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that may indent the overlying skull and also extend subcortically, with low signal intensity
on T1-weighted images, high signal on T2-weighted images that is similar to CSF, and
slightly higher signal intensity in the lesion than CSF on proton density images. Cyst
formation and enhancement with gadolinium may occur. Calcification is present in some
cases and may be more readily demonstrated with X-ray CT. Confident differentiation from
low-grade astrocytomas and ganglioglioma is not possible by MRI24.

Hypothalamic hamartomas, sometimes associated with gelastic epilepsy, precocious puberty
and cognitive impairment, are clearly demonstrable using MRI25. More subtle abnormalities
such as focal nodular heterotopia and band heterotopia may only be apparent if optimal MRI
techniques are used. Band heterotopia ‘double cortex’ is an example of a generalised MCD
that may be present in patients with mild epilepsy and normal intellect.

Focal cortical dysplasia may result in refractory partial seizures. The possibility of surgical
treatment means its identification with MRI has important consequences. Focal cortical
dysplasia is not always identified with conventional MRI and may be more easily identified
on a FLAIR sequence6,26,181, by reconstructing the imaging dataset in curvilinear planes, by
quantitative assessment of signal and texture27 and by sulcal analysis28.

Cavernomas
Cerebral cavernomas commonly underlie epilepsy and surgical removal carries up to a 70%
chance of subsequent seizure remission. Cavernomas are often not identified on X-ray CT,
but are readily apparent on MRI. Cavernomas are circumscribed and have the characteristic
appearance of a range of blood products. The central part contains areas of high signal on T1-
and T2-weighted images, reflecting oxidised haemoglobin, with darker areas on T1-weighted
images due to deoxyhaemoglobin. The ring of surrounding haemosiderin appears dark on a
T2-weighted image. There may be calcification, which usually appears dark on T1- and T2-
weighted images. There is no evidence of arteriovenous shunting. Arteriovenous
malformations with high blood flow have a different and distinctive appearance.

Granulomas
Worldwide, the commonest causes of refractory focal epilepsy are cysticercosis and
tuberculomas. These lesions have characteristic appearances on MRI that evolve with time
and which, unless calcified, may resolve and be regarded as ‘disappearing lesions’29.

Longitudinal studies of the effect of epilepsy on the brain

Voxel and anatomically-based methods may be applied in longitudinal studies to identify
subtle changes in the brain and to determine the effects of epilepsy. The majority of previous
cross-sectional studies have inferred that more severe hippocampal damage is associated with
a longer duration of epilepsy and a greater number of seizures. Longitudinal studies, however,
are necessary to ascribe cause and effect. Two recent studies have suggested atrophy of the
hippocampus occurring over three years of active epilepsy in patients attending epilepsy
clinics30,31.

A large community-based study has shown that those with a history of a prior neurological
insult had smaller neocortical volumes and an accelerated rate of brain atrophy, and that in
patients with newly diagnosed epilepsy without a history of prior insult the rate of atrophy
was no different from age-matched controls. Patients with chronic epilepsy, however, were
more likely to have had significant loss of neocortical, hippocampal or cerebellar volume
over 3.5 years32. Further, on a more sensitive voxel-based analysis, 54% of those with chronic
epilepsy, 39% of those with newly diagnosed seizures and 24% of controls had areas of brain
volume loss33. These studies implied that secondary brain damage might occur in the context