THE PENUMBRA SIGN ON T1-WEIGHTED MR-IMAGING IN SUBACUTE OSTEOMYELITIS- FREQUENCY, CAUSE AND SIGNIFICANCE

Objective: We studied the frequency and cause of a feature exhibited o n T1-weighted (T1-W) magnetic resonance (MR) imaging termed the 'penum bra sign' in a series of patients presenting with osteomyelitis and co rrelated it with the double-line sign described as a T2-weighted (T2-W ) or short tau inversion recovery (STIR) feature of both the Brodie's abscess and avascular necrosis, Materials and Methods: The clinical, r adiographic, MR imaging, microbiological and histological findings in 32 patients referred to an orthopaedic oncology service, but subsequen tly proven to have osteomyelitis, were reviewed. The presence or absen ce of a rim of tissue lining an abscess cavity typified by minor signa l hyperintensity relative to the main abscess contents on T1-W MR imag ing (the 'penumbra sign') was identified, The sign was correlated with the radiographic and other findings. Results: The penumbra sign was i dentified in 24 cases (75%) and appears to be a more sensitive sign th an the corresponding double-line sign which was evident in only 29% of these on T2-W or fast STIR images, The lesions were unilocular in 11 cases (46%) and multilocular in 13 (54%), The thickness of the penumbr a ranged from 2 to 5 mm. On histological examination the tissue compri sing the penumbra sign was found to be highly vascularized granulation tissue containing thick walled arterioles. Conclusion: The penumbra s ign is characteristically seen on T1-W MR images in subacute osteomyel itis and is due to a thick layer of highly vascularized granulation ti ssue which may not be visible as the double-line sign on T2-W or fast STIR sequences. This characteristic, but not pathognomonic, MR finding supports the diagnosis of bone infection and helps to exclude the pre sence of a tumour.