Imaging modalities shed light on intracranial cysts

Dr. Saleem and Dr. Baxi are senior registrars, Dr. Nagendra and Dr. Murali are senior consultant radiologists, and Dr. Kishore is head of the radiology department, all at the Care Hospital Hyderabad in Andhra Pradesh, India.

Any fluid-filled cavity or sac that is lined by an epithelium is a cyst, and intracranial cystic lesions are a common finding on CT and MR imaging of the brain.^1,2 These lesions contain either cerebrospinal fluid, fluid that is similar to CSF, mucus, or proteinaceous fluid. They are lined by epithelial cells, inflammatory cells, or glial cells. The attenuation characteristics of the cyst on CT and MRI and the contrast enhancement patterns depend on the cyst's contents and the composition of the wall.

Intracranial cystic lesions have been classified into five groups based on their etiology: normal variant cysts, developmental cysts, post-traumatic cysts, postinfectious and inflam¬matory cysts, and primary and secondary neoplasms with a cystic component. Patho¬genic criteria have also been used for classification.³

Normal variant cysts are isodense and isointense to CSF on all sequences on CT and MRI, respectively, and are usually detected incidentally. One such lesion is the cavum septum pellucidum (fifth ventricle), a cystic CSF cavity of the septum pellucidum of uncertain significance. This cyst occurs when the fetal septum pellucidum fails to obliterate. The resulting enclosed cavity with parallel walls can measure up to 1 cm in diameter. Another lesion in this category is the cavum vergae (sixth ventricle). This occurs in less than 1% of the adult population and is a posterior extension of the cavum septum pellucidum.

The cavum velum interpositum is a triangular CSF space, located between the bodies of the lateral ventricles. Unlike the cavum vergae, the cavum velum interpositum ends at the foramen of Monro and causes a splaying of the fornices.

Enlarged tumefactive perivascular (Virchow-Robin) spaces are pial-lined, CSF-filled spaces that accompany penetrating arteries supplying the brain parenchyma. Perivascular spaces are normally less than 5 mm in diameter. Tumefactive perivascular spaces are much larger than this and have an expanding appearance. These enlarged spaces usually occur in the basal ganglia and white matter and, less commonly, in the midbrain.

DEVELOPMENTAL CYSTS

Dandy-Walker malformation is characterized by an enlarged posterior fossa with cystic dilatation of the fourth ventricle; upward displacement of the lateral sinuses, tentorium, and torcular herophili; vermian aplasia or hypoplasia; hypoplastic or compressed brain stem; and heterotopias. In 80% of the cases, there is an associated hydrocephalus. The fourth ventricle opens dorsally into a large posterior fossa cyst with an outer wall that is continuous with the leptomeninges (Figure 1).

Extra-axial, intra-arachnoid CSF-like fluid-filled sacs that are lined by arachnoid cells are known as arachnoid cysts. This lesion does not communicate with the ventricular system. Imaging shows a smooth, sharply demarcated round/ovoid extra-axial cyst, which displaces the cortex and buckles the gray-white interface. CSF attenuation is followed in all sequences unless complicated by hemorrhage. No contrast enhancement is observed (Figure 2).

Colloid cysts are endodermal-derived unilocular cysts that are filled with viscid, gelatinous material and lined with a fibrous capsule. These lesions virtually always occur in the anterior third ventricle at the foramen of Monro. Two-thirds of colloid cysts are homogeneously hyperdense to brain parenchyma on unenhanced CT. The remaining third are isodense. Signal on MRI is variable and depends on the cyst contents. Colloid cysts are most commonly hyperintense on T1-weighted sequences and hypointense on T2-weighted sequences.

An epidermoid cyst is an ectodermal inclusion cyst that is filled with desquamated cells and keratinaceous debris. It may be either congenital or a result of trauma. Imaging reveals a well-delineated cauliflower-like cystic lesion that insinuates along CSF spaces and cisterns and encases nerves and vessels. These cysts are usually isodense to CSF on CT and isointense to CSF on T1-weighted and T2-weighted MR sequences.

Diffusion-weighted MRI, however, shows epidermoid cysts as being hyperintense to CSF, revealing that they have water restriction, which means that movement of water molecules is reduced (Figure 3). This helps to distinguish them from arachnoid cysts, which show no water restriction and appear hypointense.^1,4-7

Dermoid cysts are congenital ectodermal inclusion cysts that have a thick wall of connective tissue. They occur most commonly in or near the midline and then in the parasellar, frontobasal, and posterior fossa regions. They may rupture into the subarachnoid space. The lipid content of dermoid cysts causes a fatlike hypodensity on unenhanced CT and hyperintensity on both T1- and T2-weighted MRI sequences. Fluid-fluid or fluid-debris levels may be present within the cystic lesions. No enhancement is observed after administration of gadolinium contrast.

Rathke's cleft cyst refers to an ectodermal cyst, with variable serous or mucoid contents, that is derived from either the arachnoid membrane or epithelial remnants along Rathke's pouch. Between 20% and 25% of Rathke's cleft cysts are purely intrasellar, while about 70% are both intra- and suprasellar. Imaging appearance varies according to cyst contents. Typically, however, these cysts appear hypodense on CT and hyperintense on T1-weighted MR sequences. They are noncalcified.

POST-TRAUMATIC/POSTINFARCT

Porencephalic cysts are CSF-filled intraparenchymal lesions. These lesions, which may be congenital or acquired following an infarct, may communicate with the ventricular system. Imaging reveals a cystic intraparenchymal cavity corresponding to a specific arterial territory, with enlargement of the adjacent ventricle. Porencephalic cysts are isodense to CSF on CT and isointense to CSF on all MRI sequences.

Leptomeningeal cysts can form when there is a loculation of CSF onto or through a skull defect, typically when a linear fracture in the frontal or parietal bone (>4 mm wide) is accompanied by a dural tear. Arachnoid tissue herniates into the dural defect, and CSF pulsations produce fracture diastasis. This may consequently also be referred to as the "growing" fracture. Leptomeningeal cysts are frequently accompanied by an adjacent area of encephalomalacia. Imaging should show a cyst that has the density of CSF. It will be isointense to CSF on all MR sequences and will communicate with the subarachnoid space. This lesion should also be observed adjacent to a skull defect and have indistinct scalloped margins.

Multicystic encephalomalacia is due to a diffuse insult, caused by ischemia, infection, or developmental insults to the brain late in gestation, during birth, or after birth. Multiple cystic cavities of variable size separated by glial septations form in the necrotic area. These are encephalomalacic cysts.

POSTINFECTION/INFLAMMATION

A focal pyogenic infection of the brain parenchyma that has a thin uniform wall, homogeneous center, and surrounding florid edema is an abscess. Most abscesses occur in the supratentorial region in the frontal or parietal lobes at the gray-white interface. Up to 14% of these lesions occur infratentorially.

Imaging findings on CT vary according to the stage of the abscess. An ill-defined hypodense subcortical lesion with mass effect is seen in the early cerebritis stage. Imaging of the late cerebritis stage shows a central hypodense lesion with peripheral edema and an increased mass effect. In the early capsule stage, there will be a hypodense mass and moderate vasogenic edema with mass effect. Edema and the mass effect decrease in the late capsule stage.

Abscesses appear with a hypointense rim on T2-weighted MR sequences, hyperintense on diffusion-weighted MR sequences, and hypointense on apparent diffusion coefficient mapping. MR spectroscopy shows an increase in levels of acetate, lactate, pyruvate, alanine, succinate, and amino acids. This finding permits the differentiation of an abscess from necrotic or cystic tumors.

Neurocysticercosis, which is caused by the larva of Taenia solium, is the most common parasitic infection of the central nervous system. The lesion can be located anywhere within the CNS, though the parenchymal cortico¬medullary junction is the most common site in the brain. CT reveals the four distinct stages of neurocysticercosis. Imaging-based identification of the lesion's different stages is pathognomonic.

In the vesicular stage, when the larva is viable, CT shows a cystic lesion with a central hyperdense nodule, representing the scolex of the Taenia. No edema or enhancement is observed. The degenerating larva induces edema in the colloid vesicular stage. The lesion then appears as a focal hypodense area with ring enhancement. As the lesion heals during the granular nodular stage, it becomes isodense or mildly hyperdense with a calcified scolex. Lesion enhancement and edema are both characteristic. The final stage is reached when the lesion has healed. CT reveals a completely calcified nodule, without edema or enhancement.

Cerebral hydatid disease, which is caused by Echinococcus granulosus (the hydatid worm), is extremely rare. Hydatid cysts may be single or multiple, uni- or multiloculated, and thin- or thick-walled. More specific signs include visualization of a calcified wall, presence of daughter cysts, and membrane detachment. Hydatid cysts are typically supratentorial. They can occur anywhere within the brain but are more often located in the middle cerebral artery territory. The parietal lobe is the most frequently involved region. A mass effect is observed when imaging a hydatid cyst, but contrast enhancement is absent.⁸

Tumefactive multiple sclerosis is cystic in appearance and may mimic the appearance of neoplasms. Lesions show semilunar (horseshoe-shaped) enhancement on MRI following intravenous injection of gadolinium contrast. MRS reveals low levels of N-acetylaspartate (NAA) and a low NAA/creatine ratio. Choline and lactate levels are both elevated.

NEOPLASMS

Cystic schwannoma is a benign well-delineated round or lobulated encapsulated tumor that arises eccentrically from Schwann nerve cells. Acoustic schwannoma is the most commonly seen cerebellopontine angle tumor. Multiple schwannomas are characteristic of type 2 neurofibromatosis. Schwannomas commonly show cystic or fatty degeneration and hemorrhagic necrosis. Between 15% and 20% of schwannomas show an intralesional or peritumoral cyst.Imaging findings for a vestibular schwannoma include an extra-axial mass with enlarged ipsilateral cerebellopontine angle cistern, a CSF vascular cleft, displaced gray-white matter interface, brain stem rotation, and compression of the fourth ventricle. Schwannomas are iso- to hypodense on CT and hyperintense on T2-weighted MRI. Solid components enhance intensely after contrast administration.

Meningioma is the most common nonglial primary brain tumor to arise from specialized meningothelial cells. Most meningiomas are extra-axial, dural-based lesions with a dural tail. Around 90% occur in the supratentorium. Typical meningiomas are hyperdense on unenhanced CT and iso¬intense to gray matter on all MRI sequences. Enhancement after contrast administration is intense. True cystic meningiomas with large intratumoral cysts are uncommon, though 10% to 15% of meningiomas show cystic areas of necrosis. An alanine peak is seen on MRS.

Craniopharyngioma is a benign slow-growing tumor that arises from the vestigial craniopharyngeal duct or Rathke cleft. These lesions are typically lobulated, well-delineated cystic masses that generally occur in the sellar and suprasellar region. CT reveals a cystic lobulated suprasellar mass with a solid mural nodule. Nodular or rim calcification should be observed as well. Craniopharyngiomas have variable signal intensity on MRI, though they are most often hypointense on T1-weighted and hyperintense on T2-weighted MR sequences.

Pituitary macroadenomas make up 70% to 80% of pituitary adenomas. Macroadenomas are greater than 10 mm across and are usually endocrinologically inactive. They have a figure eight appearance with a suprasellar component. The lesions are usually isointense to cortical gray matter and enhance intensely on MRI after administration of gadolinium contrast. They generally contain necrotic, cystic, or hemorrhagic components that show variable signal intensity.

