Anatomy of the Ventricles, Subarachnoid Spaces, and Meninges PDF

A n a t o m y o f th e Ven t r i c l e s , S u b a r a c h n o i d S p a c e s , an d Meninges John A. Morris, DOa, Bruce C. Gilbert, MDb, William T. Parker, MDb, Scott E. Forseen, MDa,* KEYWORDS  Anatomy  Ventricles  Subarachnoid spaces  Meninges  Neuroanatomy  Radiology KEY POINTS  Th...

A n a t o m y o f th e Ven t r i c l e s , S u b a r a c h n o i d S p a c e s , an d Meninges John A. Morris, DOa, Bruce C. Gilbert, MDb, William T. Parker, MDb, Scott E. Forseen, MDa,* KEYWORDS  Anatomy  Ventricles  Subarachnoid spaces  Meninges  Neuroanatomy  Radiology KEY POINTS  The ventricular system is an intricate series of cerebrospinal fluid (CSF)-filled intracranial chambers with complex anatomic boundaries.  The subarachnoid space is an interval between the arachnoid mater and pia mater with free multi- directional flow of CSF. Regions of dilated subarachnoid space are termed “cisterns.”  The meninges support and protect the structures of the central nervous system (CNS) with an elab- orate triple-layered membranous framework. INTRODUCTION continuous with the central canal of the spinal cord caudal to the obex (caudal apex). The central The ventricular system is a series of intercon- canal terminates in the ventriculus terminalis (the nected cavities, derived from the central canal of “fifth ventricle,” a variably persistent embryologic the embryonic neural tube, in which cerebrospinal structure) in the conus medullaris of the spinal fluid (CSF) is produced and freely flows. In cord.1,2 conjunction with the meninges, a triple-layered concentric membranous covering, a framework is formed to maintain, support, and protect the Lateral Ventricles structures of the CNS. This article will focus on normal imaging anatomy of the ventricles, The lateral ventricles are paired C-shaped cham- meninges, and communicating subarachnoid bers contained deep within the substance of the spaces within the intracranial compartment. cerebral hemispheres, consisting of a body and Owing to space constraints, limited attention is atrium along with 3 projections into the frontal, given to anatomic variants and pathologic temporal, and occipital lobes, termed “horns.” processes. Each lateral ventricle has a capacity of 7 to 10 mL.3,4 ANATOMY OF THE VENTRICULAR SYSTEM The foramen of Monro marks the boundary be- tween the body of the lateral ventricle and the fron- The ventricular system is composed of 4 discrete tal horn, with the frontal horn extending anteriorly. cavities consisting of the paired lateral ventricles The anterior wall and roof of the frontal horn are and unpaired midline third and fourth ventricles formed by the genu of the corpus callosum, with (Fig. 1). The ventricular system is continuous with the floor formed by the rostrum. The head of the neuroimaging.theclinics.com the subarachnoid spaces through the fourth caudate nucleus forms the lateral wall, whereas ventricle outlet foramina of Luschka (lateral aper- the columns of the fornix form the inferomedial tures) and Magendie (median aperture) and wall (Fig. 2A–C). a Department of Radiology and Imaging, Medical College of Georgia at Augusta University, 1120 15th Street, Augusta, GA 30912, USA; b Neuroradiology, Neuroradiology Section, Department of Radiology and Imaging, Medical College of Georgia at Augusta University, 1120 15th Street, Augusta, GA 30912, USA * Corresponding author. E-mail address: [email protected] Neuroimag Clin N Am 32 (2022) 577–601 https://doi.org/10.1016/j.nic.2022.04.005 1052-5149/22/Ó 2022 Elsevier Inc. All rights reserved. Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 578 Morris et al Fig. 1. Volumetric illustration of the ventricular system. (Courtesy of M. Skalski, DC, San Jose, California.) The body of the lateral ventricle projects poste- 97% of term infants.7 Owing to the progressive riorly from the foramen of Monro to the splenium of fusion of the septum pellucidum leaflets, a cavum the corpus callosum where it communicates vergae is essentially always accompanied by a openly with the atrium. The roof of the ventricular cavum septum pellucidum and termed a cavum body is formed by the body of the corpus callosum septi pellucidi et vergae. with the floor formed by the thalamus. The super- The atrium or trigone is a triangular cavity where omedial and inferomedial walls are, respectively, the ventricular body communicates with the occip- formed by the septum pellucidum and body of ital and temporal horns (see Fig. 2A, D; Fig. 4). The the fornix. The lateral wall is formed by the caudate body and splenium of the corpus callosum form nucleus and thalamus. The stria terminalis, a main the roof. The tapetum (Latin for “carpet” or “tapes- outlet pathway of the amygdala, sits within the try”) is a sheetlike bundle of decussating fibers in caudothalamic groove along with the thalamostri- the splenium of the corpus callosum that arches ate vein.5 over the atrium. The tapetum continues laterally The cavum septi pellucidi (Fig. 3A) is a potential and comprises the lateral wall with the caudate cavity between the laminae of the septum pelluci- body. The floor is created by the collateral trigone, dum bounded anteriorly by the genu of the corpus a flattened triangular area overlying the collateral callosum, superiorly by the body of the corpus cal- sulcus. The sloping contour of the medial wall is losum, and posteriorly by the forniceal pillars. The formed by the calcar avis—the prominence that cavum is a normal fetal structure with typical overlies the deep end of the calcarine sulcus— fusion of the septum pellucidum laminae by 3 to and bulb of the corpus callosum, which is a 6 months after birth.6 bulging created by the forceps major.3,5 The cavum vergae (Fig. 3B) is a potential cavity The temporal horn is the longest and largest between the laminae of the septum pellucidum horn, extending anteriorly from the atrium and ter- persisting posterior to the forniceal pillars and is minating at the amygdala (Fig. 5). The medial floor thus the caudal continuation of a cavum septi pel- is formed by a prominence overlying the hippo- lucidi. The posterior border is formed by the sple- campus and separated by a thin layer of white nium of the corpus callosum with the superior matter called the alveus. The collateral eminence border formed by the body of the corpus callosum. forms the lateral part of the floor. The roof is The inferior border is formed by the transverse created by the caudate nucleus and tapetum. portion of the fornix. The septum pellucidum leaf- The tapetum also runs inferiorly to comprise the lets begin fusing from posterior to anterior in utero lateral wall and separates the lateral wall from with the cavum vergae closed in approximately the optic radiations. The medial wall is a small cleft Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  Ventricles, Subarachnoid Spaces, and Meninges 579 Fig. 2. Axial (A, B) and coronal (C, D) T1 images coregistered with a color-coded fractional anisotropy map at the level of the lateral ventricle floor (A), frontal horns (B, C), and atrium (D). AC, anterior commissure; ACR, anterior corona radiata; ALIC, anterior limb internal capsule; Atr, ventricular atrium; CG, cingulate gyrus; CH, caudate head; Chr, choroid plexus; EC, external capsule; FH, frontal horn of the lateral ventricle; Fx, fornix; G, genu of the corpus callosum; PCR, posterior corona radiata; PLIC, posterior limb of the internal capsule; PThR, posterior thalamic radiations; R, rostrum of the corpus callosum; SP, septum pellucidum; Spl, splenium of the corpus cal- losum; StrT, stria terminalis; Tap, tapetum; Th, thalamus. between the fimbria of the fornix and the inferolat- mimic pathologic conditions. Several variants, eral thalamus.3,5 including coarctation, asymmetry, and ependymi- The occipital horn curves posteriorly and medi- tis granularis, are important for the radiologist to ally from the atrium and varies in size; it may be ab- be aware of in order to prevent misinterpreting sent or may extend deep into the occipital lobe. these findings as pathology. For a detailed The tapetum forms the roof and lateral wall and description, we recommend reviewing Scelsi, separates