Diffusion is the discriminator: an abscess restricts CENTRALLY, a necrotic tumour does not.
Orient first
- The differential is short and the discriminators are specific: abscess, high-grade glioma, metastasis, demyelination (typically an INCOMPLETE ring), and less often tuberculoma or a resolving haematoma.
- DIFFUSION IS THE SINGLE MOST USEFUL SEQUENCE. Pus is viscous and cellular, so an abscess cavity is bright on high-b-value DWI and DARK on ADC. Necrotic tumour cavities generally do not restrict.
- An INCOMPLETE ring, open toward the cortex or the grey matter, is characteristic of demyelination and is a pattern worth recognising because it prevents a biopsy.
- The ring WALL matters: a smooth thin wall favours abscess; a thick irregular nodular wall favours tumour.
- PERFUSION separates tumour from abscess and from demyelination: high relative cerebral blood volume in the rim supports a high-grade tumour.
Acquire the study
- T1 pre- and post-gadolinium, T2, FLAIR, DWI with ADC, susceptibility-weighted imaging, and perfusion where available.
- Add spectroscopy where the abscess-versus-tumour question persists.
- Confirm which sequence you are on before judging any signal — see the MRI sequence primer.
- Ensure the post-contrast series is compared against a matched pre-contrast T1 with the same fat suppression.
The manoeuvre
- Assess the RING: thickness, regularity, and whether it is complete or open.
- Review high-b-value DWI and CONFIRM any restriction on the ADC map, reading over the CAVITY rather than the rim.
- Assess the T2 signal of the rim — a low-T2 rim is a recognised abscess feature.
- Review susceptibility imaging for blood products and for a dual-rim appearance.
- Assess surrounding vasogenic oedema and mass effect, and measure midline shift if present.
- Review perfusion and compare rim blood volume with normal white matter.
- Count the lesions and note their distribution, including whether they sit at the grey–white junction.
- Assess for leptomeningeal or ependymal enhancement.
What confirms it
- Central restricted diffusion confirmed on the ADC map, in a smooth thin-walled ring lesion, indicates an abscess.
- A thick irregular ring with elevated rim perfusion and no central restriction indicates a necrotic tumour.
What licenses you to exclude it
- ⚠️ Imaging narrows this differential; it does not settle it. Say what would — aspiration for an abscess, biopsy for a tumour — rather than issuing a single diagnosis.
- Treated abscesses and tumours both change appearance, so a study after antibiotics or steroids qualifies every conclusion.
- Absence of restriction does not exclude an early or partially treated abscess.
The classic misread
- Calling restriction on DWI alone without the ADC map — T2 shine-through is the classic false positive.
- Reading diffusion over the rim rather than the cavity.
- Missing demyelination by not recognising the incomplete ring.