Imaging Musculoskeletal Infection — Systematic Approach

MRI Assessment of Osteomyelitis

MRI with gadolinium contrast is the gold standard imaging modality for osteomyelitis, with sensitivity of 95% and specificity of 88%. The key MRI findings (PACES mnemonic):

Bone marrow oedema: The most sensitive but least specific finding. On T1: the normal bright fatty marrow is replaced by dark signal. On STIR/T2 fat-sat: marrow shows bright signal indicating oedema. This pattern is also seen in tumour, stress fracture, and contusion — clinical correlation is essential.

Abscess formation: On contrast-enhanced T1 fat-sat images, an abscess appears as a collection with ring enhancement (wall enhances, centre does not). This distinguishes it from phlegmon, which shows diffuse, homogeneous enhancement. Abscess identification is critical for surgical planning — abscesses typically require drainage, while phlegmon may respond to antibiotics alone.

Penumbra sign: A thin rim of T1 hyperintensity (1-2mm) immediately surrounding an intraosseous abscess (Brodie abscess). This finding is SPECIFIC for infection and helps distinguish a Brodie abscess from a bone tumour. The hyperintense rim represents granulation tissue at the interface between abscess and viable bone.

Cortical erosion and sinus tract: Cortical destruction is best assessed on contrast-enhanced images. A sinus tract appears as a linear enhancing track extending from the infected bone through soft tissues to the skin surface — this is specific for chronic osteomyelitis.

Paediatric considerations: In children under 18 months, the growth plate has transphyseal blood vessels that allow infection to cross from the metaphysis to the epiphysis, potentially causing concurrent osteomyelitis and septic arthritis. After 18 months, the growth plate acts as a barrier to infection spread.

Imaging of Prosthetic Joint Infection

PJI imaging is challenging because metalwork creates significant artefact on MRI and the distinction between aseptic loosening and infection is often unclear on radiographs. The imaging approach:

Radiographs: May show progressive osteolysis, periosteal reaction, endosteal scalloping, or hardware loosening — but these signs are NON-SPECIFIC and may be identical in aseptic loosening. Rapid progressive osteolysis within the first 2 years raises suspicion for infection.

Nuclear medicine: (1) Triple-phase bone scan: sensitive but NOT specific — positive for ANY cause of increased bone turnover (including normal post-operative remodelling for up to 12-18 months). (2) Combined labelled WBC scan + sulphur colloid scan: the most accurate nuclear medicine combination for PJI. WBC accumulation at the prosthesis WITHOUT corresponding sulphur colloid uptake (normally taken up by displaced marrow) indicates infection rather than normal marrow distribution. Sensitivity 91%, specificity 89%. (3) FDG-PET: high sensitivity (approximately 96%) but limited specificity around metalwork. Useful for ruling out chronic PJI.

MRI with metal artefact reduction: Newer sequences (MARS, SEMAC, VAT) can partially overcome metalwork artefact. MRI may show periosteal oedema, fluid collections, and synovitis, but interpretation requires expertise.

Joint aspiration: REMAINS THE GOLD STANDARD for PJI diagnosis — imaging supports but does not replace microbiological assessment. Synovial fluid analysis: WBC count, differential, culture, and increasingly alpha-defensin and leukocyte esterase strip testing.

MRI evidence supports its role as the gold standard for osteomyelitis diagnosis.

Evidence guides the complex imaging approach to musculoskeletal infection.