Penumbra sign | Radiology Signs

Answer:

The penumbra sign is a thin rim of relatively high T1 signal lining the inner margin of a bone abscess cavity, lying between the low-signal necrotic centre and the low-signal sclerotic reactive rim. It is a useful indicator of subacute osteomyelitis, the Brodie abscess, and it helps separate an abscess from a tumour, which is the central diagnostic difficulty in a child or young adult presenting with a lytic metaphyseal lesion, night pain and no fever. Reported specificity is high, in the region of 90 per cent or more, although sensitivity is moderate, so its absence does not exclude infection. The rest of the MRI picture supports the diagnosis: a target or halo appearance with a fluid-signal centre, a rim that enhances thickly and irregularly, surrounding marrow oedema, periosteal reaction and a tract or channel extending toward the physis or cortex. The differentials are osteoid osteoma, which has a small round nidus with intense surrounding sclerosis and a classic clinical response to non-steroidal drugs, Langerhans cell histiocytosis, eosinophilic granuloma and Ewing sarcoma, which produces a permeative pattern with a soft tissue mass. Treatment is antibiotics with surgical curettage when there is a walled-off cavity, and biopsy should still be obtained when imaging leaves any doubt.

Why is it called so?

The bright rim surrounding the darker abscess centre resembles the penumbra, the partially lit zone around the shadow of an eclipse.

Pathophysiology

In subacute osteomyelitis, the host contains the infection and walls it off with a layer of vascular granulation tissue lining the abscess cavity. This granulation tissue is highly cellular and vascular and contains proteinaceous material, haemorrhage and free radicals produced by macrophages, all of which shorten T1 and make the layer relatively bright on unenhanced T1-weighted images compared with both the pus in the centre and the sclerotic reactive bone outside it. The same layer enhances after contrast. Tumours do not form this specific vascular granulation lining, which is why the sign favours infection, though a rim of granulation tissue can occasionally occur around other necrotic lesions.

Alternative names: Penumbra sign of Brodie abscess

Other associated named signs: Target sign of the abscess on fluid-sensitive sequences and the serpiginous tract sign of a draining sinus

References

1. Grey AC, Davies AM, Mangham DC, Grimer RJ, Ritchie DA. The ‘penumbra sign’ on T1-weighted MR imaging in subacute osteomyelitis: frequency, cause and significance. Clin Radiol. 1998;53(8):587-92. PMID: 9744584.