it from the optic radiations. The collat- and colleagues.3 eral trigone forms the floor. Similar to the atrium, Coarctation refers to close apposition or fusion of the bulb of the corpus callosum and calcar avis 2 ventricular walls, most commonly seen in the oc- form the medial wall (Fig. 6).3,5 cipital and frontal horns.8 The cause is likely devel- opmental in nature because there is no histologic evidence of an underlying inflammatory or gliotic re- Normal Anatomic Variants action.9 When focal, a portion of the ventricle can Anatomic variations of the lateral ventricles are be isolated creating an ependyma-lined cyst common findings in healthy individuals and, referred to as a “connatal cyst.” These may be mul- though not frequently a diagnostic dilemma, may tiple and, when seen in infants, may regress.10 Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 580 Morris et al Fig. 3. (A) A cavity is present between the laminae of the septum pellucidum (white arrows) confined between the genu of the corpus callosum anteriorly and columns of the fornix posteriorly, consistent with a cavum septi pellucidi. (B) When the cavity between the septum pellucidum leaflets persists caudally to the columns of the fornix, it is termed a cavum vergae. Due to the ordered fusion of the septum pellucidum leaflets from posterior to anterior, a cavum vergae is essentially always accompanied by a cavum septum pellucidum and is termed a cavum septi pellucidi et vergae (red arrows). Size and morphologic asymmetry of the lateral Ependymitis granularis is a normal anatomic ventricles is common, particularly of the occipital finding referring to symmetric foci of high T2 signal and temporal horns.11,12 Clinical significance of anterolateral to the frontal horns, typically less than asymmetric lateral ventricles is controversial, 10 mm in extent and demonstrating a triangular although small volumetric or morphologic differ- morphology. This imaging finding is thought to ences, in the absence of obstructing lesion or relate to a loose network of axons with decreased adjacent parenchymal disease, are likely of no tissue myelin content. This porous ependyma al- clinical significance.13 lows transependymal CSF flow. Despite the name Fig. 4. Coronal T1 image coregistered with color-coded fractional anisotropy map at the level of the lateral ventricle atrium. Atr, atrium of the lateral ventricle; CA, calcar avis; CS, calcarine sulcus; CST/PThR, corticospinal tract/posterior thalamic radiations; CTr, collateral trigone; FM/BCC, forceps major/bulb of the corpus callosum; IFO/ILF, inferior fronto-occipital fasciculus/inferior longitudinal fasciculus; PCR, posterior corona radiata; Tap, tapetum. Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  Ventricles, Subarachnoid Spaces, and Meninges 581 Fig. 5. Sagittal T1 image coregistered with color-coded fractional anisotropy map at the level of the lateral ventricle atrium and temporal horn. Inset: coronal T1 image through the anterior temporal horns. A, amygdala; Alv, alveus; Atr, atrium of the lateral ventricle; CoE, collateral eminence; CS, collateral sulcus; CT, caudate tail; Fim, fimbria of the fornix; Hpc, hippocampus; Occ, occipital horn of the lateral ventricle; StrT, stria terminalis; Tap, tapetum; TemH, temporal horn of the lateral ventricle. implicating an inflammatory process, there is no Third Ventricle histologic evidence that this is the case.14 This The third ventricle (Fig. 8) is a funnel or slit-shaped finding may mimic pathologic condition including midline cavity with an estimated capacity of 1 mL, transependymal edema relating to obstruction varying by age and gender.4 The third ventricle (typically not confined to the frontal horns and ven- communicates with the lateral ventricles anterosu- tricles would be enlarged), demyelinating lesions periorly through the foramina of Munro and with (rarely symmetric or triangular in morphology), or the fourth ventricle posteroinferiorly through the chronic microvascular ischemic changes (usually cerebral aqueduct of Sylvius. not symmetric or abutting the ventricles). The anterior wall of the third ventricle extends from the foramina of Monro superiorly to the optic Foramina of Monro chiasm inferiorly. From superior to inferior, it is The 2 interventricular foramina of Monro (Fig. 7) made up of the foramina of Monro, anterior are bilateral channels that connect the lateral ven- commissure, lamina terminalis, optic recess, and tricles to the third ventricle. They are cylindrical optic chiasm. The optic chiasm is positioned at conduits with a crescentic-to-oval cross section the junction of the anterior wall and floor of the that flare out at their proximal and distal ends third ventricle.19,20 and slope caudally from lateral to medial and ante- The posterior wall of the third ventricle extends rior to posterior with a cross-sectional area of from the suprapineal recess superiorly to the approximately 5 mm2. The left foramen is typically aqueduct of Sylvius inferiorly. From superior to slightly larger than the right.15 The foramina and inferior, the posterior wall of the third ventricle is are bounded by the forniceal pillars anteriorly formed by the suprapineal recess, habenular and anterior pole of the thalamus posteriorly.16 commissure, pineal body, pineal recess, posterior The structures that pass through include choroid commissure, and aqueduct of Sylvius.19,20 plexus, branches of the choroidal artery, and inter- The roof of the third ventricle extends from the nal cerebral, thalamostriate, superior choroidal, foramina of Monro anteriorly to the suprapineal and septal veins.17 Matys and colleagues18 have recess posteriorly. It is made up of 4 layers (supe- recently described in detail the age-related neuro- rior to inferior): fornix, superior layer of the tela imaging anatomic features of foramen of Monro choroidea, vascular layer, and inferior layer of the and also suggested renaming each structure as tela choroidea. The body of the fornix makes up the interventricular canalicilus. the anterior portion of the superior layer and the Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 582 Morris et al Fig. 6. Sagittal T1 image coregistered with color-coded fractional anisotropy map at the level of the lateral ventricle occipital horn. ACR, anterior corona radiata; Atr, atrium of the lateral ventricle; Cing, cingulum; CTr, collateral trigone; IFO/UNC, inferior fronto-occipital fasciculus/uncinate fasciculus; MCP, middle cerebellar peduncle; Occ, occipital horn of the lateral ventricle; PCR, posterior corona radiata; PLIC, posterior limb of the internal capsule; StrT, stria terminalis. crura of the fornices and hippocampal commis- The velum interpositum is a subarachnoid space sure form the posterior portion of the superior located between the layers of the tela choroidea. It layer.19,20 has a triangular shape with apex located just behind The tela choroidea is a loose trabecular pial tis- the foramina of Monro and posterior inferior attach- sue forming 2 layers above and below the vascular ment just above the pineal body. The velum interpo- layer. The vascular layer is composed of the inter- situm is typically a closed space but when it nal cerebral veins and medial posterior choroidal communicates with the quadrigeminal cistern, it is arteries. Choroid plexus projects inferiorly from referred to as the cavum veli interpositi (Fig. 9).21 the lower layer of the tela choroidea at the roof of The floor of the third ventricle spans from the op- the third ventricle and from components of the tic chiasm anteriorly to the margin of the aqueduct tela choroidea that project into the lateral of Sylvius posteriorly. The anterior part of the third ventricles.19,20 ventricular floor is made up of diencephalic Fig. 7. Foramina of Monro (dashed white arrows) as viewed from a coronal perspective on a three-dimensional model (A) connecting the lateral ventricles superiorly to the third ventricle inferiorly. Foramina as viewed on ob- lique sagittal (B), axial (C), and coronal (D) constructive interference in steady state (CISS) magnetic resonance (MR) images. ColF, columns of the fornices; LatV, lateral ventricle; Thal, thalamus; V3, third ventricle. Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  Ventricles, Subarachnoid Spaces, and Meninges 583 Fig. 8. Oblique coronal (left) and sagittal (right) CISS MR images of the third ventricle. AC, anterior commissure; Aqs, aqueduct of Sylvius; Cho, choroid plexus; ForM, foramen of Monro; Fx, fornix; HC, habenular commissure; HT, hypothalamus; ICV, internal cerebral vein; Inf, pituitary infundibulum; IR, infundibular recess; LV, lateral ventricle; MB, mammillary body; MI, massa intermedia; OC, optic chiasm; OR, optic recess; PB, pineal body; PC, posterior commissure; Pit, pituitary gland; PR, pineal recess; SPR, suprapineal recess; TC, tuber cinereum; Tec, tectum; Teg, tegmentum; Tel, tela choroidea, Thal, thalamus; V3, third ventricle; V4, fourth ventricle. structures, and the posterior part of the floor is via the pituitary (infundibular) stalk. The NH, origi- made up of mesencephalic structures. The floor nating from neuroectoderm, is therefore neuro- of the third ventricle, from anterior to posterior, is nally connected to the median eminence (ME) made up of the inferior surface of the optic chiasm, and tuber cinereum of the hypothalamus via the pi- pituitary infundibulum, tuber cinereum, mammil- tuitary stalk. However, the glandular anterior pitui- lary bodies, posterior perforated substance, and tary (adenohypophysis or pars distalis plus the midbrain tegmentum.19,20 pars tuberalis, which wraps around the pituitary The floor of the third ventricle is formed by hypo- stalk) is distinct and arises from an invagination thalamic structures. The hypothalamus is below of the oral ectoderm (Rathke pouch). the thalamus, forming the ventral part of the dien- The lateral walls of the third ventricle are formed cephalon, and is also a component of the limbic by the hypothalamus inferiorly and thalamus supe- system. The hypothalamus is divided into 3 re- riorly. The hippocampal sulcus extends from the gions (supraoptic, tuberal, and mammillary) from foramina of Monro to the aqueduct of Sylvius anterior to posterior; and 3 areas (periventricular, and separates the medial surfaces of the hypothal- medial, and lateral) from medial to lateral. These amus and thalamus. The massa intermedia (inter- regions and areas contain the hypothalamic thalamic adhesion, adhesio interthalamica) is a nuclei. The hypothalamus is highly interconnected connection between the thalami that contains with other parts of the brain, especially the brain- commissural fibers and neurons. It is present in stem and its reticular formation. In the limbic sys- 80% of people, is variable in size, and is typically tem, it has connections to other structures, located in the anterior superior quadrant of the including the amygdala and septal nuclei located third ventricle. Its precise function is unknown. rostral to the anterior commissure, and is also con- The outline of the lateral walls has the appearance nected with areas of the autonomic nervous of a bird’s head, with beak formed by the optic and system. infundibular recesses and the head formed by the The pituitary gland is composed of 2 lobes, lateral surface of the thalamus.19,20 anterior and posterior, with an intermediate lobe (pars intermedia) that joins the 2 regions but this Cerebral aqueduct (of Sylvius) is avascular and almost absent in humans. An intraglandular or hypophyseal cleft lies between The cerebral aqueduct connects the third ventricle the anterior and intermediate lobes. The portion to the fourth ventricle and measures 15 to 18 mm of the anterior third ventricle floor forming the in length with a highly variable cross-sectional infundibulum ends in the posterior lobe of the pitu- area.22 It is located within the midbrain and itary gland (neurohypophysis [NH] or pars nervosa) bounded circumferentially by a column of Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 584 Morris et al Fig. 9. Axial (A) and sagittal (B) T2 MR images of the lateral and third ventricles. (A) There is enlargement of the CSF space between the lateral ventricle atria (arrows) with splaying of the fornices. (B) On sagittal images this is confirmed to represent dilatation of the velum interpositum (cavum veli interpositi, arrows) as positioned below the splenium of the corpus callosum/columns of the fornices and above the internal cerebral veins. The apex points anteriorly but remains posterior to the foramina of Monro. periaqueductal gray matter. The tegmentum sits longitudinally oriented vertical centerline that sep- anteriorly with the tectum posteriorly. The superior arates the floor into equal halves. The sulcus limi- opening of the aqueduct (aditus aquaeducti) is tans is a discontinuous longitudinal sulcus that triangular in shape with the base positioned separates each half of the floor into the ME and dorsally and bounded by the posterior commis- the lateral vestibular area. The ME is a raised strip sure. Two small, rounded bulges are visible at that borders the midline and contains, from cranial the ventral aditus separated by a median sulcus. to caudal, the facial colliculus and 3 triangular They are likely caused by protrusion of the rubral areas that overly the nuclei of the hypoglossal nuclei into the aqueductal lumen and are thus and vagus nerves, as well as the area postrema. identified as the rubral eminences. The median The stacked arrangement of these 3 paired trian- sulcus persists along the floor of the aqueduct gular areas gives the caudal floor a feather or and continues as the median sulcus in the rhom- pen nib appearance on gross inspection and has boid fossa of the fourth ventricle. There are 2 con- been termed the calamus scriptorius. strictions within the aqueduct corresponding to The locus ceruleus is at the rostral tip of each the superior and inferior colliculi with an inter- sulcus limitans (see Fig. 10). The sulcus limitans vening dilatation termed the ampulla that corre- is deepest at the pontine and medullary portions sponds to the tegmental sulcus. Beyond the of the floor, forming 2 distinct dimples, the superior second constriction, the chamber dilates abruptly and inferior fovea, respectively. At the level of the into the fourth ventricle.23 Detailed neuroimaging superior fovea, there is an elongated swelling of anatomy and morphometry of the aqueduct is the ME that is formed by the abducens nucleus also available in Matys and colleagues.24 and ascending portion of the facial nerve root, the facial colliculus. The hypoglossal triangle over- lies the hypoglossal nucleus and is medial to the Fourth Ventricle inferior fovea and the vagal triangle is caudal to The fourth ventricle is the tent-shaped midline ter- the inferior fovea overlying the dorsal nucleus of minal compartment of the intracranial ventricular the vagal nerve. The area postrema is a tongue- system with an estimated CSF capacity of 1 to shaped projection from the lower aspect of the 2 mL.4 The fourth ventricle has a floor, a roof, ME immediately rostral to the obex (see Fig. 10; and 2 lateral recesses. Fig. 11). The floor of the fourth ventricle, known as the The vestibular area of the floor of the fourth rhomboid fossa, has a characteristic diamond ventricle is lateral to the sulcus limitans at the level shape with the rostral point at the cerebral aque- of the junction of caudal pons and rostral medulla, duct, the caudal point at the obex, and lateral overlying the vestibular nuclei. The dorsal. points formed by 2 lateral recesses at the ponto- cochlear nucleus, and cochlear portion of the ves- medullary junction (Fig. 10). The dorsal surface of tibulocochlear nerve form the auditory tubercle. In the pons forms the rostral two-thirds of the floor the lateral part of the vestibular area. and the inferior one-third is formed by the dorsal The roof of the fourth ventricle resembles a surface of the medulla. The median sulcus is a rhomboid-based pyramid with the edges of the Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  Ventricles, Subarachnoid Spaces, and Meninges 585 midline and inferior medullary velum laterally (Fig. 13). These structures blend inferiorly with the tela choroidea, which forms the most caudal part of the roof. The tela choroidea attaches to ridges on the inferolateral floor that meet at the