Presentation
Left knee pain and swelling.
Patient Data
Distal lateral femoral condyle metaphyseal abscess showing T1 hyperintense rim of granulation tissue (penumbra) around the low signal fluid-filled abscess cavity with surrounding marrow edema of low signal. On post-contrast series, there's an intense enhancement of the rim and the surrounding marrow edema, associated with lateral cortical erosion and an extra-osseous abscess as well as reactive synovitis evident by diffuse synovial thickening and enhancement.
CT showed a lytic lesion (Brodie's abscess)with lateral cortical defect and an extra-osseous abscess.
The first image shows the penumbra rim eliciting a high signal on non-enhanced T1 WI and intense post-contrast enhancement (yellow arrows).
The second image shows the characteristic four-layered appearance of an abscess consisting of
- a center with low T1 and high T2 signal (white arrow)
- an inner ring with muscle signal intensity on T1 and low intensity on T2 (yellow arrow)
- an outer ring of fibrotic reaction with low signal intensity on T1 and T2 (blue arrow)
- a peripheral halo of bone marrow edema with low signal intensity on T1 and variable signal intensity on T2 (red arrow)
Case Discussion
The patient had leukocytosis on CBC and elevated ESR and CRP levels. He tested negative for TB.
The patient received antibiotic treatment. The patient improved after antibiotic treatment and his pain resolved.
The penumbra sign is a thin rim of granulation tissue that lines the abscess cavity in cases of subacute osteomyelitis. It has a characteristic of a high signal on unenhanced T1 WI compared to the central hypointense fluid-filled abscess cavity and the surrounding bone marrow edema. It shows an intense post-contrast enhancement.
This rim might appear hyperintense secondary to high protein content similar to cerebral abscesses.
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