Table 51.1.

Magnetic resonance imaging (MRI) features of hippocampal sclerosis (HS) by visual analysis of MRI

Hippocampal atrophy

The most specific and reliable feature: determined by comparing the hippocampal size on each side on all available coronal slices. Small asymmetries can be present due to normal variation or a tilted position in the scanner, and should not be considered as abnormal. It is important to evaluate the shape of the hippocampus as well. The normal hippocampus has an oval shape. In the presence of hippocampal sclerosis it is usually flattened and inclined

Increased T2/FLAIR signal

May be insufficient to diagnose HS in isolation. It is important to compare both hippocampi and the signal intensity of the nearby cortex to avoid false positives. Clearly asymmetric hyperintense signal is more reliable

Loss of internal structure

Usually associated with atrophy and hyperintense T2/FLAIR signal. The loss of normal internal hippocampal structure is a consequence of neuronal loss and gliosis

Asymmetry of the horns of the lateral ventricles

Usually present when the lesion occurs later in life, and therefore, it is variable and may lead to false lateralization

Atrophy of the anterior temporal lobe

Often present, but nonspecific

Atrophy of the ipsilateral fornix and mammillary bodies

Secondary to the neuronal loss of the hippocampus, and considered secondary signs. Seen in cases of pronounced hippocampal atrophy

T2 mapping (relaxometry)

An objective method for measuring abnormal T2 signal which may be difficult to detect visually

FLAIR, fluid-attenuated inversion recovery.