obex called the taeniae. The tela choroidea of the inferior fourth ventricle has 3 openings, the paired lateral foramina of Luschka and the midline caudal foramen of Magendie, which communicate with the subarach- noid spaces of the cerebellopontine angle cisterns and foramen magnum, respectively.26,27 The lateral recesses are lateral tunnel-like exten- sions of the fourth ventricle at the level of the pon- tomedullary junction (widest segment of the floor) that are formed by the junction of the roof and floor (Fig. 14). The floor and a sheetlike layer of neural tissue that extends laterally from the floor to join the tela choroidea, the rhomboid lip, forms the ventral wall of the lateral recess. The caudal margin of the cerebellar peduncles forms the rostral wall. The peduncle of the flocculus crosses the dorsal margin of the lateral recess and inter- connects the inferior medullary velum and the floc- culus. The tela choroidea extends from the taenia of the inferolateral floor to attach on the edge of Fig. 10. Floor of the fourth ventricle. The cerebellar the peduncle of the flocculus to form the caudal peduncles have been sectioned and the cerebellum wall of the lateral recess. The biventral lobule of removed in order to expose the floor of the fourth the cerebellum is dorsal to the lateral recess. The ventricle. (Adapted from Rhoton AL. Relationships flocculus lies superior to the extreme lateral of the Inferior Medullary Velum, Dentate Nucleus, margin of the lateral recess. Several cranial nerves Tonsil, and Cerebellomedullary and Cerebellomesen- are also intimately associated with the lateral cephalic Fissures. The Neurosurgical Atlas. Available recess with the glossopharyngeal and vagus at: https://www.neurosurgicalatlas.com/ nerves arising ventral to the lateral recess (see neuroanatomy/relationships-of-the-fourth- Fig. 14 inset). The facial nerve arises rostral to ventricular-floor-and-cerebellar-peduncles. Accessed April 7, 2022; with permission.) the lateral recess and the vestibulocholear nerves runs across the lateral recess floor.28–30 pyramid base representing the ventricular borders and the cerebellar fastigium forming the apex and Choroid Plexus dividing the roof into superior and inferior parts25 (see Fig. 11; Fig. 12). The superior part of the Choroid plexus is present within the lateral, third, roof is formed at the midline by the superior med- and fourth ventricles and absent within the frontal ullary velum, a thin lamina of white matter between and occipital horns of the lateral ventricles and ce- the superior cerebellar peduncles and rostral to rebral aqueduct. the lingula of the cerebellum. The lateral wall of Choroid plexus is the primary source of CSF, the superior part of the roof is formed by the supe- producing 500 to 600 mL per day (0.4 mL/min). rior cerebellar peduncle. The fibers of the superior At any given time, there is approximately 150 mL cerebellar peduncle arise in the dentate nucleus total CSF volume, with 125 mL within the sub- and track along the medial surface of the middle arachnoid space and 25 mL within the ventricles31 cerebellar peduncle to form the superior part of amounting to a turnover of 4 times per day. Other the lateral wall. The inferior part of the lateral wall less prominent sources of CSF include brain inter- is formed by the fibers of the inferior cerebellar stitial fluid (up to 20% of CSF),32 ventricular epen- peduncle. The inferior part of the roof of the fourth dyma, and brain capillaries.33 ventricle projects ventral and slightly caudal from Within the lateral ventricles, the choroid plexus the fastigium to attach on the taeniae of the infero- runs along a cleft between the fornix and thalamus lateral floor. The inferior part of the roof adjacent to called the choroidal fissure. The choroidal fissure the fastigium is formed by the nodulus at the forms a C-shape extending from the foramina of Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 586 Morris et al Fig. 11. Midsagittal T1-weighted image through the fourth ventricle. APos, area postrema; CLobC, central lobule of the cerebellum; Fas, fastigium; FM, foramen of Magendie; IMV, inferior medullary velum; LingC, lingula of the cerebellum; Med, medulla; NodC, nodule of the cerebellum; Obx, obex; PonB, basis pontis; PonT, pontine tegmentum; SMV, superior medullary velum; Tct, tectum; Teg, tegmentum; TonC, cerebellar tonsil; V4, fourth ventricle. Fig. 12. Oblique sagittal CISS image through the fourth ventricle. AqS, aqueduct of Sylvius; Cho, choroid plexus; CistM, cisterna magna; FM, foramen of Magendie; IMV, inferior medullary velum; LingC, lingula of the cere- bellum; Med, medulla; PonB, basis pontis; PonT, pontine tegmentum; SMV, superior medullary velum; SupCC, su- perior cerebellar cistern; Tct, tectum; Teg, tegmentum; Th, thalamus; TonC, cerebellar tonsil; V3, third ventricle; V4, fourth ventricle. Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  Ventricles, Subarachnoid Spaces, and Meninges 587 Fig. 13. Axial T1-weighted image through the fourth ventricle at the level of the pons. CM, corpus medullare; CN VII/VIII, cranial nerves VII and VIII; CPAC, cerebellopontine angle cistern; FColl, facial colliculi; MCP, middle cere- bellar peduncle; NodC, nodule of the cerebellum; PonB, basis pontis; PrePC, prepontine cistern; PymV, pyramid of the vermis; TonC, cerebellar tonsil; V4, fourth ventricle. Monro to its inferior terminal point, which is termed fourth ventricle, there are 2 prominent inverted the “inferior choroidal point.” Within the atria, there L-shaped tufts of choroid along the roof. These is a prominent triangular tuft called the glomus. typically protrude prominently from the foramina The tela choroidea is an invagination of the pia of Luschka into the cerebellopontine angle cis- mater and ependyma, which gives rise to the terns to form the flower baskets of Bochdalek.16,34 choroid plexus within the choroidal fissure and along the roof of the third ventricle.3 Choroid Circumventricular Organs of the Brain plexus within the third ventricle is often hypoplas- There are 7 described midline circumventricular or- tic and not apparent on imaging studies. Within the gans (CVOs) that are neuroendocrine structures Fig. 14. Axial T1-weighted image through the PMDJ at the foramen of Luschka. Inset: axial CISS through the su- perior medulla at the foramen of Luschka. Biven, biventral lobule of the cerebellum; CM, corpus medullare; CN IX, cranial nerve IX; FC, foramen cecum of the medulla; FL, foramen of Luschka; FlC, cerebellar flocculus; ICP, infe- rior cerebellar peduncle; LR, lateral recess of the fourth ventricle; PMDJ, pontomedullary junction; PymV, pyramid of the vermis; TonC, cerebellar tonsil; V4, fourth ventricle; VA, vertebral artery; Vac, confluence of the vertebral arteries into the basilar; VenA, ventricular aperture of the lateral recess. Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 588 Morris et al Table 1 Characteristics of circumventricular organs of the brain Type Location Primary Functions AP Sensory Floor of the distal fourth ventricle at the Chemoreceptor trigger zone for level of the obex (only paired CVO) vomiting. Autonomic cardiorespiratory control OVLT Sensory Anterior wall of third ventricle Osmoregulation via angiotensin and vasopressin pathways. Possible mediator of inflammatory reactions to plasma molecules SFO Sensory Underside of the fornix at the Osmoregulation and fluid balance via confluence of the foramina of Monro angiotensin and vasopressin pathways SCO Secretory Ventral surface of the posterior Not well understood; likely plays a role commissure adjacent to the cerebral in water homeostasis and electrolyte aqueduct balance via aldosterone NH Secretory Posterior portion of the pituitary Release of vasopressin and oxytocin (originating from the floor of the third ventricle) ME Secretory Small protuberance on the tuber Collection of releasing and inhibiting cinereum posterior to the regulatory hypothalamic hormones infundibular stalk (CRF, GnRH, TRH, GHRH, DA) PG Secretory Posterior wall of third ventricle Regulation of circadian rhythms via melatonin Abbreviations: AP, area postrema; CRF, corticotropin-releasing factor; DA, dopamine; GHRH, growth