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ventricular system overview :: Brain Imaging

Gross and Imaging Anatomy

Ventricles and Choroid Plexus
Basic embryology: Early in embryonic development, the forebrain cavity divides into two lateral ventricles, which develop as outpouchings from the rostral third 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 fourth 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 (SAS).

The ventricular system is comprised of four interconnected CSFfilled, ependymal-lined cavities that lie deep within the brain.

The paired lateral ventricles communicate with the third ventricle via the Y-shaped foramen of Monro.

The third ventricle communicates with the fourth ventricle via the cerebral aqueduct (of Sylvius).

In turn, the fourth ventricle communicates with the SAS via its outlet foramina (the midline foramen of Magendie and the two lateral foramina of Luschka).

Lateral ventricles

 Each lateral ventricle has a body, atrium, and three projections ("horns").

The roof of the frontal horn is 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 body of 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 atrium contains the choroid plexus glomus and is formed by the confluence of the body with the temporal and occipital horns.

The temporal horn extends 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 horn is 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 two long arms extending towards each lateral ventricle and a short inferior common stem that connects with the roof of the third ventricle.

Third ventricle

The third 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 third ventricle.

The floor of the third ventricle is formed by several critical anatomic structures. From front to back these include the optic chiasm, hypothalamus with the tuber cinereum and infundibular stalk, mammillary bodies, and roof of the midbrain tegmentum.

The third ventricle has two 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 third ventricle.

A variably sized interthalamic adhesion (also called the massa intermedia) lies between the lateral walls of the third 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 third ventricle with the fourth ventricle.

Fourth ventricle

 The fourth ventricle is a roughly diamondshaped 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 fourth ventricle has five distinctly shaped recesses.
The posterior superior recesses are paired, thin, flat, CSF-filled pouches that cap the cerebellar tonsils.
The lateral recesses curve anterolaterally from the fourth ventricle, extending under the brachium pontis (major cerebellar peduncle) into the lower cerebellopontine angle cisterns. The lateral recesses
transmit choroid plexus through the foramina of Luschka into the adjacent subarachnoid spaces.

The fastigium is a triangular, blind-ending, dorsal midline outpouching that points towards the cerebellar vermis.

The fourth 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 and the production of CSF

The choroid plexus is comprised of highly vascular papillary excrescences with a central connective tissue core coated by an ependymaderived 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, the glomus, 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 then dives through the interventricular foramen (of Monro) and curves posteriorly along the roof of the third ventricle.

The choroid plexus, in the body of the lateral ventricle, curls around the thalamus into the temporal horn, where it fills the choroidal fissure and lies superomedial to the hippocampus.

CSF is predominantly, but not exclusively, secreted by the choroid plexuses.

Brain interstitial fluid, ependyma, and capillaries may also play a poorly defined role in CSF secretion.

The choroid plexus epithelium secretes CSF at the rate of about 0.2-0.7 mL/minute or 600-700 mL/day.

The mean CSF volume is 150 ml, with 25 ml in the ventricles and 125 ml in subarachnoid spaces.

CSF flows through the ventricular system and passes through the exit foramina of the fourth 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.

Not all CSF is produced in the choroid plexus.

Drainage of brain interstitial fluid is a significant extrachoroidal source.

CSF plays an essential role in the maintenance of brain interstitial fluid homeostasis and regulation of neuronal functioning.

Cisterns and Subarachnoid Spaces

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 cistern is 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.

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.

The perimesencephalic(ambient) cisterns are thin wings of CSF that extend posterosuperiorly from the suprasellar cistern to the quadrigeminal cistern.

They wraparound the midbrain and contain the trochlear nerves, P2 PCA segments, superior cerebellar arteries, and 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 vein of Galen. An anterior extension, the velum interpositum, lies below the fornix and above the third 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, the anterior inferior cerebellar arteries (AICAs), and the trigeminal and abducens nerves (CN5 and CN6).

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., PICAs) and the hypoglossal nerve (CN12).

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 posterior inferior cerebellar artery (PICA).

The cisterna magna merges imperceptibly with the SAS of the upper cervical spinal canal.

The cerebellopontine angle cisterns (CPAs) lie between the pons/cerebellum and the petrous temporal bone.

Their most important contents are the trigeminal, facial, and vestibulocochlear nerves (CN5, CN7, and CN8).

Other structures found here include the petrosal veins and AICAs.

The CPA cisterns are contiguous inferiorly with the cerebellomedullary cisterns, sometimes termed the "lower" cerebellopontine angle 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 (CN9, CN10,and CN11).

A tuft of 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. 

Whole brain FLAIR is especially useful for evaluating potential abnormalities in the SASs. 

Spin dephasing with pulsatile CSF flow is common and can mimic intraventricular 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 two 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.

