Arachnoid granulation

A.Prof Frank Gaillard et al.

Arachnoid granulations, also known as a Pacchionian granulation, are projections of the arachnoid membrane (villi) into the dural sinuses that allow CSF entrance from the subarachnoid space into the venous system.

They most frequently occur in a parasagittal location with the transverse and superior sagittal sinuses being the most common locations. The granulations typically occur next to the entrance of a superficial draining cortical vein into a sinus (similiar to colonic diverticuli occuring next to penetrating vessels). They increase in size and number with age and are seen in approximately two-thirds of patients.

They are most commonly encountered in radiological practice as incidental osteolytic, sharply circumscribed indolent-appearing lucencies on skull CT or X-rays, or a filling defect in dural venous sinuses, which can be mistaken for dural venous thrombosis.

The granulations are typically of CSF density and protude into the calvarium or a dural venous sinus causing a filling defect. They may simulate a dural venous sinus thrombosis but are usually easy differentiated given their round well defined shape and classic location.

Signal characteristics are generally those of CSF 2:

  • T1: low signal intensity
  • T2: high signal intensity; iso- or even slightly hyperintense to CSF
  • FLAIR: should attenuate
  • T1 C+(Gd): no enhancement

Giant granulations (>10 mm) may show atypical MRI signal characteristics, with higher T1 and T2 signal than CSF and incomplete FLAIR signal suppression 5. Rarely these may cause sinus obstruction 6.

It is named after Antonio Pacchioni, an Italian anatomist (1665-1726), who wrote extensively on the anatomy of the dura mater, published in 1705.

Neuroanatomy
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Article information

rID: 1818
Section: Anatomy
Synonyms or Alternate Spellings:
  • Pacchionian bodies
  • Pacchionian granulations
  • Pacchionian granulation
  • Arachnoid granulations
  • Pacchion
  • Arachnoid villi

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    Case 6: on left
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    Case 7
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    Case 8: a large one within the sagittal sinus
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    Case 9: giant granulation
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    Case 10: multiple pacchionian granulations
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