Annotated image
This annotated T2 axial image through a tuberculoma nicely demonstrates a dual rim sign.
The outer low-signal rim is believed to be due to paramagnetic free radical production by macrophages in the abscess capsule whereas the high-signal inner rim, deep to the enhancing capsule, correlates microscopically with granulation tissue.
When seen this clearly and continuously surrounding a centrally necrotic lesion, it is strongly suggestive of an abscess rather than a tumor. In this instance the presence of low T2 signal central caseating necrotic material allows the high inner rim to be seen very well.
Case courtesy of Ammar Ashraf, Radiopaedia.org, rID: 79483