INTRODUCTION
Cysts are common brain imaging findings. They can be divided in non-neoplastic and neoplastic lesions (Fig.1).
Fig. 1: Classification of intracaranail cystic lesions.
The questions below may help the diagnostic approach.
· Is the cyst extra-axial or intra-axial?
· Is it supra- or infratentorial?
· If it is extra-axial, is it midline or off-midline (Fig. 2)?
· If it is intra-axial, is it parenchymal or intraventricular (Table 1)?
Fig. 2: Extra-axial cysts.Differential diagnosis.
Table 1: Classification of intracranial cysts according to location.
ENLARGED PERIVASCULAR SPACES (VIRCHOW-ROBIN)
These are pial-lined interstitial fluid-filled structures that accompany penetrating arteries and veins. They are common and considered a normal variant.
Imaging features
Typically less than 5 mm in diameter and isointense to CSF at all sequences (Fig. 3).
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Fig. 3: Brain MR (A) axial T1-WI (B) axial T2-WI demonstrate multiple small cysts representing enlarged perivascular space in the basal ganglia.
CHOROID PLEXUS CYSTS
Nonneoplastic epithelial-lined cysts of the choroid plexus. Usually bilateral and commonly located in the lateral ventricular atria. They are asymptomatic and found incidentally.
Imaging features
These cysts are iso- to hyperattenuated to CSF on nonenhanced CT.
Peripheral calcification is common and they may enhance.
Signal intensity is variable. Most are iso- or hyperintense to CSF on precontrast T1-WI and may show rim or nodular enhancement. Usually hyperintense to CSF on T2 WI and do not become completely hypointense on FLAIR. Two-thirds show restriction on diffusion (Fig.4).
Fig. 4: Axial brain MR images (A) T2-WI (B) FLAIR (C) diffusion and (D) ADC map demonstrate bilateral choroid plexus cysts in the atria of the lateral ventricules (arrows). Note restricted diffusion on the right (yellow arrows).
Differential diagnosis
Ependymal cyst and villous hyperplasia of the choroid plexus.
PINEAL CYST
Pineal cysts and cystic degeneration of the pineal gland with some residual pineal parenchyma are seen in up to 10% of cases at routine imaging studies.
Imaging features
Unilocular fluid-filled mass within the pineal gland. They can have rim or nodular calcium in the cyst wall. Rim or nodular enhancement is also common. On T1-WI, 55%–60% are slightly hyperintense to CSF. Most do not appear hypointense on FLAIR and 60% enhance (Fig. 5).
Fig. 5: Brain MR axial images (A) T2-WI (B) FLAIR and (C) T1 post contrast demonstrate a small pineal cyst (arrows). The cyst is hyperintense on FLAIR.
Differential Diagnosis
Pineocytoma which is more likely to have solid components.
Other cysts (arachnoid and, rarely, epidermoid).
EPENDYMAL CYSTS
They are rare, benign, ependymal-lined cysts of the lateral ventricle or juxtaventricular region of the temporoparietal region and frontal lobe. Most are incidental.
Imaging features
Usually a non enhancing thin-walled CSF-containing cyst (Fig. 6).
Fig. 6: Ependymal cyst. Axial T2 brain MRI shows a cyst in the left lateral ventricle.
Differential Diagnosis
Choroid plexus and arachnoid cysts, neurocysticercosis and asymmetric ventricles.
ARACHNOID CYSTS
Benign, congenital, intraarachnoidal space-occupying cysts that do not communicate with the ventricular system. Most are supratentorial and 50 to 60% are found in the middle cranial fossa.
Imaging features
They appear as extra-axial fluid collection isointense with CSF on all sequences with no restriction on DWI and no enhancement (Fig. 7).
Fig. 7: Brain MR images (A) sagittal T1-WI (B) axial T2-WI (C) FLAIR and (D) axial T1-WI post contrast show an arachnoid cyst in the cerebellar cistern (arrows).
COLLOID CYSTS
They are benign mucin-containing cysts (15%–20% of intraventricular masses). More than 99% are found wedged in the foramen of Monro.
Imaging features
The best diagnostic clue is its location at the foramen of Monro. It usually appears as a hyperattenuated mass on nonenhanced CT (Fig. 8). On T1-WI, two thirds are hyperintense. The majority are isointense to brain on T2-WI.
Fig. 8: Axial brain CT (A) coronal CT reconstruction (B) and brain MR axial T2-WI (C) demonstrate a colloid cyst in the foramen of Monro (arrows). The cyst is hyperdense on CT and hypointense on T2-WI.
Differential Diagnosis
The most common “lesion” mistaken for a colloid cyst is CSF flow artifact.
EPIDERMOID CYST
They are congenital inclusion cysts. The most common location is the cerebellopontine angle cistern (40%–50%).
Imaging features
On CT, they appear as hypoattenuated masses resembling CSF without enhancement. Calcification is present in 10%–25% of cases.
At MR most are isointense or slightly hyperintense to CSF on both T1- and T2-WI, do not suppress completely on FLAIR, show restriction on diffusion and do not enhance (Fig. 9).
Fig. 9: Epidermoid cyst. Brain MR axial images (A) T1-WI (B) T2-WI (C) FLAIR (D) T1 post contrast (E) diffusion and (F) ADC map show a nonenhancing cystic mass in the left cerebellopontine angle (arrows) with restricted diffusion.
Differential Diagnosis
Especially with arachnoid cyst which is isointense to CSF at all sequences and do not restrict on diffusion.
DERMOID CYST
Congenital ectodermal inclusion cysts. They are extremely rare and usually occur in the midline sellar, parasellar or frontonasal regions.
