Arachnoid cyst (frontal)

Case contributed by Dr Rajalakshmi Ramesh


Restrained driver involved in a high speed head-on collision with another car.

Patient Data

Age: 20 years
Gender: Female

Ventricles and sulci are within normal limits. Incidental note of a right frontal extra-axial CSF density lesion, with no contrast enhancement. It measures 49 x 12mm, is ovoid in morphology and scallops the inner table of the skull. Appearances are consistent with an arachnoid cyst but can be confirmed with MRI outside of the acute setting. No acute intracranial abnormality, including hemorrhage identified. No calvarial or skull base fracture. Bones, paranasal sinuses and orbits unremarkable.


Right frontal lobe extra-axial lesion measures 50 x 16 x 44 mm (AP x RL x CC) and demonstrates CSF signal on all sequences with no evidence of diffusion restriction. The right frontal bone is remodeled with thinning of the cortex. Mass effect present with effacement of adjacent sulci but no evidence of midline shift or hydrocephalus.

Conclusion: Right frontal extra-axial lesion demonstrates features characteristic of an arachnoid cyst.

The patient was managed conservatively, and has remained under radiological surveillance since her vehicular accident. Her images below are taken from her two-year follow-up. 


Comparison is made with previous studies. An extra-axial lesion is demonstrated overlying the right frontal lobe. It follows CSF on all sequences and has maximum measurements of 45 mm (AP) x 15mm (trans) x 41 mm (SI), stable. No evidence of diffusion restriction. Remodeled bone overlying the lesion again noted. Localized mass-effect with effacement of the underlying cerebral sulci. No effacement of the ventricular system or evidence of midline shift. No abnormal susceptibility.

Conclusion: Stable findings with features consistent with a right frontal arachnoid cyst.

Case Discussion

This case illustrates the typical radiological features of an cerebral arachnoid cyst which are benign, congenital, intra-arachnoidal space-occupying lesions that contain a clear, colorless fluid identical to CSF 1,2. They are thought to arise during fetal development of the subarachnoid space by rupture of the webbed arachnoid or by splitting of the arachnoid membrane during its delamination from the overlying dura 2,3. Arachnoid cysts represent about 1% of intracranial mass lesions 1-4.

There is strong predilection for intracranial arachnoid cysts in the middle cranial fossa (anterior to the temporal lobes), followed by frontal convexity, posterior fossa and the suprasellar region 2,3. Less common locations are within the interhemispheric fissure, over the cerebral convexity, the choroidal fissure, cisterna magna, quadrigeminal cistern, and the vermian fissures 1,4.

Imaging characteristics on CT include 1:

  • sharply demarcated extra-axial cyst that can displace or deform adjacent brain matter
  • scalloping of the adjacent calvaria
  • no identifiable internal architecture
  • no contrast enhancement

MRI characteristic of an arachnoid cyst include 1:

  • the cyst typically has the same signal intensity as CSF on all sequences
  • displacement of surrounding structures
  • no contrast enhancement

Case courtesy of Dr. Frank Gaillard

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Case information

rID: 34453
Published: 22nd Feb 2015
Last edited: 14th Aug 2019
Inclusion in quiz mode: Included

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