The circulation of cerebrospinal fluid (CSF) is crucial for maintaining homeostasis for the optimal functioning of the multiple complex activities of the brain and spinal cord, including the disposal of metabolic waste products of brain and spinal cord activity into the cerebral venous drainage. Throughout the brain, the arachnoid mater forms small outpouchings or diverticula that penetrate the dura mater and communicate with the dural venous sinuses. These outpuchings are called arachnoid granulations or arachnoid villi, and most are found within the dural sinuses, primarily in the transverse sinuses and superior sagittal sinus, but can occasionally be seen extending into the inner table of the calvarium.1,2
The amount of arachnoid granulations seen in bone, particularly around the superior sagittal sinus, may increase with age.2 Arachnoid granulations are generally small but the largest ones can be seen on gross examination during intracranial procedures or autopsy.3 Magnetic resonance imaging (MRI) can detect arachnoid granulations, which are characterized as T1 hypointense and T2 hyperintense (CSF isointense), well-circumscribed, small, nonenhancing masses within the dural sinuses or in the diploic space (Figure 1). Even small arachnoid granulations < 1 mm in length can be detected.2
Smaller arachnoid granulations have been described histologically as entirely covered by a dural membrane, thus creating a subdural space that separates the body of the arachnoid granulation from the lumen of the accompanying venous sinus.4 However, larger arachnoid granulations may not be completely covered by a dural membrane, thus creating a point of contact between the arachnoid granulation and the venous sinus.4 Larger arachnoid granulations are normally filled with CSF, and their signal characteristics are similar to CSF on imaging.5,6 Arachnoid granulations also often contain vessels draining into the adjacent venous sinus.5,6
When larger arachnoid granulations are present, they may permit the protrusion of herniated brain tissue. There has been an increasing number of reports of these brain herniations into arachnoid granulations (BHAGs) in the literature.7-10 While these herniations have been associated with nonspecific neurologic symptoms like tinnitus and idiopathic intracranial hypertension, their true clinical significance remains undetermined.10,11 This article presents 5 cases of BHAG, discusses their clinical presentations and image findings, and reviews the current literature.