“Unforgettable” – a pictorial essay on anatomy and pathology of the hippocampus

Insights Imaging (2017) 8:199–212 DOI 10.1007/s13244-016-0541-2 PICTORIAL REVIEW BUnforgettable^ – a pictorial essay on anatomy and pathology of the hippocampus Sven Dekeyzer 1,2,3 & Isabelle De Kock 2 & Omid Nikoubashman 1 & Stephanie Vanden Bossche 2 & Ruth Van Eetvelde 2,3 & Jeroen De Groote 2 & Marjan Acou 2 & Martin Wiesmann 1 & Karel Deblaere 2 & Eric Achten 2 Received: 19 September 2016 / Revised: 18 December 2016 / Accepted: 20 December 2016 / Published online: 20 January 2017 # The Author(s) 2017. This article is published with open access at Springerlink.com Abstract The hippocampus is a small but complex anatomical structure that plays an important role in spatial and episodic memory. The hippocampus can be affected by a wide range of congenital variants and degenerative, inflammatory, vascular, tumoral and toxic-metabolic pathologies. Magnetic resonance imaging is the preferred imaging technique for evaluating the hippocampus. The main indications requiring tailored imaging sequences of the hippocampus are medically refractory epilepsy and dementia. 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. Teaching points • Knowledge of normal hippocampal anatomy helps recognize anatomic variants and hippocampal pathology. • Refractory epilepsy and dementia are the main indications requiring dedicated hippocampal imaging. • Pathologic conditions centered in and around the hippocampus often have similar imaging features. * Sven Dekeyzer 1 Department of Diagnostic and Interventional Neuroradiology, University Hospital, RWTH Aachen University, Pauwelsstr. 30, 52074 Aachen, Germany 2 Department of Radiology, University Hospital (UZ) Ghent, De Pintelaan 185, 9000 Ghent, Belgium 3 Department of Medical Imaging, Onze-Lieve-Vrouw Hospital (OLV) Aalst, Moorselbaan 164, 9300 Aalst, Belgium • Clinical information is often necessary to come to a correct diagnosis or an apt differential. Keywords Hippocampus . Epilepsy . Dementia . Herpes simplex encephalitis . MRI Abbreviations AD Alzheimer’s dementia DNET Dysembryoblastic neuroepithelial tumour IHI Incomplete hippocampal inversion HSE Herpes simplex encephalitis LE Limbic encephalitis MTA Mesial temporal atrophy MTS Mesial temporal sclerosis Anatomy, embryology, arterial supply and function Normal hippocampal anatomy The hippocampus is a bilaminar gray matter structure located medially in the temporal lobe that protrudes over the temporal horn of the lateral ventricle and occupies the medial region of its floor (Figs. 1 and 2). The hippocampus consists of two interlocking gray matter folds, the cornu ammonis (or hippocampus proper) and the dentate gyrus. In the axial plane, the hippocampus resembles a seahorse (hence, its name) and it arches around the mesencephalon (hence, the term Bmesiotemporal^). In the axial and sagittal plane, it can be divided into three parts: (1) the head or anterior segment; (2) the body or intermediate segment; and (3) the tail or posterior segment. White matter fibres from the hippocampus accumulate on its superior surface to form the alveus. White matter fibres from the alveus 200 Fig. 1 Anatomy of the hippocampal formation on 3-T axial T2 (a) and sagittal 3D-MPRAGE images (b). Zoomed-in 3-T coronal T2-weighted images at the level of the hippocampal head (c) and the hippocampal tail (d). The hippocampal body is shown in detail in Fig. 2. 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. To easily recognize the different portions of the hippocampus, we can use the mesencephalon (4). 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. The hippocampal head is the only portion of the hippocampus not covered by the choroid plexus (7). 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). 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). then gather medially into thickened bundles as the fimbria, which are continuous posteriorly with the fornix [1]. Fig. 2 Anatomy of the hippocampal formation at the level of the hippocampal body on 3-T coronal T2. The hippocampal formation consists of the cornu ammonis or hippocampus proper, which can histologically be divided in the four Sommer sectors CA1–CA4, and the dentate gyrus (DG). A small hippocampal cyst (Hs) reflects the location of the largely obliterated hippocampal sulcus. A = alveus, Insights Imaging (2017) 8:199–212 Fig. 3 Arterial supply of the hippocampal body and tail. Orange = P1, red = P2 and purple = P3 segment of the posterior cerebral artery. The anterior hippocampal artery is hidden in the uncal sulcus and is shown in Fig. 4 Based on its cellular composition, the cornu ammonis is divided into four parts, the so-called Sommer’s sectors CA1 to CA4. 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. The subiculum is the medial and superior edge of the parahippocampal gyrus and its site of union with the cornu ammonis[1].The hippocampus is surrounded by several fissures which are collectively referred to as the perihippocampal fissures. The transverse fissure of Bichat is the lateral extension of the ambient cistern which separates the thalamus superiorly from the parahippocampal gyrus inferiorly. The superolateral extension of the transverse fissure is the choroidal fissure. The Ac = Ambient cistern, B = basal vein of Rosenthal, C = tail of caudate nucleus, ChF = choroid fissure, CS = collateral suclus, DG = dentate gyrus, P = posterior cerebral artery, PHG = parahippocampal gyrus, Sub = subiculum, T = temporal horn of the lateral ventricle, Tb = transverse fissure of Bichat Insights Imaging (2017) 8:199–212 Fig. 4 Arterial supply of the hippocampal head. B = basal Rosenthal vein, 1 = temporal horn of the lateral ventricle, 2 = uncal recess of the lateral ventricle, 3 = hippocampal digitations, 4 = uncal sulcus. Both the anterior hippocampal artery, originating from the trunk or branches of the posterior cerebral artery, and th (...truncated)