Spondylodiscitis (Spinal Infection)
Spondylodiscitis — infection of the intervertebral disc and adjacent vertebral endplates — is diagnosed primarily on MRI, but nuclear medicine is decisive when MRI is contraindicated, equivocal or degraded by spinal hardware, and for confirming treatment response. FDG PET/CT is the radionuclide test of choice, with pooled sensitivity ≈95% and specificity ≈91%. Importantly, labelled-leucocyte imaging is unreliable in the spine — vertebral infection is often “cold” (photopenic) — so white-cell studies should not be used here.
Pyogenic spondylodiscitis usually arises from haematogenous seeding, most commonly by Staphylococcus aureus; tuberculous and brucellar forms occur in endemic settings. Presentation is often indolent — back pain with variable fever and raised inflammatory markers — so imaging is central to diagnosis, to defining disease extent (epidural or paravertebral abscess), and to guiding biopsy and treatment duration.
Clinical context & when to image
- Confirming suspected spondylodiscitis when MRI is contraindicated (e.g. certain implants) or non-diagnostic.
- Resolving equivocal MRI — particularly early disease, or separating infection from degenerative (Modic) endplate change.
- Mapping disease extent and detecting occult distant foci — FDG PET/CT surveys the whole body in one study.
- Assessing response to antibiotic therapy and informing treatment duration.
- Problem-solving around spinal instrumentation, where MRI is degraded by metal artefact.
Imaging strategy
- MRI is the first-line imaging modality of choice for suspected spinal infection.
- FDG PET/CT is the preferred radionuclide test and the principal second-line study.
- Three-phase bone scintigraphy is sensitive but non-specific — now largely a screening adjunct.
- ⁶⁷Ga-citrate SPECT/CT was historically paired with the bone scan for the spine but has been largely replaced by FDG PET/CT.
- Labelled-leucocyte imaging is not recommended for diagnosing vertebral infection (see “Why labelled WBC fails”); EANM allows it only for paravertebral soft-tissue infection or very early (under 1 week) disease.
Protocol
- Standard whole-body FDG PET/CT after adequate fasting and glycaemic control.
- Cover the whole spine and, where relevant, the sacroiliac joints and any hardware.
- Correlate directly with recent MRI, and with CT for endplate destruction and hardware position.
- Dedicated SPECT/CT or diagnostic contrast-enhanced CT can add anatomical detail where needed.
How to read it
- Infection: increased FDG uptake spanning the disc and the two adjacent vertebral endplates, often with paravertebral or epidural soft-tissue uptake.
- Degenerative (Modic) change usually shows lower-grade uptake confined to the endplates, without disc-centred or soft-tissue involvement.
- Actively search for epidural and psoas/paravertebral abscess — these change management.
- On treatment, falling intensity and extent of uptake supports a good response.
Diagnostic performance
FDG PET/CT has consistently high accuracy for spinal infection. A bivariate meta-analysis (12 studies, 396 patients) reported pooled sensitivity 94.8% (95% CI 88.9–97.6) and specificity 91.4% (78.2–96.9). In a retrospective comparative series of 68 patients (FDG PET/CT sensitivity 96%, specificity 95%; MRI 67% and 84%), FDG PET/CT out-performed MRI in early disease (MRI accuracy 58% within two weeks of symptoms vs 82% thereafter).
| Modality | Sensitivity | Specificity | Role in the spine |
|---|---|---|---|
| MRI | ≈67% overall in one series (lower very early) | ≈84% | First-line; may be falsely negative in the first ~1–2 weeks |
| FDG PET/CT | ≈95% (pooled 94.8%) | ≈91% (pooled 91.4%) | Preferred radionuclide test; images through hardware; monitors response |
| Labelled WBC | Low (≈39%); cold/photopenic in ~54% | ≈98% for increased uptake; photopenia non-specific (≈52%) | Unreliable — not recommended |
| ⁶⁷Ga-citrate (± bone scan) | Moderate | Moderate | Historical; superseded by FDG |
Why labelled WBC fails in the spine
- Vertebral osteomyelitis characteristically appears photopenic (“cold”) on labelled-leucocyte imaging — reported in about 54% of cases.
- Reported sensitivity is low (around 39% when only increased activity is counted as positive).
- Proposed mechanisms include microvascular occlusion with necrosis, and a waning neutrophil response in more chronic (longer-symptom) disease.
- Because normal marrow and infection both alter white-cell distribution, cold defects are non-specific — a “cold” spine can still be infected.
Pitfalls
- Modic type 1 degenerative endplate change can be FDG-avid and mimic infection — correlate with MRI morphology.
- Recent surgery or instrumentation causes post-operative FDG uptake for a period; pattern and timing matter (EANM/SNMMI: image preferably 3–4 months after surgery).
- Vertebral fracture, tumour and inflammatory arthropathy can also be FDG-avid.
- Do not use labelled-WBC imaging to exclude spinal infection.
Evidence & guidelines
- The EANM/SNMMI joint guideline (FDG PET in infection and inflammation, v2.0) lists suspected spondylodiscitis — with or without spinal hardware (preferably 3–4 months after surgery) — among its indications, particularly when MRI is contraindicated or inconclusive.
- Meta-analytic data support high pooled sensitivity and specificity of FDG PET/CT for spinal infection, with particular value early and for treatment monitoring.
- IDSA guidance on native vertebral osteomyelitis frames MRI as the primary imaging test, with nuclear imaging where MRI is limited.