hormone-releasing factor; GnRH, gonadotropin-releasing hormone; ME, median eminence; NH, neurohypophysis; OVLT, organum vasculo- sum of the lamina terminalis; PG, pineal gland; SCO, subcommissural organ; SFO, subforniceal organ; TRH, thyrotropin-releasing hormone. characterized by a rich network of permeable cap- intracranial vasculature are transmitted through illaries with an incomplete blood–brain barrier.35 the subarachnoid space. Within the subarachnoid These organs serve a homeostatic function be- space, CSF flows over the surface of the brain and tween blood, CSF, and brain parenchyma by way down the length of the spinal cord. CSF is of sensory and secretory functions. The CVOs resorbed via arachnoid granulations (Pacchionian include the organum vasculosum of the lamina ter- bodies), projections of the arachnoid membrane minalis (OVLT), subforniceal organ (SFO), area into the dural sinuses that allow CSF to pass postrema (AP), NH, ME, and pineal gland (PG). from the subarachnoid space into the venous sys- The subcommissural organ (SCO) is inconsistently tem, accounting for most CSF resorption. A small listed as a CVO because it lacks a high concentra- amount of CSF may also enter the lymphatic sys- tion of fenestrated capillaries making its blood– tem via the nasal cribriform plate in association brain barrier less permeable than other CVOs; how- with olfactory nerve roots37,38 or in association ever, its function as a secretory neuroendocrine with spinal nerve roots.38 structure has been used as an argument for classi- The subarachnoid cisterns are compartments fication as a CVO. The CVOs are commonly classi- within the subarachnoid space where there is a fied as being sensory (AP, OVLT, SFO) or secretory pool of CSF within an expanded pia-arachnoid in- (SCO, NH, ME, PG) in nature. See Table 1 for an terval. Although described as distinct compart- overview of CVO characteristics. A more detailed ments, in reality, they are continuous or discussion of imaging features by Horsburgh and separated by porous membranes, trabeculae, Massoud36 is recommended for further reading. and septa39; major partitions are discussed in the meninges section of this article. For ease of orga- SUBARACHNOID SPACES AND CISTERNS nization, the major named subarachnoid cisterns presented below are organized in a roughly The subarachnoid space is an interval between the anatomic fashion from inferior to superior divided arachnoid mater and pia mater with free multidi- by infratentorial and supratentorial compartments rectional flow of CSF that allows for equalization (Table 2). Major contents of the subarachnoid cis- of composition. Additionally, cranial nerves and terns are detailed in Table 3. Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  Ventricles, Subarachnoid Spaces, and Meninges 589 Table 2 Subarachnoid cisterns by intracranial compartment Infratentorial At the Level of the Tentorium Supratentorial Cisterna magna Interpeduncular Crural (paired) Cerebellomedullary (paired) Quadrigeminal Suprasellar Premedullary Ambient (paired) Carotid (paired) Prepontine Oculomotor (paired) Cerebellopontine angle Sylvian (paired) (paired) Superior cerebellar Lamina terminalis Subdiaphragmatic Pericallosal INFRATENTORIAL the anterior surface of the medulla. The cranial Cisterna Magna boundary is at the pontomedullary junction (at the level of the foramina of Luschka) and is sepa- The largest of the subarachnoid cisterns is the rated from the prepontine cistern by the medial cisterna magna (Figs. 15 and 16), bounded supe- pontomedullary membrane. The cistern commu- riorly by the cerebellar vermis, anteriorly by the nicates freely with the anterior spinal cistern infe- dorsal surface of the medulla, posteriorly by the in- riorly. The lateral extent is defined by dense ner table of the occipital bone, and inferiorly by the arachnoid trabeculae crossing the subarachnoid foramen magnum where it communicates freely space anterior to the CN IX–X–XI complex. The with the posterior spinal cistern. CSF drains into premedullary cistern contains the distal vertebral the inferior aspect of the cisterna magna from fora- arteries terminating as the basilar origin, PICA, men of Magendie. The posterior inferior cerebellar and CN XII.39,40 arteries (PICAs) enter the cisterna magna and commonly divide into lateral and medial trunks supplying the cerebellar hemispheres and vermis, Prepontine Cistern respectively. The cisterna magna communicates The prepontine cistern (see Fig. 15; Fig. 17) is superiorly with the superior cerebellar cistern and located ventral to the pons and dorsal to the clivus. laterally with the cerebellomedullary cisterns via The superior boundary is formed by the mesence- the cerebellomedullary fissure.39,40 phalic leaf of the Liliequist membrane, inferiorly by the medial pontomedullary membrane (at the Cerebellomedullary Cisterns origin of the basilar artery), and laterally by the The cerebellomedullary cisterns (see Fig. 16) are anterior pontine membranes. The basilar artery is occasionally included under the umbrella of the found within the prepontine cistern and gives rise cisterna magna or referred to as the inferior cere- to the anterior inferior cerebellar artery (AICA), bellopontine angle cisterns. The cisterns are whereas CN VI traverses in the anterior pontine limited superiorly by the lateral pontomedullary membrane.39,40 membrane that crosses the subarachnoid space between CN VIII and CN IX just caudal to the pon- Cerebellopontine Angle Cisterns tomedullary junction. The inferior border is formed The cerebellopontine angle cisterns (see Fig. 17) by the foramen magnum. The premedullary cistern are triangular in shape on axial images and lies anteriorly and is separated by arachnoid trabe- bounded superiorly by the tentorium cerebelli, culations anterior to CN IX, CN X, and CN XI. As posteriorly by the anterior surface of the cere- well as CN IX, X, and XI, the PICA is contained bellum, anteriorly by the anterior pontine mem- within the cerebellomedullary cisterns. The cis- brane (separating from the prepontine cistern terns are continuous with the cisterna magna post- and containing CN VI), laterally by the petrous tem- eroinferiorly via the cerebellomedullary poral bone (including the internal acoustic fissure.39,40 meatus), and inferiorly by the lateral pontomedul- lary membrane, which serves as a boundary be- Premedullary Cistern tween the cerebellopontine angle and The premedullary cistern (see Figs. 15 and 16) cerebellomedullary cisterns. CN V, CN VII, CN lies between the lower aspect of the clivus and VIII, portions of the AICA, the superior cerebellar Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 590 Morris et al Table 3 Major contents of the subarachnoid cisterns Cistern Common Major Contents Ambient Nerves: CN IV Arteries: Posterior cerebral artery (PCA) P2p segment, lateral posterior choroidal, medial posterior choroidal, short and long circumflex, thalamogeniculate, inferior temporal, parieto-occipital Carotid Arteries: Internal carotid, ophthalmic, anterior choroidal, posterior cerebral, posterior communicating, middle cerebral, anterior cerebral Cerebellomedullary Nerves: CN IX, CN X, CN XI Arteries: Vertebral, PICA Cerebellopontine Nerves: CN V, CN VI, CN VII, CN VIII Arteries: SCA and AICA, both commonly bifurcating into rostral and caudal trunks Veins: tributary veins converge to form the superior petrosal vein Cisterna magna Arteries: PICA, commonly dividing into medial and lateral trunks Crural Arteries: PCA P2a segment, peduncular perforating, anterior choroidal, medial posterior choroidal, short and long circumflex, thalamogeniculate, inferior temporal Interpeduncular Nerves: CN III (in lateral wall) Arteries: Posterior thalamoperforating, basilar bifurcation, PCA origin, PComm, SCA, medial posterior choroidal, short and long circumflex Veins: Peduncular, posterior communicating, median anterior pontomesencephalic Lamina terminalis Arteries: ACA A1 and A2 segments, recurrent artery, anterior communicating, orbitofrontal; perforators supplying the chiasm, anterior third ventricle, and anterior hypothalamic area Oculomotor Nerves: CN III Pericallosal Arteries: ACA A2–A5 segments, callosomarginal, frontopolar, internal