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Gross and Imaging Anatomy

Ventricles and Choroid Plexus
Basic embryology: Early in embryonic development, the forebrain cavity divides into 2 lateral ventricles, which develop as outpouchings from the rostral 3rd ventricle and are connected to it by the interventricular foramen (a.k.a. foramen of Monro). In the coronal plane, these form a central H-shaped "monoventricle." The cerebral aqueduct develops from the midbrain vesicle. The 4th ventricle develops from a cavity within the hindbrain and merges caudally with the central canal of the spinal cord.
Anatomic overview: The brain CSF spaces include both the ventricular system and subarachnoid spaces (SASs). The ventricular system is comprised of 4 interconnected CSF-filled, ependymal-lined cavities that lie deep within the brain. The paired lateral ventricles communicate with the 3rd ventricle via the Y-shaped foramen of Monro. The 3rdventricle communicates with the 4th ventricle via the cerebral aqueduct (of Sylvius). In turn, the 4thventricle communicates with the SAS via its outlet foramina (the midline foramen of Magendieand the 2 lateral foramina of Luschka).
Lateral ventricles: Each lateral ventricle has a body, atrium, and 3 projections ("horns"). The roof of the frontal hornis formed by the corpus callosum genu. It is bordered laterally and inferiorly by the head of the caudate nucleus. The septi pellucidi is a thin, bilayered membrane that extends from the corpus callosum genu anteriorly to the foramen of Monro posteriorly and forms the medial borders of both frontal horns.
The bodyof the lateral ventricle passes posteriorly under the corpus callosum. Its floor is formed by the dorsal thalamus and its medial wall is bordered by the fornix. Laterally, it curves around the body and tail of the caudate nucleus.
The atriumcontains the choroid plexus glomus and is formed by the confluence of the body with the temporal and occipital horns. The temporal hornextends anteroinferiorly from the atrium and is bordered on its floor and medial wall by the hippocampus. Its roof is formed by the tail of the caudate nucleus. The occipital hornis surrounded entirely by white matter fiber tracts, principally the geniculocalcarine tract and the forceps major of the corpus callosum.
Foramen of Monro is a Y-shaped structure with 2 long arms extending towards each lateral ventricle and a short inferior common stem that connects with the roof of the 3rd ventricle.
3rd ventricle: The 3rd ventricle is a single, slit-like, midline, vertically oriented cavity that lies between the thalami. Its roof is formed by the tela choroidea, a double layer of invaginated pia. The lamina terminalis and anterior commissure lie along the anterior border of the 3rd ventricle. The floor of the 3rd ventricle is formed from front to back by the optic chiasm, the hypothalamus with the tuber cinereum and infundibular stalk, mammillary bodies, and the roof of the midbrain tegmentum.
The 3rd ventricle has 2 inferiorly located CSF-filled projections: The slightly rounded optic recess and the more pointed infundibular recess. Two small recesses, the suprapineal and pineal recesses*,* form the posterior border of the 3rd ventricle. A variably sized interthalamic adhesion (also called the massa intermedia) lies between the lateral walls of the 3rd ventricle. The massa intermedia is not a true commissure.
Cerebral aqueduct is an elongated tubular conduit that lies between the midbrain tegmentum and the quadrigeminal plate. It connects the 3rd ventricle with the 4th ventricle.
4th ventricle: The 4th ventricle is a roughly diamond-shaped cavity that lies between the pons anteriorly and the cerebellar vermis posteriorly. Its roof is covered by the anterior (superior) medullary velum above and the inferior medullary velum below.
The 4th ventricle has 5 distinctly shaped recesses. The posterior superior recesses are paired, thin, flat, CSF-filled pouches that cap the cerebellar tonsils. The lateral recessescurve anterolaterally from the 4th ventricle, extending under the brachium pontis (major cerebellar peduncle) into the lower cerebellopontine angle cisterns (CPAs). The lateral recesses transmit choroid plexus through the foramina of Luschka into the adjacent SAS. The fastigiumis a triangular, blind-ending, dorsal midline outpouching that points towards the cerebellar vermis. The 4th ventricle gradually narrows as it courses inferiorly, forming the obex. Near the cervicomedullary junction, the obex becomes continuous with the central canal of the spinal cord.
Choroid plexus, CSF, and brain interstitial fluid (ISF): The choroid plexus is comprised of highly vascular papillary excrescences with a central connective tissue core coated by an ependyma-derived secretory epithelium. The embryonic choroid plexus forms where the infolded tela choroidea contacts the ependymal lining of the ventricles, thus developing along the entire choroidal fissure.
The largest mass of choroid plexus, theglomus, is located in the atrium of the lateral ventricles. The choroid plexus extends anteriorly along the floor of the lateral ventricle, lying between the fornix and thalamus. It extends anteroinferiorly from the glomus into the temporal horn, where it fills the choroidal fissure and lies superomedial to the hippocampus. It also dives through the interventricular foramen (of Monro) and curves posteriorly along the roof of the 3rd ventricle.
The choroid plexus has 2 major functions: CSF production and maintenance of the blood-CSF barrier.
