Arachnoid cyst CT
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American Roentgen Ray Society Images of Arachnoid cyst CT |
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
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Overview
CT
Arachnoid cysts are cystic extra-axial lesions of the brain. On CT scans they are observed with a similar density to CSF, with sharp nonenhancing borders. The main differential diagnosis is the epidermoid cyst which present with irregular borders, while the arachnoid cyst is sharp but smooth. While the CT can is useful, an MRI is more precise on establishing its origin and location.
The majority of these cysts (50-60%) are found in the floor of the middle cranial fossa, while one-fourth to one-third of occur in the posterior fossa, particularly in the retrocerebellar, cerebellopontine, and quadrigeminal plate cisterns. They may also occur in the spinal cord.
On CT scans, they are seen as a structure isodense to CSF, with sharp nonenhancing borders. The walls of an arachnoid cyst are smooth on CT, as opposed to those of an epidermoid cyst, which are most often irregular. MRI can demonstrate the exact location, extent, and relationship of the cyst, as well as differentiate arachnoid from epidermoid cysts. CSF signal is seen within the well-defined cyst on all sequences. Epidermoids also may have a signal identical or very similar to that of arachnoid cyst on T1-weighted and T2-weighted images. To differentiate this from arachnoid cysts FLAIR as well as diffusion weighted sequences are used. On the FLAIR sequence, , an arachnoid cysts follow the CSF signal, so no signal seen, while there is a higher signal with FLAIR in epidermoid cysts. With DWI, diffusion is reduced within epidermoids, which then appear brighter than CSF, but for arachnoid cysts the signal intensity is low. In some instances though, arachnoid cysts may contain proteinaceous fluid or blood, and signal loss on DWIs may not be marked, which can cause diagnostic confusion. Plain radiographic findings are nonspecific and are not valuable offer in the diagnosis of arachnoid cysts, except for changes in skull contour due to large cysts. Large arachnoid cysts should be considered for serial scans because they may show progressive enlargement, and patients may become candidates for surgical consideration. Indications for surgical intervention include hydrocephalus and increased intracranial pressure. Surgery for asymptomatic cysts is controversial.