They may be hyperintense on FLAIR and 60-80% appear quite bright on DWI.

A strongly enhancing choroid plexus mass in a child is most likely a choroid plexus papilloma

With the exception of the fourth 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.

Colloid cyst is the only relatively common pathology here.

It is rare in children and 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 ependymoma, papilloma, and metastasis are rare.

Third ventricle mass

Again, the most common "lesion" in this location is either CSF flow artifact or a normal structure (the
massa intermedia).

Colloid cyst is the only common lesion that occurs in the third ventricle; 99% are wedged into the
foramen of Monro.

Extreme vertebrobasilar dolichoectasia can indent the third 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 third ventricle in adults are also uncommon, though an intraventricular
macroadenoma and chordoid glioma are examples.

Neurocysticercosis occurs here but is uncommon.

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).

Fourth ventricle mass

 Pediatric masses are the most common intrinsic abnormalities of the fourth 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 three and can mimic medulloblastoma.

Metastases to the choroid or ependyma are probably the most common fourth ventricle neoplasm of adults.

Primary neoplasms are rare.

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 fourth ventricle, lying behind the pontomedullary junction.

A newly described rare neoplasm, rosette-forming glioneuronal tumor, is a midline mass of the fourth ventricle. It has no particular distinguishing imaging features and, although it may appear aggressive, it is a benign (WHO grade I) lesion.

Hemangioblastomas are intraaxial masses but may project into the fourth ventricle.

Epidermoid cysts and neurocysticercosis cysts can be found in all ages.

Subarachnoid Spaces and Cisterns

Overview: The subarachnoid spaces 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, are often nonspecific. Patient age is also helpful though generally of secondary importance.

Normal variants

CSF flow-related artifacts are common, especially in the basal cisterns on FLAIR imaging.

Mega cisterna magna may be considered a normal variant, as is a cavum velum interpositum (CVI).

A CVI is a thin, triangularshaped CSF space between the lateral ventricles that lies below the fornices and above the third ventricle. Occasionally a CVI may become quite large.

Suprasellar cistern mass

Common masses in adults are upward extensions of macroadenoma, meningioma, and aneurysm.

The two 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-IAC masses.

Meningioma, epidermoid cyst, aneurysm, and arachnoid cyst together represent about 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 about 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 fourth ventricle originates in the obex and lies behind the medulla.

FLAIR hyperintensity

 Hyperintense sulci and subarachnoid spaces are seen with MR artifacts, as well as a variety of lesions.

Pathologic FLAIR hyperintensity is typically related to blood (e.g., subarachnoid hemorrhage), protein (meningitis), or cells (pia-subarachnoid space metastases).

Less commonly, gadolinium-based contrast agents in patients with blood-brain barrier leakage or renal failure can cause FLAIR hyperintensity.

Rare causes of FLAIR hyperintensity include ruptured dermoid cyst, moyamoya ("ivy" sign), and acute cerebral ischemia.

Contrast enhancement helps distinguish meningitis and metastases from subarachnoid hemorrhage and CSF artifacts.

Imaging Gallery

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 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 subarachnoid space 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 (curved black arrows) of the lateral ventricle are separated by a tiny septi pellucidi (blue arrow). Note the foramen of Monro (curved blue arrows) connecting the lateral ventricles to the 3rd ventricle (opened blue arrow)

Axial T2 MR at the level of the cerebral aqueduct (blue arrow) demonstrates the infundibular recess of the 3rd ventricle (White arrow) , mammillary bodies (Curved white arrow), interpeduncular cistern (Black arrow), and the quadrigeminal cisterns (curved blue arrows).

Axial T2 MR at the level of the 4th ventricular outlet shows the foramen of Magendie (Curved arrow)  and foramina of Luschka (arrows)

Sagittal T2 SPACE MR image shows the normal flow void due to CSF flow at the cerebral aqueduct (blue arrow) and foramen of Magendie (curved blue arrow). Note the chiasmatic (white arrow) and infundibular recesses (black arrow) of the 3rd ventricle and fastigium (open blue arrow) 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 (curved blue arrow) 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 image in a patient with hydrocephalus demonstrates a prominent pseudomasses (curved blue arrow) of the 3rd ventricle caused by pulsatile CSF.

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

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

Axial DWI shows characteristic large choroid plexus cysts (blue arrows) 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. 

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

Axial FLAIR MR in a patient with acute subarachnoid hemorrhage due to aneurysm rupture shows high signal in the left sylvian fissure (curved blue arrow) and posterior cortical sulci (blue arrows)

Axial FLAIR MR in a patient with chronic renal disease who received IV gadolinium 48 hours prior shows marked FLAIR hyperintensity (curved blue arrows) 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).

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