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.
The penumbra is isointense to muscle on T1, enhances on contrast administration, and is hypointense on T2. The penumbra sign can help distinguish an 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 centre 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 oedema with low signal intensity on T1 and variable signal intensity on T2
- 1. Grey AC, Davies AM, Mangham DC et-al. The 'penumbra sign' on T1-weighted MR imaging in subacute osteomyelitis: frequency, cause and significance. Clin Radiol. 1998;53 (8): 587-92. - Pubmed citation
- 2. Martí-bonmatí L, Aparisi F, Poyatos C et-al. Brodie abscess: MR imaging appearance in 10 patients. J Magn Reson Imaging. 3 (3): 543-6. - Pubmed citation
- 3. Mcguinness B, Wilson N, Doyle AJ. The "penumbra sign" on T1-weighted MRI for differentiating musculoskeletal infection from tumour. Skeletal Radiol. 2007;36 (5): 417-21. doi:10.1007/s00256-006-0267-1 - Pubmed citation
- 4. Davies AM, Grimer R. The penumbra sign in subacute osteomyelitis. Eur Radiol. 2005;15 (6): 1268-70. doi:10.1007/s00330-004-2435-9 - Pubmed citation