CSF is predominantly, but not exclusively, secreted by the choroid plexuses. Brain ISF, ependyma, and capillaries all play a potential role in CSF secretion. Drainage of brain ISF is a significant extrachoroidal source of CSF. The choroid plexus epithelium secretes CSF at the rate of ~ 0.4 mL/minute or 500-600 mL/day. In adult humans, there are 280 mL of ISF and 140 mL of CSF, of which 30 mL are in the ventricle, 80 mL in the cerebral SAS, and 30 mL in the spinal SAS.
CSF plays an essential role in the maintenance of brain ISF homeostasis and regulation of neuronal functioning.
Traditional model of CSF****homeostasis:****CSF flows through the ventricular system and passes through the exit foramina of the 4th ventricle into the SAS. The bulk of CSF resorption is through the arachnoid villi along the superior sagittal sinus. CSF also drains into lymphatic vessels around the cranial cavity and spinal canal.
Updated model of CSF and ISFhomeostasis: Brain perivascular spaces and paravascular spaces play a critical role in CSF homeostasis. The perivascular spaces from a key component of the brain's "protolymphatic" or "glymphatic" system. ISF circulation likely occurs through the water-selective aquaporin (AQP) channels of the glymphatic system, a key factor in regulating extracellular space water homeostasis. AQP4 is highly expressed in the atrocytic end-feet.
Cisterns and SASs
Overview: The SASs lie between the pia and arachnoid. The sulci are CSF-filled spaces between the gyral folds. Focal expansions of the SASs form the brain CSF cisterns. These cisterns are found at the base of the brain around the brainstem, tentorial incisura, and foramen magnum. Numerous pial-covered septa cross the SAS from the brain to the arachnoid. All SAS cisterns communicate with each other and the ventricular system, providing natural pathways for disease spread (e.g., meningitis, neoplasms). The brain cisterns are conveniently grouped into supra-, peri-, and infratentorial cisterns. All contain numerous important critical structures, such as vessels and cranial nerves.
Supratentorial/peritentorial cisterns: The suprasellar cistern lies between the diaphragma sellae and the hypothalamus. Critical contents include the infundibulum, optic chiasm, and circle of Willis.
The interpeduncular cisternis the posterior continuation of the suprasellar cistern. Lying between the cerebral peduncles, it contains the oculomotor nerves as well as the distal basilar artery and proximal segments of the posterior cerebral arteries (PCAs). Important perforating arteries, the thalamoperforating and thalamogeniculate arteries, arise from the top of the basilar artery and cross the interpeduncular cistern to enter the midbrain.
Theperimesencephalic****(ambient) cisternsare thin wings of CSF that extend posterosuperiorly from the suprasellar cistern to the quadrigeminal cistern. They wrap around the midbrain and contain the trochlear nerves, P2 PCA segments, superior cerebellar arteries, and the basal vein of Rosenthal.
The quadrigeminal cistern lies under the corpus callosum splenium, behind the pineal gland and tectal plate. It connects with the ambient cisterns laterally and the superior cerebellar cistern inferiorly. The quadrigeminal cistern contains the pineal gland, trochlear nerves, P3 PCA segments, proximal choroidal arteries, and the vein of Galen. An anterior extension, the velum interpositum, lies below the fornix and above the 3rd ventricle. The velum interpositum contains the internal cerebral veins and medial posterior choroidal arteries.
Infratentorial cisterns: The unpaired posterior fossa cisterns that lie in the midline are the prepontine, premedullary, and superior cerebellar cisterns, as well as the cisterna magna. The lateral cisterns are paired and include the cerebellopontine and cerebellomedullary cisterns.
The prepontine cistern lies between the upper clivus and the "belly" of the pons. It contains numerous important structures, including the basilar artery, anterior inferior cerebellar arteries (AICAs), and trigeminal and abducens nerves (CNV and CNVI).
The premedullary cistern is the inferior continuation of the prepontine cistern. It lies between the lower clivus in front and the medulla behind. It extends inferiorly to the foramen magnum and contains the vertebral arteries and branches [e.g., posterior inferior cerebellar arteries (PICAs)] and the hypoglossal nerve (CNXII).
The superior cerebellar cistern lies between the straight sinus above and the vermis below. It contains the superior cerebellar arteries and veins. It connects superiorly through the tentorial incisura with the quadrigeminal cistern and inferiorly with the cisterna magna. The cisterna magna lies below the inferior vermis between the medulla and the occiput. It contains the cerebellar tonsils and the tonsillohemispheric branches of the PICA. The cisterna magna merges imperceptibly with the SAS of the upper cervical spinal canal.
The CPAs lie between the pons/cerebellum and the petrous temporal bone. Their most important contents are the trigeminal, facial, and vestibulocochlear nerves (CNV, CNVII, and CNVIII). Other structures found here include the petrosal veins and AICAs. The CPA cisterns are contiguous inferiorly with the cerebellomedullary cisterns.
The cerebellomedullary cisterns extend laterally around the medulla and are continuous with the cisterna magna below and the CPAs above. They contain the vagus, glossopharyngeal, and spinal accessory nerves (CNIX, CNX, and CNXI). A tuft of the choroid plexus exits each foramen of Luschka into the cerebellomedullary cistern. The flocculus of the cerebellum that projects into this cistern can appear very prominent. The flocculus and choroid plexus are normal contents of the cerebellomedullary cisterns and should not be mistaken for pathology.

