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Imaging of brain tumours

First decide whether the tumour is intra-axial or extra-axial, then use age, location, enhancement, diffusion and perfusion — meningioma with a dural base, metastases at the grey–white junction, glioblastoma with a ring and necrosis, lymphoma with restricted diffusion, and posterior fossa tumours in children.

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Why it matters in the exam

Signs of an extra-axial tumour: a broad dural base, a "dural tail", fluid or vessels between tumour and brain (CSF cleft) and inward displacement of the cortex. Meningioma is the commonest extra-axial tumour in adults: uniform enhancement, calcifications and hyperostosis of the bone. Vestibular schwannoma extends into the internal auditory canal ("ice-cream cone"); bilateral in NF2. Intra-axial tumours in adults: metastases — multiple, at the grey–white junction, with large oedema; glioblastoma — a thick irregular ring with necrosis, sometimes crossing the corpus callosum ("butterfly"); primary lymphoma — periventricular, uniform enhancement, hyperdense on CT and restricted diffusion because of high cellularity; steroids before biopsy may make it disappear.

Under WHO 2021 gliomas are also classified by mutation: IDH-mutant astrocytoma, oligodendroglioma (IDH mutation and 1p/19q codeletion) — usually frontal and calcified — and IDH-wild-type glioblastoma. The T2-FLAIR mismatch sign (high T2 signal that suppresses on FLAIR except at the rim) points to IDH-mutant astrocytoma without 1p/19q codeletion. Perfusion: rCBV is high in high-grade tumour and tumour recurrence, and low in radiation necrosis. Spectroscopy: choline rises and NAA falls in tumour; lipid–lactate in necrosis. After anti-angiogenic treatment enhancement falls even without true response (pseudoresponse); after chemoradiation enhancement may rise temporarily (pseudoprogression).

In children most tumours are in the posterior fossa: pilocytic astrocytoma — a cerebellar cyst with an enhancing mural nodule; medulloblastoma — a midline (vermian) mass, hyperdense on CT, with restricted diffusion and a tendency to spread through CSF — hence MRI of the whole spine; ependymoma — in the fourth ventricle, "squeezing" through the foramina, with calcifications. Craniopharyngioma in a child — a cystic, calcified suprasellar mass. In the pituitary: microadenoma below 10 mm, seen as a less-enhancing area on a dynamic study; macroadenoma 10 mm or more. Ring-enhancing lesions: abscess (thin smooth ring with central restricted diffusion), glioblastoma, metastasis and demyelination (open ring). Epidermoid cyst — restricted diffusion and FLAIR that does not suppress, unlike an arachnoid cyst.

Key points

• Extra-axial: dural base, CSF cleft — meningioma.
• Metastases at the grey–white junction.
• Glioblastoma — ring and necrosis; lymphoma — restricted diffusion.
• High perfusion — tumour; low — radiation necrosis.
• Children: pilocytic, medulloblastoma, ependymoma.

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Sources

• WHO Classification of Tumours of the Central Nervous System, 5th edition (2021) — Gliomas, glioneuronal and neuronal tumours; Embryonal tumours
• RANO Criteria for Response Assessment in Neuro-Oncology (RANO 2.0), 2023 — Pseudoprogression and pseudoresponse
• ACR Appropriateness Criteria: Brain Tumors (Workup and Follow-up), 2019 — MRI with contrast, perfusion and spectroscopy

Sources are given for further reading; the text is written afresh and not copied from them.

Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.

See also: Gliomas and glioblastoma in adults