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Arachnoid cyst (posterior fossa)

Case contributed by: Dr Frank Gaillard

Presentation:

Non-specific headaches

Patient Data:

Age: Young adult
Gender: Male

MRI brain

Modality: MRI

MRI through the posterior fossa demonstrates a large right sided extra-axial CSF intensity mass lesion. It follows CSF on all sequences, including FLAIR and DWI/ADC. There is significant mass effect on the adjacent cerebellar tissue and remodelling and expansion of the adjacent skull is evident. 

High resolution T2 images (FIESTA) demonstrates that this lesion is bounded by a very thin membrane, best seen bulging across the midline towards the left. 

Sagittal images demonstrate upward bowing and thinning of the corpus callosum suggesting hydrocephalus. 

Annotated FIESTA

Modality: Annotated image

The membrane of the arachnoid cysts can be seen bulging to the left of the midline (blue arrows). Remodelling of the skull is also present (pink arrows). 

Case Discussion:

This case illustrates many of the features of arachnoid cysts, although actual visualisation of the membrane as clearly as this is not always achieved. 

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