Imaging Recommendations

MR: Thin-section 3D T2WI or FIESTA/CISS best detail CSF within the ventricular system, SASs, and basal cisterns, and exquisitely delineates their contents. FLAIR MR is especially useful for evaluating potential abnormalities in the SASs. Spin dephasing with pulsatile CSF flow is common and can mimic pathology, especially in the basal cisterns and around the interventricular foramen. Incomplete CSF suppression with "bright" CSF can mimic pathologic SASs.

Differential Diagnosis Approach

Ventricles and Choroid Plexus
Overview: Approximately 10% of intracranial neoplasms involve the cerebral ventricles, either primarily or by extension. An anatomy-based approach is most effective as there is a distinct predilection for certain lesions to occur in one ventricle or cistern and not others. Age is also a helpful consideration. Specific imaging findings, such as signal intensity, enhancement, and the presence or absence of calcification are relatively less important than location and age.
Normal variants: Asymmetry of the lateral ventricles is a common normal variant, as is flow-related CSF pulsation artifact. A cavum septi pellucidi (CSP) is a common normal variant, seen as a CSF cleft between the 2 leaves of the septum pellucidum. An elongated, finger-like posterior continuation of the CSP between the fornices, a cavum vergae (CV), may be associated with a CSP.
Lateral ventricle mass: Choroid plexus cysts (xanthogranulomas) are a common, generally age-related, degenerative finding with no clinical significance. They are nonneoplastic noninflammatory cysts, usually bilateral with rim calcification, may be hyperintense on FLAIR MR, and 60-80% appear bright on DWI. A strongly enhancing choroid plexus mass in a child is most likely a choroid plexus papilloma. With the exception of the 4th ventricle, a choroid plexus mass in an adult is usually meningioma or metastasis, not a choroid plexus papilloma.
Some lateral ventricle lesions display a distinct predilection for specific sublocations within the lateral ventricles. An innocent-appearing frontal horn mass in a middle-aged or older adult is most often a subependymoma. A "bubbly" mass in the body of the lateral ventricle is usually a central neurocytoma. Neurocysticercosis cysts can occur in all ages and in virtually every CSF space.
Foramen of Monro mass: The most common "abnormality" here is a pseudolesion caused by CSF pulsation artifact. A colloid cyst is the only relatively common pathology here. It is rare in children and is typically a lesion of adults. Flow artifact can mimic a colloid cyst, but mass effect is absent. In a child with an enhancing mass in the interventricular foramen, tuberous sclerosis with subependymal nodule &/or giant cell astrocytoma should be a consideration. Masses such as an ependymoma, papilloma, and metastasis are rare.
3rd ventricle mass: Again, the most common "lesion" in this location is either CSF flow artifact or a normal structure (the massa intermedia). A colloid cyst is the only common lesion that occurs in the 3rd ventricle; 99% are wedged into the foramen of Monro. Extreme vertebrobasilar dolichoectasia can indent the 3rd ventricle, sometimes projecting upward as high as the interventricular foramen, and should not be mistaken for colloid cyst.
Primary neoplasms in children are uncommon here but include choroid plexus papilloma, germinoma, craniopharyngioma, and a sessile-type tuber cinereum hamartoma. Primary neoplasms of the 3rd ventricle in adults are also uncommon, though an intraventricular macroadenoma and chordoid glioma are examples.
Cerebral aqueduct: Other than aqueductal stenosis, intrinsic lesions of the cerebral aqueduct are rare. Most are related to masses in adjacent structures (e.g., tectal plate glioma).
4th ventricle mass: Pediatric masses are the most common intrinsic abnormalities of the 4th ventricle. Medulloblastoma, ependymoma, and astrocytoma predominate. Atypical teratoid/rhabdoid tumor (AT/RT) is a less common neoplasm that may occur here. It usually occurs in children under the age of 3 and can mimic medulloblastoma.
