Introduction
Brain tumours are a common topic in neuroradiology, and in the oral exam you are expected to explain how the main tumour types look on CT and MRI, how to separate them from other lesions, and what imaging tells the clinician about surgery, complications and follow-up. Here the focus is on imaging signs and their interpretation; the physics and sequences of the modalities are covered in the imaging modalities topic.
Classification and the main tumour types
Brain tumours are classified by cell of origin (glial, meningeal, neuronal, embryonal, pituitary, metastatic) and by WHO grade from I to IV. Location and patient age are often the first clues in the radiological report. Adults usually have supratentorial gliomas and metastases; children more often have posterior fossa tumours.
Key imaging features on CT and MRI
- Non-contrast CT is the first examination in emergencies: it shows mass effect, midline shift, haemorrhage, hydrocephalus and calcification, and detects a hyperdense lesion such as a medulloblastoma or a haemorrhagic metastasis.
- MRI with and without gadolinium is the method of choice for characterising a tumour. Compare T1 and T2/FLAIR signal, look at the enhancement pattern (solid, ring, homogeneous or none), and check the perilesional oedema.
- Diffusion-weighted imaging helps: high cellularity (lymphoma, medulloblastoma, some abscesses) restricts diffusion, while most high-grade gliomas and metastases do not.
- Perfusion MRI shows increased relative cerebral blood volume in high-grade tumours and helps distinguish recurrence from radiation necrosis.
- Intra-axial versus extra-axial is the first distinction to state: intra-axial tumours arise from brain parenchyma and extra-axial tumours (meningioma, schwannoma, pituitary adenoma) from the meninges, nerves or sella.
- Peritumoural oedema is a sign of activity and malignancy in many lesions, and its extent is often greater for metastases and glioblastoma than for meningioma.