Hippocampus is a complex structure and forms a part of limbic system,
critical to human memory,
emotions and behaviour.
Bilateral hippocampal lesions result in amnesia.
The advent of surgical procedures that can cure medically intractable seizures by excision of hippocampal abnormalities has spurred extensive research directed toward MR analysis of this region.
Our purpose is to introduce the reader to these techniques.
This paper is divided into three parts: normal hippocampal anatomy,
MR imaging techniques and lesions affecting the hippocampus.
We present a collection of cases from congenital,
inflammatory,
infective,
metabolic,
vascular,
degenerative and neoplastic conditions affecting the hippocampus.
ANATOMY:
The hippocampus is part of the inner limbic gyrus on the medial surface of the cerebral hemispheres and has a curved shape resembling that of a seahorse.
It is subdivided into three segments:
Anterior head- oriented transversely with morphological digitations
Body- cylindrical and oriented parasagittally.
The white matter tracts of the fimbria and the alveus are located superiorly.
Posterior tail- narrows and curves around splenium to form the indusium grisium.
On anatomical coronal sections,
the hippocampus is usually shown at the mid body,
where it comprises two interlocking U shaped gray matter structures: the hippocampus proper- superolateral upside down U (Ammon’s horn) and the dentate gyrus- inferomedial U.
The former consists of four zones: CA1,
CA2,
CA3,
and CA4.
The hippocampal sulcus/ fissure is a sulcus separating the dentate gyrus from the subiculum.
The Cornu Ammonis is continuous inferolaterally with the subiculum (the inferior most structure in the hippocampal formation) and the cortex of the parahippocampal gyrus.
Relations of the hippocampus- medially the ambient cistern,
superiorly the choroid fissure and temporal horn,
inferiorly the parahippocampal gyrus and the temporal horn laterally.
The amygdala is separated from the hippocampus by the uncal recess of the temporal horn and is located anterior and superior to it.
White matter fibres from the hippocampus accumulate on the superior surface to form the sheet-like alveus,
which is covered by the ependyma of the temporal horn of the lateral ventricle,
and overlying choroid plexus.
White matter fibres from the alveus then gather into bundles as the fimbria,
which are continuous posteriorly with the fornix.
The hippocampus receives arterial blood mainly from the posterior cerebral artery.