Metastases to the choroid or ependyma are probably the most common 4th ventricle neoplasm of adults. Choroid plexus papilloma does occur here as well as in the CPA cistern. Subependymoma is a lesion of middle-aged adults that is found in the inferior 4th ventricle, lying behind the pontomedullary junction. A newly described rare neoplasm, rosette-forming glioneuronal tumor, is a midline mass of the 4th ventricle. It has no particular distinguishing imaging features and, although it may appear aggressive, it is a benign (WHO grade 1) lesion. Hemangioblastomas are intraaxial masses but may project into the 4th ventricle. Epidermoid cysts and neurocysticercosis cysts can be found in all ages.
SASs and Cisterns
Overview: The SASs are a common site of pathology that varies from benign congenital lesions (such as arachnoid cyst) to infection (meningitis) and neoplastic involvement ("carcinomatous meningitis"). Anatomic location is key to the differential diagnosis, as imaging findings, such as enhancement and hyperintensity on FLAIR MR, are often nonspecific.
Normal variants: CSF flow-related artifacts are common, especially in the basal cisterns on FLAIR MR. Mega cisterna magna may be considered a normal variant, as is a cavum velum interpositum (CVI). A CVI is a thin, triangular-shaped CSF space between the lateral ventricles that lies below the fornices and above the 3rd ventricle. Occasionally, a CVI may become quite large.
Suprasellar cistern mass: Common masses in adults are upward extensions of macroadenoma, meningioma, and aneurysm. The 2 most common suprasellar masses in children are astrocytoma of the optic chiasm/hypothalamus and craniopharyngioma.
Cerebellopontine angle mass: In adults, vestibular schwannoma accounts for almost 90% of all CPA-internal auditory canal (IAC) masses. A meningioma, epidermoid cyst, aneurysm, and arachnoid cyst together represent ~ 8% of lesions in this location. All other less common entities, such as lipoma, schwannomas of other cranial nerves, metastases, neurenteric cysts, etc., account for ~ 2%.
In the absence of neurofibromatosis type 2, vestibular schwannomas are very rare in children. CPA epidermoid and arachnoid cysts may occur in children. Extension of ependymoma laterally through the foramina of Luschka may involve the CPA.
Cystic-appearing CPA masses comprise their own special differential diagnosis. While vestibular schwannoma with intramural cysts can occur, it is less common than epidermoid and arachnoid cysts. Neurocysticercosis may occasionally involve the CPA. Large endolymphatic sac anomaly (IP-2) shows a CSF-like mass within the posterior wall of the temporal bone. Hemangioblastoma and neurenteric cysts are other less common cystic masses that occur in the CPA.
Cisterna magna mass: Tonsillar herniation, whether congenital (Chiari 1) or secondary to posterior fossa mass effect or intracranial hypotension, is the most common "mass" in this location. Nonneoplastic cysts (arachnoid, epidermoid, dermoid, neurenteric) may also occur here.
Neoplasms in and around the cisterna magna, such as meningioma and metastasis, are typically anterior to the medulla. Subependymoma of the 4th ventricle originates in the obex and lies behind the medulla.
FLAIR MR hyperintensity: Hyperintense sulci and SASs are seen with MR artifacts as well as a variety of lesions. Pathologic FLAIR MR hyperintensity is typically related to blood (e.g., subarachnoid hemorrhage), protein (meningitis), or cells (pia-SAS metastases). Less commonly, gadolinium-based contrast agents in patients with blood-brain barrier leakage or renal failure can cause FLAIR MR hyperintensity.
Rare causes of FLAIR MR hyperintensity include a ruptured dermoid cyst, moyamoya (ivy sign), and acute cerebral ischemia. Contrast enhancement helps distinguish meningitis and metastases from subarachnoid hemorrhage and CSF artifacts.

