Extra axial

Extra-axial is a descriptive term to denote lesions external to the brain parenchyma, in contrast to intra-axial which describes lesions within the brain substance.

Often it is trivially easy to distinguish an intra-axial from an extra-axial mass. In many cases, especially when the mass is large and associated with parenchymal changes, such as oedema, localisation can be more difficult. A number of features are helpful in suggesting that a mass or lesion is extra-axial, including:

  subarachnoid space
      CSF cleft sign
      widening of adjacent subarachnoid space/cistern
       intervening pial arteries or veins
  brain parenchyma
      absence of a claw sign
      intervening cortex between mass and white matter
      white matter buckling sign
  bone and meninges
      

dural tail sign

erosion or invasion/destruction of bone

hyperostosis ¹⁾.