frontal, paracentral, parietal Premedullary Nerves: CN XII rootlets (in posterior wall) Arteries: Vertebral, anterior spinal Prepontine Arteries: Basilar, AICA origin Quadrigeminal Nerves: CN IV Arteries: Trunks and branches of PCA (P3 segment) and SCA Veins: Convergence of internal cerebral, basal veins, and other tributaries of the vein of Galen Subdiaphragmatic Pituitary gland Superior cerebellar Arteries: Median and paramedian branches of the SCA Veins: Superior vermian Suprasellar Nerves: CN II, optic chiasm Arteries: Carotid perforating branches including superior hypophyseal and infundibular Sylvian Arteries: MCA branches, lenticulostriate, distal recurrent artery branches Veins: Superficial Sylvian artery (SCA), the vein of the cerebellopontine in this space. The cerebellopontine angle cisterns fissure as it ascends to reach the superior petrosal are continuous with the fourth ventricle via the vein, and Bochdalek’s flower basket are contained foramina of Luschka.39,40 Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  Ventricles, Subarachnoid Spaces, and Meninges 591 Superior Cerebellar Cistern The superior cerebellar cistern (see Fig. 15) is located between the superior surface of the cere- bellum and the tentorium and communicates ante- riorly with the quadrigeminal cistern and posteriorly with the cisterna magna below the confluence of the dural venous sinuses. The cistern tapers laterally and blends into the sub- arachnoid space over the cerebellar hemispheres. Terminal branches of the SCA are found within the superior cerebellar cistern.39,40 Subdiaphragmatic Cistern The subdiaphragmatic cistern (see Fig. 19) is a variably present (seen on MR imaging in approxi- mately 70% of patients) small but distinct sub- Fig. 15. Sagittal CISS image through the posterior fossa. AntSpC, anterior spinal cistern; CistM, cisterna arachnoid space with considerable variability in magna; FM, foramen of Magendie; IP, interpeduncu- structure. The roof of the cistern is formed by the lar cistern; PostSpC, posterior spinal cistern; PreM, pre- diaphragma sellae, the floor by the superior aspect medullary cistern; PreP, prepontine cistern; Quad, of the pituitary gland, the lateral walls by the arach- quadrigeminal cistern; SupCC, superior cerebellar noid extending laterally through the medial walls of cistern; V3, third ventricle; V4, fourth ventricle. the cavernous sinus, and the medial walls by the infundibular stem. The cistern communicates with the suprasellar cistern by way of the ostium of the diaphragma.41 COMMUNICATING THROUGH THE TENTORIAL INCISURA Interpeduncular Cistern The interpeduncular cistern (see Fig. 15; Figs. 18–19) is conical in shape on axial images. Located between the cerebral peduncles and the posterior perforated substance, the interpe- duncular cistern serves as the confluence of the supratentorial and infratentorial subarachnoid space and straddles the tentorial incisura. Boundaries are formed by the Liliequist mem- brane superiorly (diencephalic leaf) and inferiorly (mesencephalic leaf), which separates the inter- peduncular cistern from the suprasellar and pre- pontine cisterns, respectively. Anteriorly the cistern is limited by the origin of the Liliequist membrane and posteriorly by the posterior perfo- rated substance. It communicates with the crural and ambient cisterns laterally beyond the medial borders of the cerebral peduncles. The cistern contains the bifurcation of the basilar artery, Fig. 16. Axial CISS image through the posterior fossa proximal (peduncular) segments of the posterior at the level of the medulla. CistM, cisterna magna; CMCs, cerebellomedullary cistern; CMF, cerebellome- cerebral and superior cerebellar arteries, and dullary fissure; CN XII, cranial nerve XII; Med, medulla; posterior communicating arteries. CN III courses PICA, posterior inferior cerebellar artery; PreM, pre- within the lateral wall of the cistern and projects medullary cistern; TonC, cerebellar tonsils; VA, verte- between the posterior cerebral and superior cere- bral artery. bellar arteries.39,40,42 Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 592 Morris et al divided by the superior cerebellar membrane. Contents include the posterior cerebral artery, SCA, the basal vein, and CN IV. The crural cisterns are occasionally labeled as the anterior ambient cistern in which case the ambient cistern proper is labeled as the posterior ambient cistern.40,42 SUPRATENTORIAL Crural Cisterns The crural cisterns (see Fig. 18) are situated around the ventrolateral aspect of the midbrain be- tween the cerebral crus and the uncus of the tem- poral lobe. They communicate dorsally with the Sylvian cistern and ambient cistern. Contents include the anterior choroidal artery, the medial portion of the posterior choroidal artery, and the basal vein.40,42 Suprasellar Cistern (Chiasmatic Cistern) The suprasellar cistern (see Figs. 18 and 19) is a midline cistern anterior to the interpeduncular Fig. 17. Axial CISS image through the posterior fossa cistern that surrounds the pituitary infundibulum at the level of the cerebellopontine angle. AICA, ante- and optic chiasm. The cistern communicates rior inferior cerebellar artery; BA, basilar artery; CN VI, with the cistern of the lamina terminalis superiorly, cranial nerve VI; CN VII/VIII, cranial nerves VII and VIII; CPAC, cerebellopontine angle cistern; ICAp, petrous the middle cerebral artery cisterns leading to the portion of the internal carotid artery; PonB, basis pon- Sylvian cisterns anterolaterally, and the interpe- tis; PreP, prepontine cistern; V4, fourth ventricle. duncular cistern posteriorly (partially separated by Liliequist membrane). The origin of the anterior cerebral arteries is contained in the suprasellar Quadrageminal Cistern cistern.40,42 The midline quadrigeminal cistern (see Figs. 15, 18, and 19) is located between the colliculi Carotid Cisterns rostrally, the splenium of the corpus callosum su- The carotid cisterns (see Figs. 18 and 19) are periorly, the superior surface of the cerebellum bounded by the anterior portion of the uncus later- inferiorly, and the great cerebral vein of Galen ally, the lateral edge of the optic chiasm medially, caudally, roughly encompassing the pineal region. superiorly by the anterior perforated substance, The lateral walls are formed by the pulvinar anteri- and inferiorly by the posterior clinoid process orly and infrasplenial occipital cortex posteriorly. and cavernous sinus. The cistern is separated The cistern communicates with the posterior from the suprasellar cistern medially by the medial aspect of the pericallosal cistern superiorly, the carotid membrane. The middle cerebral artery cis- ambient cisterns inferolaterally, and posteriorly terns leading to the Sylvian cisterns are found with the superior cerebellar cistern. The quadrige- laterally, interpeduncular cistern posteromedially, minal cistern contains posterior cerebral, posterior lamina terminalis cistern superomedially, and the choroidal, and superior cerebellar arteries as well crural cisterns posteriorly. There is commonly a as CN IV. The confluence of the vein of Galen confluent area between the carotid, interpeduncu- and inferior sagittal sinus into the straight sinus is lar, and crural cisterns without membranous sepa- contained within the cistern.39,40 ration. The carotid cisterns primarily contain the internal carotid arteries (ICA) and proximal ICA Ambient Cisterns branches.40,42 The ambient cisterns (see Figs. 18 and 19) are Oculomotor Cisterns found posterolateral to the midbrain. In conjunc- tion with the crural cisterns, they act as a connec- The oculomotor nerve arises from the ventral tion between the quadrigeminal cistern and the midbrain and has a “free” portion, a segment interpeduncular cistern and are considered to within the oculomotor cistern, and a segment have supratentorial and infratentorial components where it is incorporated into the fibrous lateral Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  Ventricles, Subarachnoid Spaces, and Meninges 593 Fig. 18. Axial CISS images at the level of the basal cisterns with (left) and without (right) color overlays. Amb, ambient cistern; BBA, bifurcation of the