In musculoskeletal radiology, the penumbra sign represents a rim of vascularized granulation tissue surrounding a bone abscess cavity, with a higher T1 signal intensity than the cavity itself 1. Presence of the penumbra sign can help distinguish acute osteomyelitis from other malignant and benign bone lesions.
The penumbra is isointense to muscle on T1, enhances on contrast administration, and is hypointense on T2.
The penumbra is the same as the inner ring described by Marti-Bonmati 2. They found that bone abscesses had a characteristic four-layered appearance:
a center with low T1 and high T2 signal
an inner ring with muscle signal intensity on T1 and low intensity on T2
an outer ring of fibrotic reaction with low signal intensity on T1 and T2, and
a peripheral halo of bone marrow edema with low signal intensity on T1 and variable signal intensity on T2
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