Decide the compartment first (a CSF cleft, a dural base and buckled grey matter mean extra-axial), then use enhancement, diffusion and perfusion to separate glioma, metastasis, lymphoma and meningioma — and report mass effect before anything else.
Orient first
- Extra-axial masses (meningioma, schwannoma) sit outside the brain: a CSF cleft, displaced cortical vessels, grey matter buckled inward, a broad dural base.
- Intra-axial masses in adults are most often metastases (often multiple, at the grey–white junction, with oedema out of proportion) or high-grade glioma (single, infiltrative, crossing the corpus callosum).
- Primary CNS lymphoma is periventricular, markedly restricted in diffusion and homogeneously enhancing; steroids before biopsy can make it vanish.
Acquire the study
- T1 pre- and post-contrast (3D), T2, FLAIR, DWI/ADC, SWI; perfusion (DSC) when glioma versus metastasis or treatment effect is the question.
The manoeuvre
- Compartment on T2 and post-contrast T1: CSF cleft, dural tail, cortical buckling — extra-axial or intra-axial.
- Number and location: multiple lesions at the grey–white junction favour metastases.
- Enhancement pattern on post-contrast T1: ring, solid, none; thick irregular ring with central necrosis in high-grade glioma.
- DWI/ADC: very low ADC in a solid lesion suggests lymphoma (or a highly cellular tumour); a restricting ring cavity suggests abscess.
- FLAIR around the lesion and DSC perfusion: raised perfusion in the surrounding FLAIR signal favours infiltrative glioma over metastatic oedema.
- Mass effect: midline shift in mm, uncal or subfalcine herniation, hydrocephalus.
What confirms it
- A compartment and a pattern that fit one diagnosis, with the differential ranked.
What licenses you to exclude it
- A normal contrast-enhanced MRI excludes a clinically significant intracranial mass.
The classic misread
- Calling a ring-enhancing lesion a tumour without looking at DWI — an abscess restricts in the cavity.