Abstract
Arachnoid cysts are benign intra‐arachnoid collections of cerebrospinal fluid and comprise around 1% of the intracranial masses. Unless complicated with hemorrhage, they are similar to cerebrospinal fluid in signal intensity in most cases. Diffusion‐weighted magnetic resonance imaging (MRI) reveals that they have no water restriction and distinguishes them from epidermoid cysts, which show water restriction. Arachnoid cysts of the craniospinal junction are rare lesions, with only seven cases reported in the literature. Imaging findings of all craniospinal arachnoid cysts reveal a large posterior fossa arachnoid cyst extending through the foramen magnum to the level of the upper spine. We present MRI findings of a 27‐year‐old female patient with a craniospinal arachnoid cyst.