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References

Selected References

Images

Selected Images

Schematic 3D representation of the ventricular system, viewed in the sagittal plane, demonstrates the normal appearance and communicating pathways of the cerebral ventricles.

Sagittal midline graphic through the interhemispheric fissure depicts the subarachnoid spaces (SASs) with CSF (blue) between the arachnoid (purple) and pia (orange). The central sulcus separates the frontal lobe (anterior) from the parietal lobe (posterior). The pia mater is closely applied to the brain surface, whereas the arachnoid is adherent to the dura. The ventricles communicate with the cisterns and SAS via the foramina of Luschka and Magendie. The cisterns normally communicate freely with each other.

Axial T2 MR demonstrates normal anatomy at the level of the lateral ventricles. The frontal horns of the lateral ventricle are separated by a tiny septi pellucidi . Note the foramen of Monro connecting the lateral ventricles to the 3rd ventricle .

Axial T2 MR at the level of the cerebral aqueduct demonstrates the infundibular recess of the 3rd ventricle , mammillary bodies , interpeduncular cistern , and the quadrigeminal cisterns .

Axial T2 MR at the level of the 4th ventricular outlet shows the foramen of Magendie and foramina of Luschka .

Sagittal FEISTA MR demonstrates the lateral ventricle , chiasmatic and infundibular recesses of the 3rd ventricle, the cerebral aqueduct , and the fastigium of the 4th ventricle. The floor of the 3rd ventricle in formed from front to back by the optic chiasm, hypothalamus, mammillary bodies, and roof of the midbrain tegmentum.

Axial NECT (L) and axial T2 MR (R) shows normal asymmetry of the lateral ventricles with the right being larger than the left. The septi pellucidi are slightly bowed and displaced across the midline. When there is lateral ventricular asymmetry, it is important to scrutinize the region of foramen of Monro to exclude any obstructing pathology.

Axial FLAIR MR in a patient with hydrocephalus demonstrates focal hyperintensity in the 3rd ventricle due to a pseudomasses caused by pulsatile CSF.

Axial T2 MR shows a large ventricular mass in the frontal horn and anterior body of the right lateral ventricle. There is dilatation of the posterior body of the right lateral ventricle and displacement of the septi pellucidi to the left. On histopathology, this was a central neurocytoma.

Axial FLAIR MR demonstrates an intraventricular neurocysticercosis in the posterior 3rd ventricle with dilatation of the anterior 1/3 and lateral ventricles. Note the mild periventricular interstitial edema .

Axial DWI (L) and axial FLAIR MR (R) shows characteristic large choroid plexus cysts in the atria of both lateral ventricles. Choroid plexus cysts, often called choroid plexus xanthogranuloma, are nonneoplastic noninflammatory cysts and are often bright on DWI.

Sagittal T1 C+ MR in a 8-year-old child with ataxia demonstrates a large, heterogeneously enhancing mass centered in the 4th ventricle. Pathology revealed a classic medulloblastoma, Wnt-activated.

Axial FLAIR MR in a patient with acute cortical subarachnoid hemorrhage due to cortical venous thrombosis shows high signal in the left frontal sulci.

