Mechanism 12 of 16

Phagocytosis and cell labelling

Two routes to the same place. Either the tracer is a particle taken up by cells whose job is to eat particles, or you remove the patient’s own leucocytes, label them ex vivo, return them, and let them find the infection themselves.

Sulfur colloid distributes across the reticuloendothelial system in a fixed ratio — roughly 85% liver, 10% spleen, 5% marrow — and every classic finding is a departure from it. A colloid shift towards spleen and marrow means portal hypertension. Focal nodular hyperplasia contains Kupffer cells and so is normal or hot, while an adenoma and a metastasis have none and are cold.

Labelled white cells work differently: nothing about the label directs them anywhere. The cells home by chemotaxis, which is why the study is specific for neutrophilic infection and why it is unreliable in the spine and in chronic low-grade or granulomatous disease. Normal marrow uptake is the problem to be solved, and the answer is a paired sulfur colloid marrow scan — infection is white cell activity without matching colloid activity.

colloid / labelled cellphagocytosed or homedphagocyte

Two different jobs. On the left, the colloid is simply eaten — distribution follows the reticuloendothelial system, and a shift towards spleen and marrow is a portal-hypertension sign, not a tracer failure. On the right, lipophilic oxine ferries indium across the leucocyte membrane, transchelates onto cytoplasmic protein and leaves; the labelled cell then finds the infection on its own.

The agents

Tc-99m sulfur colloid—

SPECT / planar

Tc-99m · t½ 6.01 h · particles 0.1–1 µm (filtered to 0.1–0.2 µm for nodes)

Handle: Reticuloendothelial system — Kupffer cells, splenic and marrow macrophages
Trapping: Phagocytosed. Particle size decides destination: larger particles stay in the liver, smaller ones migrate through lymphatics to nodes.
Use: Liver–spleen scan, FNH versus adenoma, splenosis and accessory spleen, GI bleeding, oral gastric emptying, sentinel node, LeVeen shunt, marrow scan paired with a labelled white cell study.
Pitfall: Colloid shift to spleen and marrow indicates cirrhosis or portal hypertension. Lung uptake occurs with excess aluminium in the eluate or with severe liver disease. Heat-damaged red cells are more specific than colloid for splenic tissue.

In-111 oxine leucocytes—

SPECT / planar

In-111 · t½ 2.8 d · 171 and 245 keV γ · image at 18–24 h

Handle: The patient’s own neutrophils, labelled outside the body
Trapping: Lipophilic 8-hydroxyquinoline carries In-111 across the cell membrane, where the indium transchelates onto cytoplasmic proteins and the oxine diffuses out. The labelled cell then migrates by chemotaxis.
Use: Fever of unknown origin, osteomyelitis — especially the diabetic foot and prosthetic joints, paired with a sulfur colloid marrow scan — and inflammatory bowel disease.
Pitfall: Normal liver, spleen and marrow uptake is why the marrow subtraction study exists: infection is white cell uptake without matching colloid uptake. No bowel or renal excretion, unlike the HMPAO label. Poor in the spine, where vertebral osteomyelitis is often photopenic. Steroids, hyperglycaemia and antibiotics reduce sensitivity.

Tc-99m HMPAO leucocytes—

SPECT / planar

Tc-99m · t½ 6.01 h · image at 1–4 h

Handle: Same cells, Tc-99m label
Trapping: HMPAO carries Tc-99m into the leucocyte, where it converts to the hydrophilic form and is retained.
Use: Same indications with better counting statistics, better resolution and same-day imaging.
Pitfall: There is physiologic bowel, gallbladder and urinary activity with this label. For suspected inflammatory bowel disease, image early — activity on delayed images may be excreted rather than inflammatory.

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