Examqo › Examqo library › Nuclear medicine › Renal, GI, brain, infection and paediatrics

Infection and inflammation imaging — FDG, labelled leucocytes and gallium

Each agent has a target cell: labelled leucocytes migrate to a focus of neutrophils (abscess, osteomyelitis), FDG is taken up by any active inflammatory cell (fever of unknown origin, vasculitis, spine), and bone scintigraphy only shows that bone is reacting.

לקריאה בעברית

Why it matters in the exam

The exam asks which study to choose for each situation. Fever of unknown origin (FUO): FDG PET/CT is the preferred study after initial work-up, and finds a diagnosis in about half of patients — infection, tumour or inflammatory disease. Large-vessel vasculitis (giant cell arteritis, Takayasu): uptake in the aorta and its branches equal to or above the liver; the study is done before steroids or at the latest about 3 days after, because they lower sensitivity quickly. In S. aureus bacteraemia PET finds secondary foci and changes treatment.

Labelled leucocytes (In-111 oxine or Tc-99m HMPAO) are the gold standard for prosthetic joint infection, together with a marrow scan (colloid): uptake on leucocytes without matching marrow uptake = infection. In the diabetic foot too they separate osteomyelitis from Charcot, combined with SPECT/CT. In the spine leucocytes often show a non-specific "cold" defect, so in spondylodiscitis FDG is preferred and MRI is the first study; the common blood culture organism is S. aureus. Tc-99m HMPAO is excreted into bowel and kidneys after a few hours, so in the abdomen In-111 is better.

Bone scintigraphy around a prosthesis: normal increased uptake persists for months to a year or more (longer in the knee than the hip), so a positive scan is non-specific, but a negative scan almost excludes infection or loosening. Gallium-67 is an old agent largely replaced by FDG. FDG also helps in vascular graft infection, cyst infection in polycystic kidney disease, sarcoidosis and the search for a fever source in patients with devices. Limitations: after surgery there is inflammatory uptake, and FDG does not separate infection from tumour.

Key points

• FUO — FDG PET/CT.
• Vasculitis — FDG before steroids or within 3 days.
• Prosthetic joint — leucocytes and marrow; incongruence is infection.
• Spine — MRI and FDG, not leucocytes.
• A negative bone scan around a prosthesis almost excludes infection.

Summary table

A free exam question

In the full version for subscribers

• A simple algorithm
• Memory aid
• Exam traps
• 5 review cards
• Enough for IMLE

The full version is open to subscribers of: Nuclear medicine, Internal medicine, Orthopedic surgery.

Subscriber? Read the full topic Plans and prices 25 free questions

Sources

• EANM/SNMMI Guideline for FDG PET/CT in Inflammation and Infection, 2013 — Indications including FUO, vasculitis and spondylodiscitis
• EANM Guidelines for the Interpretation of Labelled Leucocyte Scintigraphy, 2018 — Acquisition protocols and interpretation criteria
• Joint EANM/SNMMI/ASNC/SVS Procedural Recommendation for FDG PET/CT Imaging in Large-Vessel Vasculitis, 2018 — Timing relative to glucocorticoids and grading

Sources are given for further reading; the text is written afresh and not copied from them.

Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.