Axial FLAIR MR in a patient with chronic renal disease who received IV gadolinium 48 hours prior shows marked FLAIR hyperintensity in the cortical sulci. Sulcal hyperintensity on FLAIR can be caused by pia-subarachnoid metastases, blood, protein (meningitis), high oxygen content, and retained contrast (renal failure, as in this case).

Additional Images

Axial T1 C+ MR in an 18-month-old child with severe hydrocephalus shows a choroid plexus papilloma (CPP). The intensely enhancing frond-like projections and location in the atrium of the left lateral ventricle are both classic findings.

Axial T1 C+ MR in a middle-aged woman shows a smoothly lobulated, intensely enhancing choroid plexus mass . CPP in adults is rare, except for the 4th ventricle. Meningioma was found at surgery.

Axial T1 C+ FS MR in a 72-year-old man with declining mental state shows a nonenhancing mass in the frontal horn of the left lateral ventricle. This is an incidental finding, most likely a subependymoma.

Axial T1 C+ MR in a 33 year old with headaches shows an inhomogeneously enhancing bubbly lesion in the body of the left lateral ventricle. The appearance and location distinguish a central neurocytoma from subependymoma and other possible lateral ventricular masses, such as a meningioma.

Axial FLAIR MR in a 3 year old with seizures shows a hyperintense mass at the interventricular foramen. Note the flame-shaped subcortical hyperintensities . This is tuberous sclerosis with a subependymal giant cell astrocytoma.

Axial FLAIR MR in a 65 year old with a "thunderclap" headache shows a foramen of Monro mass that is a colloid cyst. Other than CSF flow artifact, colloid cysts are the most common lesion found in this location. They are common in adults but relatively rare in children.

Coronal T1WI MR shows prominent pseudomasses of the 3rd and lateral ventricles caused by pulsatile CSF in and around the interventricular foramen (of Monro). Note the propagation of phase artifact across the image.

Axial T1WI C+ MR shows large lateral 3rd ventricles with "blurred" margins from transependymal CSF flow. A cysticercus cyst with scolex causes obstructive hydrocephalus. Intrinsic 3rd ventricle masses are less common than lateral or 4th ventricular lesions.

Sagittal T1WI C+ MR in a 2 year old with ataxia, nausea, and vomiting shows a lobulated enhancing mass in the 4th ventricle . Fourth ventricle masses in children are usually primitive neuroectodermal tumor or ependymoma, less often atypical teratoid-rhabdoid tumor which was found at surgery.

Sagittal T1WI C+ MR in a 52-year-old woman with episodic headaches, nausea, and vomiting shows an intensely enhancing 4th ventricle mass. This proved to be choroid plexus papilloma.

Axial FLAIR MR shows multifocal sulcal hyperintensities caused by aneurysmal subarachnoid hemorrhage.

Axial FLAIR MR shows artifactual hyperintensity in the occipital sulci secondary to incomplete CSF suppression. A repeat scan (not shown) was normal. Sulcal hyperintensity on FLAIR is nonspecific and can be caused by pia-subarachnoid metastases, blood, protein (meningitis), high oxygen content, retained contrast (renal failure), and artifact (as in this case).

Sagittal T2 SPACE MR shows the normal flow void due to CSF flow at the cerebral aqueduct and foramen of Magendie . Note the chiasmatic and infundibular recesses of the 3rd ventricle and fastigium of the 4th ventricle.

Axial T2 MR shows normal asymmetry of the lateral ventricles with the right being larger than the left. The septi pellucidi are slightly bowed and displaced across the midline. When there is lateral ventricular asymmetry, it is important to scrutinize the region of foramen of Monro to exclude any obstructing pathology.

Axial FLAIR MR in a patient with hydrocephalus demonstrates a prominent pseudomasses of the 3rd ventricle caused by pulsatile CSF.

Sagittal T1 C+ MR shows a large homogenously enhancing 4th ventricular mass , which on pathology was a meningioma. There is dilatation of the ventricular system proximal to the mass.

Axial DWI MR shows characteristic large choroid plexus cysts in the atria of both lateral ventricles within the choroid plexus glomi. Choroid plexus cysts, often called choroid plexus xanthogranuloma, are nonneoplastic noninflammatory cysts. Between 60-80% appear quite bright on DWI, as in this case.

Axial FLAIR MR in a patient with acute subarachnoid hemorrhage due to aneurysm rupture shows high signal in the left sylvian fissure and posterior cortical sulci .

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