Cerebrospinal fluid is synthesized and produced by the choroid plexus in the lateral, third, and fourth ventricles where it circulates to the subarachnoid space and eventually returns to the venous blood via the arachnoid garnulations (AG). Histologically, AG are composed of dense collagenous connective tissue admixed with clusters of arachnoid cells and a network of delicate vascular space filled with CSF from the contiguous subarachnoid space (thus low density on CT and high T2 signal on MRI). This cell layer was continuous with the underlying arachnoid membrane.

AG are growths of arachnoid membrane into the dural sinuses which have a wide variability in the site, number, size and morphology. Occasionally, these extend partly up to the inner table of the skull as so called foveolae granulations, and even expand into the diploic space and eventually involve the outer table, mimicking malignant osteolytic lesions. It was thought that some AGs hypertrophy was in response to increasing CSF volume and pressure. The most common site for AG is at the superior sagittal sinus. They are rarely reported in the occipital and temporal bones.

Most cases are usually asymptomatic and incidental findings on imaging studies.