Specific Imaging Findings

Arachnoid granulations (AGs, Pacchionian bodies or granulations) are protrusions of subarachnoid space into the cerebral venous sinus lumen. On dedicated imaging, AGs may be found in 60–90% of individuals, most commonly adjacent and/or within the superior sagittal, transverse, and straight sinuses. The detection rate primarily depends on image quality/resolution and active search for AGs – they are present in at least 20% of modern routine head CTor MRI scans, most commonly within the lateral transverse sinuses adjacent to venous entrance sites. They may be single or multiple, oval or smoothly lobulated structures that follow the CSF density and signal intensities. Internal septa and vessels may lead to linear areas of different appearance and AGs may also contain calcifications. On post-contrast images they are well-defined, filling defects wholly or partly within a venous sinus. A majority of AGs, based on their location and size, produce smooth calvarial remodeling. On MR or CT venography, displacement, distortion, and narrowing of the sinus lumen may be seen. With high-resolution post-contrast imaging one or more veins are seen to enter AGs in almost all cases. On DWI, AGs are iso-intense to normal brain tissue, while they are CSF-like on ADC maps. Large AGs, over 1 cm in size, may lead to punched out calvarial defects from the inner into the outer table on CT images. Nonfluid signal intensity is present on at least some, most commonly FLAIR, images in the vast majority of these AGs and varies from absent/hypointense (flow-voids) to gray matter isointense (stromal tissue).