|Figure 1: Plain CT of brain- Axial section|
Meningiomas are extra-axial tumours and represent the most common tumour of the meninges. They are a non-glial neoplasm that originates from the arachnoid cap cells of the meninges. They are typically benign with a low recurrence rate but rarely can be malignant.
Typical meningiomas appear as dural based masses isointense to grey matter on both T1 and T2 weighted imaging, and demonstrate vivid contrast enhancement on both MRI and CT. There are, however, many variants some of which can vary dramatically in their imaging appearance.
Meningiomas are more common in women, with a ratio of 2:1 intracranially and 4:1 in the spine. Atypical and malignant meningiomas are slightly more common in males. They are uncommon in patients before the age of 40 and should raise suspicion of neurofibromatosis type 2(NF2) when found in young patients.
Generally follows the WHO classification for CNS tumours:
- WHO I: meningioma ~88-95 %
- WHO II:atypical meningioma (atypical, clear cell, chordoid) ~ 5-6%
- WHO III:malignant meningioma(rhabdoid, anaplastic, papillary) ~1%
- WHO IV: meningioma with sarcomatous degeneration, extremely rare
There is also a Simpson grade for meningiomas.
Meningiomas are located anywhere that meninges are found, and in some places where only rest cells are presumed to be located. Locations include:
- 85-90% supratentorial
- 45% parasagittal, convexities
- 15-20% sphenoid ridge
- 10% olfactory groove/planum sphenoidale
- 5-10% juxtasellar
- 5-10% infratentorial
- <5% miscellaneous intracranial
- intraventricular meningioma(choroid plexus)
- optic nerve meningioma
- pineal gland
- spinal: especially thoracic
- <1% "extradural"
- sinonasal cavity: most common
- intraosseous and may involve scalp
- parotid gland
Plain films no longer have a role in the diagnosis or management of meningiomas. Historically a number of features were observed, including:
- enlarged meningeal artery grooves
- hyperostosis or lytic regions
CT is often the first modality employed to investigate neurological signs or symptoms, and often is the modality which detects an incidental lesion:
- 60% slightly hyperdense to normal brain, the rest are more isodense
- 20-30% have some calcification
- 72% brightly and homogeneously contrast enhance , less frequent in malignant or cystic variants
- typical for meningiomas that abut the base of the skull
- need to distinguish reactive hyperostosis from skull vault invasion (eventually involves the outer table too)
- lytic regions: particularly in higher grade lesions
- pneumosinus dilatans
As is the case with most other intracranial pathology, MRI is the investigation of choice for the diagnosis and characterisation of meningiomas. When appearance and location is typical, the diagnosis can be made with a very high degree of certainty. In many instances however the appearances are atypical.
Meningiomas typically appear as extra-axial masses with a broad dural base. They are usually homogeneous and well circumscribed, although many variants are encountered.
Signal characteristics include:
- isointense: ~60-90%
- somewhat hypointense: 10-40% compared to grey matter
- T1 C+ (Gd): usually intense and homogeneous enhancement
- isointense: ~50%
- hyperintense: 35-40%
- usually correlates with soft textures and hypervascular tumours
- very hyperintense lesions may represent the microcystic variant
- hypointense: 10-15% compared to grey matter and usually correlates with harder texture and more fibrous and calcified contents
- DWI: atypical and malignant subtypes may show greater than expected restricted diffusion although recent work suggests that this is not useful in prospectively predicting histological grade
- MR spectroscopy: Usually it does not play a significant role in diagnosis but can help distinguish meningiomas from mimics. Features include:
- increase in alanine (1.3-1.5 ppm)
- increased glutamine/glutamate
- increased choline (Cho): cellular tumour
- absent or significantly reduced N-acetylaspartate (NAA): non-neuronal origin
- absent or significantly reduced creatine (Cr)
- MR perfusion: it has good correlation between volume transfer constant(k-tans) and histological grade
- CSF vascular cleft sign,which is not specific for meningioma, but helps establish the mass to be extra-axial; loss of this can be seen in grade II and grade III which may suggest brain parenchyma invasion
- Dural tail seen in 60-72% (note that a dural tail is also seen in other processes)
- Sunburst or spokewheel appearance of the vessels
Meningiomas typically narrow arteries which they encase. This is a useful sign to distinguish a meningioma from a pituitary macroadenoma which will not.
Oedema can be seen and correlates with size, rapid growth, location (convexity and parasagittal > elsewhere), and invasion in the case of malignant meningiomas.
- mother-in-law sign: "comes early, stays late, very dense", tumour blush
- dual blood supply from both
- pial (ICA) supplies periphery
- meningeal vessels (ECA) supplies core
- spoke wheel appearance
- dense venous filling
- preoperative embolisation: especially skull base, particles are favoured; 7-9 days prior to surgery