Brain herniation into arachnoid granulations: an underrecognized posterior fossa finding

Purpose: To evaluate the prevalence, imaging characteristics, and associated findings of brain herniation into arachnoid granulations (BHAG) using the 3D driven equilibrium (DRIVE) sequence on a 3T MRI system. Materials and methods: This retrospective study included 695 patients who underwent temporal MRI between January and May 2024. Imaging findings were reviewed by two experienced radiologists. BHAG prevalence, locations, associated parenchymal damage (gliosis/encephalomalacia), and detectability with conventional MRI sequences were assessed. Demographic data and clinical information were also recorded. Results: BHAG was identified in 7.3% of cases (51/695) in the posterior fossa. Patients with BHAG were older than those without (p=0.003), with no gender preference (p=0.259). Cerebellar herniations were most frequent (47.1%), followed by temporal (45.1%) and occipital lobe herniations (17.6%). Parenchymal damage was observed in 25.5% of patients with BHAG. BHAG was undetectable with conventional MRI sequences in 40% of cases. Conclusion: BHAG is a common incidental finding in brain MRI and presumably not associated with any symptoms. 3D-DRIVE sequence has a higher sensitivity, particularly with the high-field imaging systems, in the detection of BHAG.