The anterior hippocampal artery supplies the hippocampal head, whereas the middle and posterior hippocampal arteries vascularize the hippocampal body and tail. Usually, three arteries (or groups of arteries) arising from the main or branches of the posterior cerebral artery vascularize the hippocampus: the anterior, middle and posterior hippocampal arteries. The inferolateral extension of the transverse fissure is the hippocampal fissure, which extends between the dentate gyrus and the subiculum and is often obliterated and not visible on MRI. The superolateral extension of the transverse fissure is the choroidal fissure. The transverse fissure of Bichat is the lateral extension of the ambient cistern which separates the thalamus superiorly from the parahippocampal gyrus inferiorly. The subiculum is the medial and superior edge of the parahippocampal gyrus and its site of union with the cornu ammonis.The hippocampus is surrounded by several fissures which are collectively referred to as the perihippocampal fissures. The cornu ammonis continues inferomedially in the parahippocampal gyrus, a gray matter structure that forms the transition area between the basal and mesial areas of the temporal lobe. At the level of the hippocampal tail, the fimbriae continue posteriorly as the crux of the fornix (11) that slants upwards towards the splenium of the corpus callosum (9) and the hippocampal tail continues as the subsplenial gyri (10).īased on its cellular composition, the cornu ammonis is divided into four parts, the so-called Sommer’s sectors CA1 to CA4. The hippocampal head is separated from the amygdala (5) by the uncal recess of the lateral ventricle (7) and is characterized by small digitations separated by small sulci, the digitationes hippocampi (6). The hippocampal head is the only portion of the hippocampus not covered by the choroid plexus (7). The head (1) is located in front of the mesencephalon, the body (2) can be found at the level of the mesencephalon and the tail (3) is posterior to the mesencephalon. To easily recognize the different portions of the hippocampus, we can use the mesencephalon (4). 1 = hippocampal head, 2 = hippocampal body, 3 = hippocampal tail, 4 = mesencephalon, 5 = amygdala, 6 = hippocampal digitations, 7 = temporal horn of the lateral ventricle, 8 = uncal recess of the lateral ventricle, 9 = splenium of the corpus callosum, 10 = subsplenial gyri, 11 = crura of the fornices.
The hippocampal body is shown in detail in Fig. Zoomed-in 3-T coronal T2-weighted images at the level of the hippocampal head ( c) and the hippocampal tail ( d). Clinical information is often necessary to come to a correct diagnosis or an apt differential.Īnatomy of the hippocampal formation on 3-T axial T2 ( a) and sagittal 3D-MPRAGE images ( b).Pathologic conditions centered in and around the hippocampus often have similar imaging features.Refractory epilepsy and dementia are the main indications requiring dedicated hippocampal imaging.Knowledge of normal hippocampal anatomy helps recognize anatomic variants and hippocampal pathology.The purpose of this pictorial review is threefold: (1) to review the normal anatomy of the hippocampus on MRI (2) to discuss the optimal imaging strategy for the evaluation of the hippocampus and (3) to present a pictorial overview of the most common anatomic variants and pathologic conditions affecting the hippocampus. The main indications requiring tailored imaging sequences of the hippocampus are medically refractory epilepsy and dementia. Magnetic resonance imaging is the preferred imaging technique for evaluating the hippocampus.
The hippocampus can be affected by a wide range of congenital variants and degenerative, inflammatory, vascular, tumoral and toxic-metabolic pathologies. The hippocampus is a small but complex anatomical structure that plays an important role in spatial and episodic memory.