Localizing Axial vs Extra-Axial Masses
Axial masses arise from the central nervous system parenchyma (brain or spinal cord), while extra-axial masses originate outside the neural tissue from meninges, skull, or other non-parenchymal structures—differentiation relies on specific imaging features including mass margins, CSF clefts, gray-white matter interface displacement, and relationship to dural surfaces.
Key Imaging Features for Differentiation
Extra-Axial Mass Characteristics
Extra-axial masses demonstrate several distinctive imaging features that allow confident diagnosis:
- CSF cleft sign: A thin rim of cerebrospinal fluid between the mass and brain parenchyma is pathognomonic for extra-axial location 1
- Buckling of gray-white matter interface: The cortical ribbon remains intact and is displaced inward by the mass rather than being infiltrated 1
- Dural tail or broad dural base: Extra-axial masses typically show continuity with the dura or tentorium 1
- Sharply defined margins: Extra-axial lesions have well-demarcated borders separating them from brain tissue 1
- Asymmetric widening of basal subarachnoid spaces: Particularly evident in posterior fossa lesions 1
- Bone changes: Hyperostosis or bone destruction at the site of dural attachment suggests extra-axial origin 1
Axial (Intra-Axial) Mass Characteristics
Axial masses show fundamentally different imaging patterns:
- Loss of gray-white matter differentiation: The mass infiltrates or replaces normal parenchyma 2
- Finger-like projections: Intra-axial masses often have irregular, infiltrative margins extending into surrounding brain tissue 2
- Vasogenic edema: Typically more extensive with intra-axial masses, though extra-axial masses can also cause edema 2
- No CSF cleft: The mass arises directly from brain parenchyma without intervening CSF 2
Common Extra-Axial Tumor Types
Meningiomas are the most common extra-axial brain tumors, representing approximately one-third of all intracranial neoplasms, and typically present as slowly growing dural-based masses 2. Other extra-axial masses include:
- Acoustic neuromas and other schwannomas 1
- Hemangiopericytomas 2
- Dural-based metastases 2
- Cholesteatomas 1
Imaging Protocol Recommendations
CT Imaging Approach
- Multiple-cut studies improve visualization of the fourth ventricle and basal cisterns, which is critical for accurate localization 1
- Evaluate for bone changes including hyperostosis or destruction 1
- Assess cisternal changes in association with fourth ventricle displacement 1
MRI Considerations
MRI provides superior soft tissue contrast for differentiating axial from extra-axial masses 2. Key sequences should evaluate:
- T1-weighted images with and without contrast to assess dural enhancement
- T2-weighted images to identify CSF clefts
- FLAIR sequences to evaluate vasogenic edema patterns
Critical Diagnostic Pitfalls
Avoid assuming all dural-based masses are benign meningiomas—atypical meningiomas, hemangiopericytomas, and dural metastases can have identical imaging appearances but require different management strategies 2.
- Some gliomas can present with primary or secondary extra-axial involvement, mimicking extra-axial tumors 2
- Benign meningiomas may be difficult to differentiate from more aggressive subtypes based on imaging alone 2
- The presence of a CSF cleft is the single most reliable feature for confirming extra-axial location 1
Special Considerations for Axial Skeleton Masses
When evaluating masses along the axial skeleton (spine), mass-like extramedullary hematopoiesis should be considered, particularly in patients with predisposing conditions 3. These lesions: