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Imaging modalities shed light on intracranial cysts

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).

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