Arachnoid cyst. Magnetic resonance imaging (MRI) scan of an axial (horizontal) section through the brain of a 28 year old patient. The front of the head is at top. The scan shows a large arachnoid cyst (at left, black) in the right Sylvian fissure of the brain. This cyst is an abnormal collection of cerebral spinal fluid (CSF), which is encased in arachnoidal brain cells and collagen. It is benign but can cause a variety of symptoms, including seizures and vomiting. The Sylvian fissure is associated with language and speech, so the cyst may cause speech disturbances. Treatment is by surgically removing the cyst.