Large arachnoid cyst. Coloured magnetic resonance imaging (MRI) scan of a parasagittal section of the head of a 25-year-old patient with a large arachnoid cyst (left, dark grey). This cyst is present in the Sylvian fissure of the left cerebral hemisphere of the patient's brain. It is an abnormal collection of cerebrospinal fluid (CSF), which is likely to have been caused by a split of the arachnoid membrane (a layer of tissue that surrounds and protects the brain). Despite being benign (non-cancerous), the size and location of arachnoid cysts can cause many symptoms - the Sylvian fissure is associated with language and speech, so this cyst may cause speech disturbances.