Insights Imaging 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 Kock2 & Omid Nikoubashman1 & Stephanie Vanden Bossche 2 & Ruth Van Eetvelde2,3 & Jeroen De Groote2 & Marjan Acou2 & Martin Wiesmann1 & Karel Deblaere2 & Eric Achten2 Received: 19 September 2016 /Revised: 18 December 2016 /Accepted: 20 December 2016 # The Author(s) 2017. This article is published with open access at Springerlink.com Abstract • Clinical information is often necessary to come to a correct The hippocampus is a small but complex anatomical structure diagnosis or an apt differential. that plays an important role in spatial and episodic memory. The hippocampus can be affected by a wide range of congen- Keywords Hippocampus . Epilepsy . Dementia . Herpes ital variants and degenerative, inflammatory, vascular, tumoral simplex encephalitis . MRI and toxic-metabolic pathologies. Magnetic resonance imaging is the preferred imaging technique for evaluating the hippo- campus. The main indications requiring tailored imaging se- Abbreviations quences of the hippocampus are medically refractory epilepsy AD Alzheimer’sdementia and dementia. The purpose of this pictorial review is three- DNET Dysembryoblastic neuroepithelial tumour fold: (1) to review the normal anatomy of the hippocampus on IHI Incomplete hippocampal inversion MRI; (2) to discuss the optimal imaging strategy for the eval- HSE Herpes simplex encephalitis uation of the hippocampus; and (3) to present a pictorial over- LE Limbic encephalitis view of the most common anatomic variants and pathologic MTA Mesial temporal atrophy conditions affecting the hippocampus. MTS Mesial temporal sclerosis Teaching points • Knowledge of normal hippocampal anatomy helps recognize Anatomy, embryology, arterial supply and function anatomic variants and hippocampal pathology. • Refractory epilepsy and dementia are the main indications Normal hippocampal anatomy requiring dedicated hippocampal imaging. • Pathologic conditions centered in and around the hippocam- The hippocampus is a bilaminar gray matter structure lo- pus often have similar imaging features. cated medially in the temporal lobe that protrudes over the temporal horn of the lateral ventricle and occupies the me- dial region of its floor (Figs. 1 and 2). The hippocampus * Sven Dekeyzer consists of two interlocking gray matter folds, the cornu [email protected] ammonis (or hippocampus proper) and the dentate gyrus. In the axial plane, the hippocampus resembles a seahorse 1 Department of Diagnostic and Interventional Neuroradiology, (hence, its name) and it arches around the mesencephalon University Hospital, RWTH Aachen University, Pauwelsstr. 30, (hence, the term Bmesiotemporal^). In the axial and sagittal 52074 Aachen, Germany plane, it can be divided into three parts: (1) the head or 2 Department of Radiology, University Hospital (UZ) Ghent, De anterior segment; (2) the body or intermediate segment; Pintelaan 185, 9000 Ghent, Belgium and (3) the tail or posterior segment. White matter fibres 3 Department of Medical Imaging, Onze-Lieve-Vrouw Hospital (OLV) from the hippocampus accumulate on its superior surface Aalst, Moorselbaan 164, 9300 Aalst, Belgium to form the alveus. White matter fibres from the alveus Insights Imaging 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 Fig. 3 Arterial supply of the hippocampal body and tail. Orange = P1, (d). The hippocampal body is shown in detail in Fig. 2. 1 = hippocampal red = P2 and purple = P3 segment of the posterior cerebral artery. The head, 2 = hippocampal body, 3 = hippocampal tail, 4 = mesencephalon, anterior hippocampal artery is hidden in the uncal sulcus and is shown 5 = amygdala, 6 = hippocampal digitations, 7 = temporal horn of the in Fig. 4 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 Based on its cellular composition, the cornu ammonis is mesencephalon, the body (2) can be found at the level of the divided into four parts, the so-called Sommer’ssectorsCA1 mesencephalon and the tail (3) is posterior to the mesencephalon. The to CA4. The cornu ammonis continues inferomedially in the hippocampal head is the only portion of the hippocampus not covered by parahippocampal gyrus, a gray matter structure that forms the the choroid plexus (7). The hippocampal head is separated from the amygdala (5) by the uncal recess of the lateral ventricle (7) and is transition area between the basal and mesial areas of the tem- characterized by small digitations separated by small sulci, the poral lobe. The subiculum is the medial and superior edge of digitationes hippocampi (6). At the level of the hippocampal tail, the the parahippocampal gyrus and its site of union with the cornu fimbriae continue posteriorly as the crux of the fornix (11) that slants ammonis[1].The hippocampus is surrounded by several fis- upwards towards the splenium of the corpus callosum (9) and the hippocampal tail continues as the subsplenial gyri (10). sures 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 then gather medially into thickened bundles as the fimbria, from the parahippocampal gyrus inferiorly. The superolateral which are continuous posteriorly with the fornix [1]. extension of the transverse fissure is the choroidal fissure. The Fig. 2 Anatomy of the hippocampal formation at the level of the Ac = Ambient cistern, B = basal vein of Rosenthal, C = tail of caudate hippocampal body on 3-T coronal T2. The hippocampal formation nucleus, ChF = choroid fissure, CS = collateral suclus, DG = dentate consists of the cornu ammonis or hippocampus proper, which can gyrus, P = posterior cerebral artery, PHG = parahippocampal gyrus, histologically be divided in the four Sommer sectors CA1–CA4, and Sub = subiculum, T = temporal horn of the lateral ventricle, the dentate gyrus (DG). A small hippocampal cyst (Hs) reflects the Tb = transverse fissure of Bichat location of the largely obliterated hippocampal sulcus. A = alveus, Insights Imaging Fig. 6 Bilateral sulcal remnant cysts (a) and right-sided choroid fissure cyst (b, c). Coronal T2 shows small bilateral cysts at the apex of the hippocampal fold between the dentate gyrus and Ammon’s horn (a). Fig. 4 Arterial supply of the hippocampal head. B = basal Rosenthal Coronal FLAIR (b) and axial T2-weighted (c) images show a space- vein, 1 = temporal horn of the lateral ventricle, 2 = uncal recess of the occupying cystic lesion, iso-intense to cerebrospinal fluid, at the level lateral ventricle, 3 = hippocampal digitations, 4 = uncal sulcus. Both the of the right choroid fissure anterior hippocampal artery, originating from the trunk or branches of the posterior cerebral artery, and the uncal branch of the anterior choroidal plate along the medial wall and floor of the temporal horn artery, dive into the uncal sulcus at the level of the hippocampal head and (Fig. 5a). Gradual infolding of the various components occurs form anastomoses in the sulci between the hippocampal digitations. Here, only one of both arteries in the uncal sulcus is drawn as a result of expansion of the neocortex and unequal growth of the various hippocampal components (Fig. 5b, c). The inferolateral extension of the transverse fissure is the hippocam- infolding occurs around the hippocampal sulcus, a sulcus that pal fissure, which extends between the dentate gyrus and the develops between the dentate gyrus and cornu ammonis and subiculum and is often obliterated and not visible on MRI [1]. later shifts to a location between the dentate gyrus and subiculum (Fig. 5d). Eventually, the hippocampal sulcus be- Arterial supply of the hippocampus comes obliterated, but at times, a residual cavity, a so-called sulcal remnant cyst, may be seen (Fig. 6a). Usually, three arteries (or groups of arteries) arising from the main or branches of the posterior cerebral artery vascularize Function the hippocampus: the anterior, middle and posterior hippo- campal arteries. The anterior hippocampal artery supplies the The hippocampus plays an important role in spatial and episod- hippocampal head, whereas the middle and posterior hippo- ic memory. A possible role of the medial temporal lobe in campal arteries vascularize the hippocampal body and tail. memory formation was first suggested over a century ago by The middle and posterior hippocampal artery are richly inter- von Bechterew after presenting the neuropathological findings connected with another through the so-called longitudinal ter- from the brain of a 60-year-old man who had memory prob- minal segments that run parallel to the course of the hippo- lems during the last 20 years of his life and whose brain showed campal body (Fig. 3). The uncal branch of the anterior cho- bilateral softening of the gyrus uncinatus and the hippocampal roidal artery is usually anastomosed with the anterior hippo- formation [2]. The role of the hippocampus in episodic memory campal artery in the uncal sulcus (Fig. 4)[1]. became firmly established in 1957, when