Imaging features
It varies, depending on whether the cyst has ruptured. Unruptured cysts have the same signal as fat and do not enhance (Fig. 10). They may be heterogeneous on T2-WI. If rupture occurs fatlike droplets in the subarachnoid cisterns, sulci, and ventricles as well as pial enhancement (chemical meningitis) can be seen.
Fig. 10: Dermoid cyst. Brain CT (A) and brain MR images (B) axial T1-WI (C) axial T2-WI(D) sagittal T1-WI (E) FLAIR and (F) axial T1 post contrast show a mass in the left temporal lobe (black arrows) with hyperintense areas on T1-WI (fat components) and signs of rupture (yellow arrows).
Differential Diagnosis
Epidermoid, craniopharyngioma, teratoma, or lipoma.
Table 2: Differential diagnosis. Dermoid and epidermoid cysts.
RATHKE CLEFT CYSTS
Congenital nonneoplastic cysts arising from remnants of the embryonic Rathke cleft. They are commonly incidental intra and/or suprasellar lesions. Forty percent are completely intrasellar (Fig. 11).
Fig. 11: Sella MR images (A) coronal T1-WI (B) coronal T2-WI and (C) sagittal T1 post contrat demonstrate an intrasellar nonenhancing Rathke cleft cyst (arrows).
Imaging features
Typically seen as a nonenhancing noncalcified intra-and/or suprasellar cyst with an intracystic nodule. Half are hyperintense on T1-WI. They do not enhance.
Differential Diagnosis
Craniopharyngioma, cystic pituitary adenoma or other nonneoplastic cysts.
NEUROCYSTICERCOSIS
Caused by taenia solium and. It is the most common parasitic infection of the central nervous system (SNC). It can be located anywhere within the CNS, though the parenchymal cortico-medullary junction is the most common site.
Seizures are the common manifestation.
Imaging features
Imaging findings vary with the stage of cyst development.
In the early vesicular stage a CSF-like cyst is seen on CT and MR images. A mural nodule is often present (viable larval scolex) (Fig. 12).
In the colloidal-vesicular stage, pericystic edema and cyst wall enhancement are present.
During granular stage, surrounding edema is still present, and nodular or micro-ring enhancement is common.
In the residual stage, small calcified nodules usually without enhancement are seen.
Fig. 12: Neurocysticercosis. Brain MR axial images (A) T1-WI (B) T2-WI (C) FLAIR and (D) T1 post contrast show small cystic lesions (black arrows) some with scolex (yellow arrows). Ring enhancement is seen (blue arrows).
Differential Diagnosis
Abscess, tuberculosis, neoplasm, enlarged perivascular spaces, and other parasitic infections.
None of these has the characteristic “cyst with dot” appearance (scolex).
POSTERIOR FOSSA CYSTS
Posterior fossa cysts and cystlike malformations are frequently observed on neuroimaging studies in children.
MRI has greatly facilitated differential diagnosis (Fig. 13).
Fig. 13: Diagnostic approach of posterior fossa cysts.
MEGA CISTERNA MAGNA
It represents a normal variant.
An enlarged cisterna magna with a normal fourth ventricle.
Imaging features
The presence of a normal vermis and the absence hydrocephalus help differentiate it from isolated inferior vermian hypoplasia and Blake pouch cyst, respectively (Fig. 14).
Fig. 14: Brain MR (A) sagittal T1-WI (B) axial T2-WI weighted and (C) axial T1 post contrast show an enlarged cisterna magna (arrows). No hydrocephalus.
POSTERIOR FOSSA ARAHNOID CYST
About 10% occur in the posterior fossa (Fig.15).
Fig. 15: Brain MR axial images (A) T1-WI and (B) T2-WI show an arachnoid cyst in the right cerebellopontine angle (arrows).
BLAKE POUCH CYST
In 1989, Barkovich et al. distinguished two basic kinds of posterior fossa cystlike malformations (Fig. 16). Later Blake pouch cyst (BCP) was described.
Imaging features
Typical radiological features of BPC (Fig. 17):
(1) tetraventricular hydrocephalus
(2) cyst infra- or retrocerebellar
(3) well-developed, nonrotated cerebellar vermis
(4) cystic dilation of the fourth ventricle without cisternal communication (5) some degree of cerebellar compression
Fig. 17: Brain MR images (A) axial T1-WI (B) coronal T2-WI and (C) sagittal T2-WI show a Blake pouch cyst (yellow arrows). There is severe hydrocephalus (black arrows).
Differential diagnosis
Dandy-Walker complex, arachnoid cysts, and cystlike malformations (Fig.18).
Fig. 18: Posterior fossa malformation. Diagnostic approach.
DANDY-WALKER MALFORMATION
The most common posterior fossa malformation characterized by the classic triad of hydrocephalus, absence or agenesis of the cerebellar vermis and posterior fossa cyst communicating with the fourth ventricle.
Hydrocephalus is associated in about 90% of patients.
Imaging features
(1) hypoplasia (or, rarely, agenesis) of the cerebellar vermis, which is elevated and upwardly rotated (Fig 19).
(2) dilatation of the fourth ventricle, which may fill the entire posterior fossa.
Fig. 19: Brain MR (A) axial T1-WI (B) axial T2-WI (C) coronal T1-WI (D) sagittal T1-WI and (E) sagittal T2-WI demonstrate a huge cyst corresponding to Dandy-Walker malformation (black arrows). There is vermian hypoplasia (yellow arrow) and cerebellar rotation (blue arrow).
TAKE HOME MESSAGE
A location-based approach to intracranial cysts is helpful in establishing an appropriate differential diagnosis (fig. 20).
Fig. 20: Classification of intracranial cysts according to most common location.