Dual rim sign (brain abscess)

Last revised by Frank Gaillard on 11 May 2023

The dual or double rim sign is seen on MRI in approximately 75% of cerebral abscesses and is helpful in distinguishing an abscess from a glioblastoma

On both susceptibility weighted imaging (SWI) and T2WI it consists of two concentric rims surrounding the abscess cavity, outer one of which is hypointense, and the inner one relatively more hyperintense.

Pathology

The low-signal intensity outer ring coregisters to the enhancing inner edge on T1 weighted scans and corresponds to a fibrocollagenous capsule. The low signal is believed to be due to the production of paramagnetic free radical produced by macrophage 2.

The inner high-signal ring coregisters deep to the enhancing capsule. Microscopically this inner region, between the capsule and the necrotic center, is composed of granulation tissue containing fibroblasts, large foamy histiocytes, and inflammatory cells 1.

Radiographic features

MRI

Cerebral abscesses tend to have low signal rims (capsule) seen best on SWI but also visible on T2/FLAIR. These are typically smooth (90%) and complete (75%).

In glioblastomas, the low signal rims (thought to represent hemorrhagic products at the outer region of the necrotic core) are irregular (85%) and incomplete (85%) 1

In cerebral abscesses on SWI, immediately internal to the low-intensity rim, is a high-intensity line (granulation tissue). This is known as the dual rim sign 1

On contrast enhanced sequences, the ring enhancement corresponds to the T2 hypointense capsule in abscess, whereas it occurs peripheral to the T2 hypointense rim in glioblastoma. 

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