Computerized tomography (CT) and magnetic resonance imaging (MRI) are the primary modalities for diagnosis of cerebellopotine lesions.
The main imaging features to analyze in a cerebellopontine angle mass are: CT attenuation,
signal characteristics at MR imaging,
origin,
shape,
presence of calcifications and adjacent bone reaction.
It is important to classify CPA lesions based on its intra or extra-axial origin to narrow the differential diagnosis (Fig. 2).
CPA masses can arise from:
- Cerebellopontine angle contents (cranial nerves,
vessels,
CSF,
arachnoid tissue).
- Extension from the osseous boundaries (skull base or petrous bone).
- Exophytic brainstem or ventricular tumours.
VESTIBULAR SCHWANNOMA
Schwannomas are encapsulated,
benign,
slowly growing tumours and most of them develop from the schwann cells.
Bilateral vestibular schwannomas strongly suggest the presence of neurofibromatosis type 2.
Typically,
vestibular schwannomas are centred on the porus acusticus,
and smoothly erode its posterior edge.
The intracanicular component can extend towards the CPA giving the typical “ice cream cone” apperance.
At CT,
schwannomas enhance after contrast administration and calcification is typically not present.
On MRI show T1 iso or hipointensity to adjacent brain and T2 high signal intensity.
Most of the cases show intense and homogeneous contrast enhancement,
but heterogeneous,
and cystic enhancement patterns are possible too (Fig. 3).
MENINGIOMA
Meningioma represents the second most frequent lesion in the CPA and it is the most common intracranial extra-axial tumour in adults.
They are a non-glial slow-growing neoplasm composed of meningocytes from the arachnoid layer of the meninges.
Meningiomas ussually appear hyperdense on CT,
20% show calcification,
most of them associate bone changes (hyperostosis or scalloping) and the “dural tail sign” may appear.
In contrast with vestibular schwannomas,
they do not enlarge the porus acustic.
On MRI,
meningiomas are usually isointense to the cortex on all sequences and show intense and homogeneous enhancement after contrast administration (Fig. 4).
EPIDERMOID CYST
Epidermoid cysts are the third most common CPA masses.
They are congenital developmental intradural lesions arising from the inclusion of ectodermal elements during neural tube closure.
Epidermoid cysts typically show undulating or irregular margin.
At CT,
they appear as CSF-like attenuation lesions and may have marginal calcifications.
At MRI,
epidermoid cyst are usually isointense to CSF but can vary depending on the lipid content (“white epidermoid”).
On FLAIR sequence show heterogenous/hypersignal and show increased signal on diffusion MR imaging (mixed restricted DWI and/or T2 shine trought effect) (Fig. 5 Fig. 6 ).
ARACHNOID CYST
Arachnoid cysts are benign,
congenital,
intra arachnoidal lesions filled with CSF that do not communicate with the ventricular system.
They represent 1% of all intracranial masses and most are supratentorial in location,
only 10% are found in the posterior cranial fossa.
At CT and MRI,
they appear as well defined lesions that follow CSF pattern.
They do not enhance after contrast administration,
on FLAIR sequence they suppress completely and on DWI sequence show no restriction of diffusion ( Fig. 7 Fig. 8).
OTHER LESIONS
Many other masses can be found at or around the CPA,
but they only account for less than 1% each.
Vascular lesions,
such as vertebrobasilar dolichoectasia or aneurysms (Fig. 9) and vascular loops (Fig. 10),
can lead to cranial nerves or brainstem compression.
A few tumours arising from the skull base (Fig. 11)may extend towards the CPA and in oncologic patients,
metastasis should always be considered.
We should keep in mind that in the CPA,
metastatic disease can be show as bone lesions,
intra-axial masses,
dural metastasis and leptomeningeal carcinomatosis ( Fig. 12).