basilar artery; Car, carotid cistern; CN IV, cranial nerve IV; CP, cerebral peduncle; Cru, crural cistern; Inf, pituitary infundibulum; IP, interpeduncular cistern; LMd, diencephalic leaf of the Liliequist membrane; MPChoA, medial posterior choroidal artery; PPS, posterior perforated substance; Quad, quadrigeminal cistern; SMV, superior medullary velum; SS, suprasellar cistern; Syl, Sylvian cistern; TenC, ten- torium cerebelli; Unc, uncus; V4p, proximal fourth ventricle. wall of the cavernous sinus (see Fig. 20). The free fronto-orbital veins, and collaterals to the basal portion of the oculomotor nerve courses in an vein.40,42 intersection of arachnoid membranes at the junc- tion of the carotid, suprasellar, prepontine, inter- Cistern of the Lamina Terminalis peduncular, and cerebellopontine cisterns. The opening of the oculomotor cistern is located at The “tent-shaped” midline cistern of the lamina the roof of the cavernous sinus and is termed the terminalis (see Fig. 19) communicates inferiorly “porus,” formed by a complex network of sub- with the suprasellar cistern and superiorly with arachnoid membranes and a dural cuff. The oculo- the pericallosal cistern. The anterior boundary is motor cistern surrounds the nerve for a variable formed by the union of pia mater rostral to the distance then gradually tapers and terminates anterior communicating arteries. The lamina termi- below the tip of the anterior clinoid process as nalis of the third ventricle forms the posterior wall the nerve enters the orbit through the superior with the inferior wall formed by the optic chiasm. orbital fissure.40,42,43 Lateral boundaries are formed by the gyri recti and septal areas of the frontal lobes. Important Sylvian Cisterns (Insular Cisterns) structures traversing the cistern of the lamina ter- minalis include major components of the anterior The middle cerebral artery cisterns lead to the “T” circulation with the A1 and proximal A2 segments shaped Sylvian cisterns (see Figs. 18 and 19), of the anterior cerebral arteries, the anterior considered the transitional subarachnoid space communicating artery, the recurrent artery of between the basal cisterns and the hemispheric Heubner, the hypothalamic arteries, the origin of subarachnoid space. The cistern is bounded by the fronto-orbital arteries, and venous structures the optic tract medially with other boundaries of the lamina terminalis.40,42 formed by the anterior cerebral and anterior choroidal membranes, insular cortex, and oper- Pericallosal cistern cular cortices. Contents include the M1 segment of the middle cerebral artery and its proximal The pericallosal cistern (see Fig. 19) extends under branches as well as the middle cerebral vein, the falx cerebri between the cerebral hemispheres Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 594 Morris et al Fig. 19. Sagittal and axial CISS images centered on the supratentorial cisterns. A1, A1 segment of the ACA; A2, A2 segment of the ACA; AC, anterior commissure; Amb, ambient cistern; Aqs, aqueduct of Sylvius; Car, carotid cistern; CC, corpus callosum; CLT, cistern of the lamina terminalis; HT, hypothalamus; Inf, pituitary infundibulum; IP, interpeduncular cistern; LT, lamina terminalis; LV, lateral ventricle; MB, mammillary body; MCA, middle cere- bral artery; Mdb, midbrain; MI/Thal, massa intermedia/thalamus; OC, optic chiasm; OR, optic recess; OT, optic tract; Pc, pericallosal cistern; Pit, pituitary; Quad, quadrigeminal cistern; SdC, subdiaphragmatic cistern; SP, septum pellucidum; Syl, Sylvian cistern; TC, tuber cinereum; V3, third ventricle. and above the corpus callosum from the rostrum concentric outermost-to-innermost layers: dura to the splenium. It communicates inferiorly with mater (pachymeninges), arachnoid mater, and the cistern of the lamina terminalis. Contents pia mater that together form the leptomeninges. include the pericallosal artery.40,42 The pia mater (tender mother) is firmly adherent to the surface of the brain and loosely connected to the arachnoid layer; it is generally not visualized THE MENINGES on imaging studies under normal physiologic cir- The meninges are an intricate covering of the cen- cumstances and will not be discussed further. tral nervous system, composed of 3 distinct Dura Mater The dura mater (hard mother) is the outermost layer of the meninges and is primarily composed of fibroblasts and extracellular collagen. The dura mater arises from a sheath of somitic mesoderm, the meninx primitiva, that forms around the rostral neural tube after closure and ultimately forms the scalp, skull, and meninges. The dura mater is composed of 3 layers, the periosteal layer, the meningeal layer, and the dural border cell layer. The dural border cell layer is only visible microscopically. The periosteal layer is tightly adherent to the calvarium, particularly around the foramen mag- num. It is continuous with the pericranium (external periosteum covering the outer surface of the skull) through the cranial sutures. The periosteal layer creates sheathes that surround the cranial nerves Fig. 20. Oblique axial CISS image of the oculomotor as they course through the skull base foramina, nerves. CN IIIc, cisternal segment of cranial nerve III; transitioning to epineurium as they exit. It is also CN IIIomc, segment of cranial nerve III within the ocu- continuous with the periorbita (orbital periosteum) lomotor cistern; Por, oculomotor porus. through the superior orbital fissure. Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  Ventricles, Subarachnoid Spaces, and Meninges 595 Fig. 21. Coronal (A, C, D) and axial (B, E) T2-weighted MR images. The falx cerebri is a vertically oriented dural reflection (arrows A) and extends rostrally from the internal frontal crest and crista galli (arrows B) to the tento- rium cerebelli. The tentorium cerebelli has fixed and free edges with the free edge (arrows C, D) forming a U-shaped opening that is the sole connection between the supratentorial and infratentorial compartments. The falx cerebelli is a vertically oriented dural fold that separates the cerebellar hemispheres (arrows E). The inner meningeal layer extends through the of the falx cerebri is attached to the groove for foramen magnum and is contiguous with the spi- the superior sagittal sinus. The free edge of the nal dura mater. The periosteal and meningeal falx cerebri is closely associated with the superior layers of the dura mater are adherent to one margin of the corpus callosum and contains the another throughout most of their intracranial inferior sagittal sinus. expanse, with the exception of the dural reflec- The tentorium cerebelli is the second largest tions and the dural venous sinuses. dural reflection that forms superior and inferior compartments within the cranial cavity. It has both fixed and free edges. The fixed edges attach Dural Reflections to the anterior and posterior clinoid process ante- The dural reflections are folds of the meningeal riorly, the superior margin of the petrous temporal layer that compartmentalize the cranium and bones laterally, and the grooves for the transverse assist in dampening motion of the brain. The major sinuses and internal occipital protuberance poste- dural reflections are the falx cerebri and tentorium riorly. The free edge of the tentorium cerebelli cerebelli (Fig. 21A–D). The minor dural reflections forms a roughly U-shaped opening that is the are the falx cerebelli (Fig. 21E) and diaphragma sole connection between the supratentorial and sellae (Fig. 22).44 infratentorial compartments, the tentorial incisura The falx cerebri is the largest dural reflection that or tentorial notch. The anterior margin of the tento- is oriented vertically and separates the 2 cerebral rial incisura is the dorsum sella and the apex is hemispheres in the interhemispheric fissure. It located just posterior to the PG. The anatomic in- has a sickle shape and extends from the internal terrelationships with the tentorial incisura are dis- frontal crest and crista galli to the internal occipital played in Fig. 21 panels C and D. protuberance. Posteriorly, the falx cerebri blends The falx cerebelli (see Fig. 21E) is a vertically ori- with the tentorium cerebelli. The superior