Arachnoid cyst in the SLYVIAN FISSURE. These are relatively common and benign too.will be asymptotic lesions of the arachnoid matter in the brain and spinal cord. They can be seen commonly in intracranial comparyment or arachnoid space. These cyst are usually filled with CSF. Majority are asymptomatic so when they occur usually an enlargement which produces a mass effect of other brain structures.