The majority of hemorrhagic arachnoid cysts are correlated with trauma or aneurysmal rupture. We report on a 35-year-old woman with acute headache and an intracranial mass lesion that was hyperdense in CT and hyperintense in conventional MRI images. The history and imaging were negative for trauma or vascular anomaly. Diffusion-weighted imaging (DWI) clearly demonstrated a large right anterior and middle cranial fossa arachnoid cyst with associated subacute intracystic hemorrhage without subdural hematoma. DWI offers invaluable information for the differential diagnosis of hemorrhagic arachnoid cysts from other extra-axial space-occupying lesions.
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