Single intra-axial lesions are usually primary malignant lesions
Multiple intra-axial lesions are usually metastatic
Meningiomas - the commonest extra-axial masses - are located in close proximity to a meningeal surface
Cerebral abscess is an important differential diagnosis of a ring enhancing mass
Intracranial masses are classified either as intra-axial lesions (in the brain) or extra-axial lesions (outside the brain). The distinction is not always easy to determine.
Intra-axial lesions
Intra-axial lesions are most commonly neoplastic and malignant rather than benign. In general, a single intra-axial lesion which enhances post-contrast is most likely to be a primary malignant mass (glioma), whereas multiple intra-axial lesions are usually metastatic.
Multiple lesions were seen on both sides of the brain in this patient who had a known diagnosis of lung cancer
The post-contrast image (roll over image) shows ring enhancement of the lesions
Extra-axial lesions
Meningiomas are the most common extra-axial mass. Although benign, they may grow very large and can be surrounded by a large area of adjacent cerebral oedema, often appearing to be intra-axial on initial assessment.
Characteristics of meningiomas include a smooth edge, a rounded shape, central calcification, and bright enhancement of the whole lesion post-contrast. Meningiomas arise from the meninges, with which they remain in contact, and often have a 'dural tail' which tapers from the mass to a point on the surface of the meninges.