Hemangioblastoma is an uncommon vascular neoplasm of unknown origin. The most common location is the cerebellum, with 10% to 20% of hemangioblastomas occurring as part of von Hippel-Lindau syndrome. Lesions are typically subpial well-circumscribed tumors, 60% of which will be cystic with a mural nodule. CT depicts hemangioblastoma as a low-density cyst with a strongly enhancing mural nodule abutting a pial surface. The lesion's cystic component is hyperintense to CSF on all MRI sequences. The mural nodule has variable signal intensity characteristics and strongly enhances on administration of gadolinium contrast.

Pilocytic astrocytomas constitute almost one-third of pediatric gliomas. These lesions are predominantly seen in children and young adults, and they are characteristically located around the third and fourth ventricles. Almost 50% of tumors are found in the optic chiasm and hypothalamus, and 35% in the cerebellum. Less common locations include the cerebral hemispheres and ventricles.

CT reveals a uni- or multilocular cyst, which is slightly hyperdense to CSF and contains a mural nodule with inhomogeneous contrast enhancement. Calcifications are seen in 10% to 20% of cases. The cyst wall is slightly hyperdense and does not enhance. Pilocytic astrocytomas are hypointense on T1-weighted MR sequences and hyperintense on T2-weighted sequences. Enhancement of the solid mural nodule varies (Figure 4).

Glioblastoma multiforme is a common primary brain tumor that typically occurs in the cerebral hemispheric white matter. Lesions are poorly delineated diffusely infiltrating partially necrotic masses. A thick irregular rind of tissue is seen around a necrotic core on imaging. Signal intensity is variable on MRI and depends on the solid and cystic components. Glioblastoma multiforme tumors enhance heterogeneously following injection of gadolinium contrast.

Cystic oligodendroglioma is a slow-growing neoplasm that arises from oligodendrocytes. This lesion usually occurs in adults and is most commonly located in the frontal lobe. Cystic oligodendrogliomas generally start in the hemispheric white matter and grow toward the cortex. Foci of cystic degeneration and calcification are common. Imaging depicts this neoplasm as a heterogeneous hemispheric mass that often involves the cortex.Brain stem glioma is a nonenhancing focal glioma involving the midbrain, pons, or medulla. The brain stem is usually expansile.

Pleomorphic xanthoastrocytoma is a rare partially cystic tumor with a discrete mural nodule that occurs in children and young adults. This lesion is typically located in the superficial temporal lobe. Imaging generally shows a superficial partially cystic hemispheric mass with an enhancing mural nodule.Gangliogliomas most commonly occur in the temporal or frontal lobes, appearing as well-delineated cysts with a partially calcified mural nodule. These lesions generally are seen in children and young adults, and they can cause chronic temporal lobe epilepsy. They may also be associated with cortical dysplasia.

Metastases account for 25% to 33% of brain tumors. Although they can occur anywhere, brain metastases are most often found at the junction between gray and white matter. Between 1% and 6.6% of metastases will be cystic, and the majority will have originated from a primary lung or breast tumor.^1,2 Cystic metastases have striking surrounding edema and ringlike contrast enhancement on imaging. They are hypointense on diffusion-weighted sequences and hyperintense on apparent diffusion coefficient mapping.

CONCLUSION

A diverse range of intracranial pathology has a cystic appearance. Intracranial cysts may be classified according to either etiology or pathogenesis. Depending on the nature of the contents within the cyst and its lining, intracranial cysts exhibit diverse properties of attenuation and contrast enhancement on CT and MRI. These, together with the clinical presentation, aid diagnosis of the lesion.

It is important to remember that the location of the intracranial cyst on CT and/or MRI can help to establish an initial differential diagnosis. Contrast-enhancement patterns further narrow that diagnostic choice. Diffusion-weighted MRI, FLAIR (fluid-attenuated inversion recovery) sequences, and MR spectroscopy can assist with the final characterization of intracranial cystic lesions that have a similar radiological appearance.

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