Extra-axial is a descriptive term to denote lesions that are external to the brain parenchyma, in contrast to intra-axial which describes lesions within the brain substance.
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Radiographic features
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 edema, localization 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, invasion or destruction of adjacent bone
- hyperostosis
Examples
Examples (non-exhaustive) of extra-axial lesions include:
- neoplasms
- benign masses
- hemorrhages
- vascular
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