Infection & Inflammation ImagingIn-111 / ⁹⁹ᵐTc-HMPAO WBC · ¹⁸F-FDG

Quick Answer

Two main strategies image infection. Labeled-WBC scintigraphy (In-111-oxine or Tc-99m-HMPAO leukocytes) is specific for neutrophilic infection (abscess, prosthetic-joint infection, diabetic foot) and is often paired with bone-marrow (sulfur-colloid) imaging to distinguish infection from marrow. FDG-PET/CT is highly sensitive across a broad range of infection and inflammation and excels in fever of unknown origin (FUO), large-vessel vasculitis, spinal infection, and vascular-graft/device infection. The single highest-yield concept is the WBC + marrow incongruence rule.

Definition & Scope

Infection/inflammation imaging localizes and characterizes infective and inflammatory foci that anatomic imaging cannot resolve or that require whole-body survey. The two pillars are cellular targeting (labeled leukocytes) and metabolic targeting (FDG uptake by activated inflammatory cells).

The Two Strategies

When to Use Which

Labeled WBC + Marrow Imaging — the Core Concept

Leukocytes accumulate at sites of neutrophil-mediated infection. But normal and reactive marrow also takes up labeled WBCs, so the interpretation hinges on a paired Tc-99m-sulfur-colloid marrow scan:

• Congruent WBC + marrow uptake (same distribution) → marrow, not infection.
• Spatially incongruent uptake — WBC-positive, marrow-negative → infection.

This dual-tracer maneuver is what makes periprosthetic and complex osteomyelitis reads reliable, where marrow reconversion/displacement otherwise mimics infection.

FDG-PET/CT Applications

FDG accumulates in activated neutrophils, macrophages, and lymphocytes. High sensitivity plus whole-body coverage make it powerful for:

• Fever of unknown origin: a single survey often localizes an occult focus or an alternative diagnosis (malignancy, vasculitis).
• Large-vessel vasculitis: smooth, circumferential arterial-wall uptake (GCA/Takayasu); image before/early in steroid therapy (steroids suppress uptake).
• Spondylodiscitis and vascular-graft/prosthesis and CIED/device infection (with attention to post-implant physiologic uptake windows).
• Sarcoidosis activity and (with dietary prep) cardiac sarcoid.

Radiopharmaceutical Uptake Mechanisms

Labeled WBCs migrate to chemotactic infection foci (neutrophil-dependent — hence weaker for chronic/lymphocytic or vertebral infection). Tc-99m-HMPAO WBCs give better image quality and lower dose than In-111, but have urinary/biliary/bowel excretion complicating the abdomen (In-111 has none). FDG reflects glucose metabolism of activated inflammatory cells. Ga-67 (legacy) binds transferrin/lactoferrin.

Tracer Comparison

Reporting Checklist

• For periprosthetic/bone questions, always interpret WBC against the marrow scan (congruent = marrow; incongruent = infection).
• State timing relative to surgery/instrumentation/steroids (all confound uptake).
• For vasculitis, describe distribution and grade wall uptake; note steroid status.
• Address physiologic bowel/muscle/brown-fat FDG and WBC excretion patterns.

Common Pitfalls

• Reading WBC uptake without marrow imaging in periprosthetic questions — marrow mimics infection.
• Post-surgical/inflammatory FDG uptake mistaken for infection (respect timing windows).
• Steroids suppressing vasculitis uptake — image before/early.
• Physiologic bowel, muscle, brown-fat FDG confounding FUO reads; HMPAO-WBC GU/bowel excretion in the abdomen.

Board Pearls

The single highest-yield concept: because marrow also takes up labeled leukocytes, pair the WBC scan with a sulfur-colloid marrow scan — congruent WBC + marrow uptake = marrow (no infection); spatially incongruent (WBC-positive, marrow-negative) = infection. This is the crux of prosthetic-joint and complex-osteomyelitis reads.

Labeled WBC (± marrow) is specific for neutrophilic infection (prosthetic joint, diabetic foot, complex osteomyelitis); FDG-PET/CT is the whole-body workhorse for FUO, large-vessel vasculitis, spinal infection, and graft/device infection — more sensitive, faster, higher-resolution.

Practical cautions: post-surgical/instrumentation inflammation causes FDG false positives (timing matters), and steroids suppress vasculitis uptake (image before/early). Tc-99m-HMPAO WBC gives better images and lower dose than In-111 but has urinary/biliary/bowel excretion to account for in the abdomen (In-111 has none). Ga-67 persists as a legacy fallback (spinal osteomyelitis, sarcoid activity).

Related Pages

• Tracer: Gallium-67 (legacy roles); FDG.
• Related: Cardiac sarcoidosis (suppression-prep FDG); pitfalls: Pearls, pitfalls & normal variants.

Figure / Diagram Suggestions

• The WBC + marrow congruence/incongruence decision plate (periprosthetic infection).
• A strategy selector flow (labeled WBC vs FDG by clinical question).
• Large-vessel vasculitis FDG pattern (circumferential wall uptake) with the steroid caveat.

Self-Check (Board-Style)