An arachnoid cyst is a benign, fluid-filled sac that forms in the arachnoid membrane, one of the three layers of tissue covering the brain and spinal cord. Most arachnoid cysts are congenital, meaning they are present from birth, and many are found incidentally during brain imaging performed for other reasons. On neonatal cranial ultrasound, they appear as well-defined, anechoic structures that do not enhance on Doppler imaging because they contain cerebrospinal fluid rather than blood. Most arachnoid cysts are small, cause no symptoms, and require no treatment, though larger ones that cause pressure on the brain may occasionally need surgical drainage.
Sonography Term