2).