Osteomyelitis

Imaging choice depends on site, chronicity, and whether bone has been “violated” by surgery, fracture or a prosthesis. In previously normal bone a positive three-phase bone scan is convincing; in violated or marrow-containing bone the bone scan loses specificity and labelled-WBC (with a marrow scan) or FDG PET/CT are preferred. FDG PET/CT excels in chronic and axial (central-skeleton) disease, where white-cell imaging is least reliable.

Clinical context & when to image

• Suspected osteomyelitis with equivocal radiographs, particularly where MRI is unavailable, contraindicated or degraded by metal.
• Chronic or recurrent osteomyelitis, where separating active infection from prior change is difficult — FDG PET/CT is the most accurate single test.
• Post-traumatic and implant-associated infection (see also Prosthetic joint infection).
• Whole-body survey for suspected multifocal disease.

Imaging strategy

• MRI is first-line at most sites for anatomical detail and marrow sensitivity.
• Three-phase bone scintigraphy (⁹⁹ᵐTc-diphosphonate): very sensitive — a normal scan largely excludes osteomyelitis in unviolated bone.
• Labelled-leucocyte imaging (⁹⁹ᵐTc-HMPAO or ¹¹¹In-oxine), with a ⁹⁹ᵐTc-colloid marrow scan when equivocal, is highly specific in the appendicular skeleton and is the preferred nuclear test after recent fracture or with metalwork.
• FDG PET/CT is the most accurate single test for chronic osteomyelitis and for the axial skeleton (where WBC is unreliable — see Spondylodiscitis).
• SPECT/CT (or PET/CT) localises uptake to bone versus soft tissue.

Protocol

• Three-phase bone scan: dynamic flow, blood-pool and delayed (~2–4 h) images.
• Labelled-WBC: autologous cells labelled and re-injected; imaging with a paired marrow scan in marrow-containing bone.
• FDG PET/CT: standard whole-body acquisition with glycaemic control.
• Add SPECT/CT for anatomical localisation.

How to read it

• Three-phase bone scan: focal uptake on all three phases suggests osteomyelitis; a normal study makes it unlikely in unviolated bone.
• Labelled WBC: focal accumulation indicates neutrophilic infection; interpret against the marrow map.
• Incongruence (WBC uptake without matching marrow) supports osteomyelitis; congruent uptake reflects marrow.
• FDG: focal cortical/medullary uptake favours active osteomyelitis.

Diagnostic performance

For chronic osteomyelitis a 2005 meta-analysis found FDG PET to be the most accurate modality (pooled sensitivity 96%, specificity 91%) versus bone scintigraphy (82%/25%), leucocyte scintigraphy (61%/77%) and MRI (84%/60%). Leucocyte scintigraphy performed well in the peripheral skeleton (84%/80%) but poorly in the axial skeleton (21%/60%). A 2019 multisociety consensus considers WBC and FDG equally accurate in peripheral bone without recent fracture or hardware.

Pitfalls

• The bone scan is sensitive but non-specific — fracture, surgery, tumour and degenerative change all take up tracer.
• Recent surgery or fracture alters marrow and confounds WBC/marrow interpretation.
• Labelled WBC is unreliable in the axial skeleton (photopenia) — use FDG there.
• Correlate FDG with CT to avoid over-calling degenerative or post-surgical uptake.