margin ented dural fold that extends along the internal Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 596 Morris et al Fig. 22. Axial (A), coronal (B), and sagittal (C) T2-weighted ZOOM MR images of the diaphragma sella (dashed arrows). Axial image A demonstrates the opening of the diaphragma allowing for the passage of the infundib- ulum. Coronal postcontrast T1 MR image (D) through the sella turcica demonstrates normal enhancement of the diaphragma sella (dashed arrows). ICAc, cavernous segment of the internal carotid artery; Inf, pituitary infundib- ulum; Sella, sella turcica. occipital crest anteriorly into the posterior cere- veins, veins of the falx cerebri, emissary veins, bellar notch, partially separating the cerebellar and diploic veins. The superior sagittal sinus typi- hemispheres. It extends from the inferior margin cally drains into the sinus confluence (torcular Her- of the posterior falx cerebri superiorly to the fora- ophili) or into one of the transverse sinuses. men magnum inferiorly. The falx cerebelli contains The inferior sagittal sinus is located within the the occipital sinus and occasionally splits in 2 near falx cerebri along its free edge (see Figs. 23A the foramen magnum. and 24A). It begins at the crista galli and drains The diaphragma sellae is the smallest dural to the straight sinus. It drains the medial frontal reflection. It extends from the tuberculum sellae lobes, corpus callosum, falx cerebri, and cingulate anteriorly to the posterior clinoid processes poste- gyri. riorly, covering the sella turcica (see Fig. 22). There The inferior sagittal sinus and the vein of Galen is a small opening in the diaphragma sellae join to form the straight sinus (see Figs. 23A and through which the pituitary infundibulum projects 24A). The straight sinus is a midline structure that (see Fig. 22A). is found at the junction of the falx cerebri and ten- torium cerebelli that most commonly drains into the sinus confluence, less commonly the trans- Dural Venous Sinuses verse sinuses. Occasionally, a falcine sinus may The dural venous sinuses are formed by separa- be encountered, most commonly a communica- tion of the periosteal and meningeal layers of the tion between the superior and inferior sagittal si- dura mater, are lined by endothelium, and are nuses within the posterior aspect of the falx valveless. They drain portions of the scalp (via cerebri (see Fig. 23H).45 The falcine sinus may be emissary veins), calvarium, dura, and brain ulti- persistent or recanalized and the straight sinus mately into the internal jugular veins. CSF also may or may not be present. drains into the dural venous sinuses via the arach- The occipital sinus extends from the marginal si- noid granulations. nus (see Fig. 23G) of the foramen magnum to the The superior sagittal sinus courses along the sinus confluence along the internal occipital crest midline from the foramen cecum to the occipital (see Fig. 23B). It is the smallest of the dural venous protuberance (Figs. 23 and 24A,B). It is located sinuses and is most commonly a single vessel within the falx cerebri along its convex superior located at the midline. The occipital sinus is margin and drains the cortical veins, meningeal much less commonly duplicated or absent and Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  Ventricles, Subarachnoid Spaces, and Meninges 597 Fig. 23. Sagittal (A, H), coronal (B), and axial (C, D, E, F, G) post-contrast T1 images of the dural venous sinuses. BVP, basilar venous plexus; CS, cavernous sinus; FS, falcine sinus; ICV, internal cerebral vein; IJV, internal jugular vein; IPtS, inferior petrosal sinus; ISS, inferior sagittal sinus; JB, jugular bulb; MarS, marginal sinus; OCCs, occipital sinus; SC, sinus confluence; SiS, sigmoid sinus; SPaS, sphenoparietal sinus; SPtS, superior petrosal sinus; SSS, supe- rior sagittal sinus; StS, straight sinus; TS, transverse sinus; VA4, V4 segment of the vertebral arteries; VoG, vein of Galen. positioned just off the midline, varying in size with configuration of the transverse sinus. Most age. The variability in the morphology of the occip- commonly, the left transverse sinus is hypoplastic ital sinus and inconsistent demonstration on imag- or aplastic and the right transverse sinus is domi- ing studies can create problems in posterior fossa nant. The transverse sinus may arise from the si- surgical approaches.46,47 nus confluence, straight sinus, or directly from The transverse (lateral) sinus is a paired dural the superior sagittal sinus and drain into the sig- venous sinus that is contained within the folds of moid sinus.48 The transverse sinus transitions to the tentorium cerebelli along the inner surface of the sigmoid sinus as it exits the tentorium cerebelli the occipital bone, within a groove at the margin and receives venous drainage from the inferior of the cruciform eminence (see Figs. 23B, C and anastomotic vein of Labbe and superior petrosal 24A, B). There is significant variability in the sinus. Fig. 24. Sagittal (A) and oblique coronal (B) maximum intensity projection time-of-flight magnetic resonance venography (MRV) images. BVR, basal vein of Rosenthal; EJV, external jugular vein; ICV, internal cerebral vein; IJV, internal jugular vein; ISS, inferior sagittal sinus; SC, sinus confluence; SiS, sigmoid sinus; StS, straight sinus; SSS, superior sagittal sinus; TS, transverse sinus; VoG, vein of Galen. Downloaded for Anonymous User (n/a) at Pontifical Catholic University of Chile from ClinicalKey.com by Elsevier on April 07, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 598 Morris et al The sigmoid sinus courses inferomedially within predominantly drains into the superior and inferior an S-shaped groove (sigmoid sulcus) in the mas- petrosal sinuses. The left and right cavernous si- toid portion of the temporal bone, ultimately drain- nuses are connected anteriorly and posteriorly by ing to the jugular bulb and internal jugular vein (see the intercavernous sinuses that are located within Figs. 23C, E, F and 24A, B). The relative depths of the diaphragma sellae.49 the sigmoid sulci give an indication of sinus domi- What was termed the sphenoparietal sinus by nance, with the deeper groove on the dominant Breschet50 has been considered a misnomer and side. The sigmoid plate is thin bone that separates corresponds to the combination of the parietal the sigmoid sinus from the mastoid portion of the portion of the frontal ramus of the middle menin- temporal bone that, when dehiscent, can be asso- geal vein and the sinus of the lesser sphenoid ciated with pulsatile tinnitus of venous origin. wing (see Fig. 23D). The sinus of the lesser sphe- The superior petrosal sinus is a paired dural noid wing receives venous drainage from the mid- venous channel that originates in the cavernous si- dle meningeal vein, diploic veins of the orbital roof nus and drains to the junction of the transverse and greater sphenoid wing and the ophthalmome- and sigmoid sinuses (see Fig. 23D, E). It travels ningeal vein and drains to the cavernous sinus.51 along the petrous ridge within the superior petrosal The basilar venous plexus is located between sulcus within the attached margins of the tento- the periosteal and meningeal layers of the dura rium cerebelli and receives venous drainage from along the posterior surface of the clivus (see the cerebellum, inferior cerebral vein, and labyrin- Fig. 23F). It extends from the cavernous sinus su- thine vein. The superior petrosal sinus is closely periorly to the foramen magnum inferiorly. It has associated with the trigeminal nerve, typically trav- extensive and variable communications with the eling along the superior margin of the porus cavernous sinuses, intercavernous sinuses, supe- trigeminus. rior petrosal sinuses, inferior petrosal sinuses, The inferior petrosal sinus is a paired dural marginal sinus, and internal vertebral venous venous channel that originates from the cavernous plexus.52 sinus along the posteroinferior margin and drains to the junction of the sigmoid sinus and the jugular Arachnoid Mater bulb (see Fig. 23F). It travels in the inferior petrosal sulcus, a groove in the petrous temporal bone and The arachnoid mater is the delicate an