The penumbra sign represents a rim of vascularized granulation tissue around a bone abscess cavity that has a higher T1 signal intensity than the cavity itself 1.
It should not be confused with the ischemic penumbra of cerebral infarction.
The penumbra is isointense to muscle on T1, enhances on contrast administration, and is hypointense on T2. The penumbra sign can help distinguish acute osteomyelitis from other malignant and benign lesions.
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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