Brain cyst on MRI

Colloid cyst: anatomy and radiological presentation

Colloid cysts are smooth, spherical lesions that occur exclusively in the paraphysis region. Topographically, this is the posterior lip of the interventricular foramen of Monro, located at the border between the third and lateral ventricles.

The MRI appearance of such cysts is highly variable, as the signal intensity directly depends on the chemical composition of their contents. Calcium, cholesterol, or hemosiderin may be identified within the cyst.

• Basic sequences: The signal may be completely analogous to cerebrospinal fluid (CSF). However, it is much more common to observe a hyperintense signal on T2-weighted images and in the T1 FLAIR sequence. High intensity on FLAIR is usually due to the presence of a protein component.
• Contrast enhancement: One of the key characteristics is the lack of paramagnetic contrast agent uptake. On post-contrast T1-weighted images, the lesion remains isointense.
• CT equivalent: When performing non-contrast computed tomography, such cysts are characterized by initial hyperdensity.

The main clinical problem associated with colloid cysts is their tendency to cause hydrocephalus. This occurs due to intermittent or constant physical obstruction of the CSF outflow pathways from the lateral ventricle system.

Differential diagnosis: partially cystic lesions

In the course of radiological diagnosis, a colloid cyst must be differentiated from other intraventricular neoplasms that may have a cystic component or specific structure.

1. Central neurocytoma (Central Neurocytoma): Appears as a multilocular (with septa), partially cystic lesion. Unlike a colloid cyst, a neurocytoma actively enhances with contrast. It is closely related to the septum pellucidum and is capable of filling the bodies of both lateral ventricles, also provoking hydrocephalus with enlargement of the temporal horns.
2. Intraventricular meningioma (Meningioma): Most frequently localized in the atrium of the lateral ventricle. It presents as a well-circumscribed, globular mass. On T1 and T2-weighted images, its signal may be isointense to the cerebral cortex. Marked contrast enhancement is characteristic.
3. Choroid plexus papilloma: A lobulated tumor typically located in the atrium of the lateral ventricle. It demonstrates intense gadolinium enhancement on T1-weighted images and is also frequently accompanied by ventricular dilatation.

Complex cases: cystic metastases and abscesses

Not only true cysts or primary tumors, but also secondary lesions and inflammatory processes can have a cystic structure on MRI. Advanced MRI techniques, such as diffusion-weighted imaging (DWI), play a crucial role in differential diagnosis.

• Brain abscesses: They contain viscous fluid (pus) inside, which leads to marked restriction of water molecule diffusion. On MRI, this manifests as a bright signal on trace-weighted DWI and, correspondingly, a dark signal on apparent diffusion coefficient (ADC / ADC) maps.
• Cystic or necrotic metastases (and gliomas): Unlike abscesses, they do not typically exhibit such pronounced diffusion restriction.

It is important to remember that the presence or absence of contrast enhancement in intracerebral lesions per se is not a 100% reliable predictor of malignancy, although it critically assists in the overall characterization of the lesion.

Principles of classification of intraaxial lesions

When evaluating any mass lesions of the brain, the radiologist relies on the principles of a "layered diagnosis" (layered diagnosis) established in the WHO classification (specifically, the 2016 edition).

Modern diagnostic radiology goes hand in hand with genetics. Key DNA mutations are now included in the final diagnosis as critical prognostic factors. Thus, the IDH mutation status and the presence of 1p/19q co-deletion are recognized as the primary genetic determinants for defining a specific glioma subtype, which affects the interpretation of structural and perfusion MRI data (including the perifocal edema zone).

Class and exam questions

What else to study on this topic

• Topography of intraventricular lesions (meningioma, ependymoma, neurocytoma)
• Radiological presentation of metastases: vasogenic edema and melanoma features
• Application of MR perfusion (PWI) and spectroscopy (MRS) in neuro-oncology
• Diffusion tensor imaging (DTI) and tractography of white matter tracts
• The role of fluorodeoxyglucose PET (FDG-PET) in assessing malignancy

Other topics in the Neuroimaging section

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