11.4 Infection Imaging Procedures
Key Takeaways
- Ga-67 citrate images infection/inflammation (and some tumors) with multi-day imaging; bowel and normal lacrimal/marrow activity are pitfalls
- Labeled WBC imaging uses In-111 oxine or Tc-99m HMPAO; choose based on chronicity, anatomy (abdomen vs extremities), and logistics
- In-111 WBC: dual photopeaks (~171/245 keV), imaging typically at 4 and 24 hours (sometimes 48 h); low normal GI activity early—better for abdominal infection
- Tc-99m HMPAO WBC: 140 keV, higher counts, earlier imaging (often 0.5–4 h, sometimes delayed); biliary/bowel excretion can confound abdominal reads
- Indications include osteomyelitis, IBD, and fever of unknown origin; antibiotics, chronic vs acute processes, and wrong-patient labeling are critical pitfalls
11.4 Infection Imaging Procedures
Quick Answer: Ga-67 for infection/inflammation (multi-day). Labeled WBCs—In-111 oxine vs Tc-99m HMPAO—for osteomyelitis, IBD, FUO. Know imaging times, normal distribution, and pitfalls (antibiotics, chronic vs acute, labeling ID).
Ga-67 for Infection
Ga-67 citrate localizes in inflammatory and infected tissue via transferrin/lactoferrin pathways and leukocyte-related mechanisms (teaching level). It remains useful when labeled WBC imaging is impractical (leukopenia, some spinal infections historically) or when protocol prefers Ga-67.
| Item | Teaching notes |
|---|---|
| Activity | Often ~5 mCi class IV (range ~3–6+ mCi per protocol) |
| Collimator / energies | Medium energy; multiple photopeaks |
| Imaging | Frequently 24–72 h (infection protocols may image earlier and later) |
| Normal activity | Liver, spleen, marrow, bowel, salivary/lacrimal, nasopharynx, genitalia |
| Strengths | Chronic infection, some opportunistic infections, discitis/osteomyelitis problem cases |
| Weaknesses | Slow, bowel confusion, less specific; largely superseded by WBC/MRI/PET in many pathways |
Tech tips: schedule multi-day returns; consider bowel prep if ordered; use SPECT/CT for spine and complex sites; document recent surgery (wound uptake).
Labeled White Blood Cell Imaging
Autologous leukocytes are separated, labeled, and reinjected so they migrate to sites of active infection/inflammation. Labeling technique (Domain III) must be flawless; procedure-day focus is indication, timing, biodistribution, and pitfalls.
In-111 Oxine WBCs vs Tc-99m HMPAO WBCs
| Feature | In-111 oxine WBC | Tc-99m HMPAO WBC |
|---|---|---|
| Label | In-111 oxine | Tc-99m exametazime (HMPAO) |
| Half-life / energy | t½ 2.8 d; ~171 & 245 keV | t½ 6 h; 140 keV |
| Typical adult activity | ~0.3–0.5 mCi class | ~5–10+ mCi class |
| Usual imaging times | ~4 h and 24 h (± 48 h) | ~30 min–4 h; optional later |
| Normal distribution | Spleen > liver > marrow; minimal early bowel/urinary | Spleen, liver, marrow; renal, bladder, biliary → bowel with time |
| Abdomen/pelvis infection | Often preferred (less physiologic GI activity early) | GI excretion can mimic or mask IBD/abscess |
| Extremities / osteomyelitis | Excellent delayed imaging | Excellent counts and resolution early |
| Same-day convenience | Multi-day | Better same-day logistics |
Both require same-patient reinjection only, aseptic handling, and assay of labeling efficiency per SOP.
Indications
| Indication | How WBC imaging helps |
|---|---|
| Osteomyelitis (appendicular skeleton, complicated hardware) | Focal WBC uptake with complementary bone marrow (sulfur colloid) imaging to separate marrow expansion from infection |
| Inflammatory bowel disease | Active bowel segments accumulate labeled WBCs (In-111 often preferred for abdomen) |
| Fever of unknown origin / occult infection | Survey for abscess or focal pyogenic infection |
| Vascular graft / soft-tissue infection | Focal uptake along graft or collection |
| Prosthetic joint infection workups | Part of multi-modality algorithms |
Bone marrow pairing: In the axial skeleton and around prostheses, Tc-99m sulfur colloid marrow scan plus WBC scan distinguishes discordant infection (WBC+/marrow−) from congruent marrow packing.
Imaging Workflow (Tech)
- Confirm CBC/WBC count adequacy for labeling; verify two patient identifiers on blood and dose.
- Coordinate pharmacy/lab pickup and reinjection timing—cell viability falls with delay.
- Inject labeled cells IV; flush well; never administer through filters that trap cells unless protocol says otherwise.
- Image at protocol times with correct collimator (ME for In-111; LEHR for Tc-99m).
- Include whole-body or limited FOV as ordered; add SPECT/CT for complex anatomy.
- For osteomyelitis near marrow-rich bone, schedule/compare marrow imaging when ordered.
Normal Distribution Differences — Exam Favorites
| Observation | Likely explanation |
|---|---|
| Hot spleen on both agents | Normal (splenic sequestration of cells) |
| Early renal/bladder on HMPAO-WBC | Expected secondary excretion pathway |
| Progressive bowel on delayed HMPAO-WBC | Physiologic biliary/GI excretion—not automatically IBD |
| Focal bowel on In-111 WBC at 24 h without migration pattern control | Suspect true inflammation—physician interprets |
| Lung uptake early after reinjection | Can reflect cell activation/damage during labeling |
Pitfalls
Antibiotics
Ongoing antibiotic therapy may decrease sensitivity by reducing leukocyte recruitment and organism load. Document antibiotic type and duration; do not cancel unilaterally—flag for the interpreting physician. Some protocols prefer imaging before prolonged therapy when clinically safe.
Chronic vs Acute Infection
Labeled WBCs excel in acute pyogenic processes rich in neutrophils. Chronic low-grade infection, some fungal/TB processes, and vertebral osteomyelitis may be falsely negative or better suited to MRI, Ga-67, or FDG depending on pathway. Exam stems that stress “chronic nonpyogenic” often point away from classic WBC positivity.
Other Traps
| Pitfall | Result |
|---|---|
| Wrong-patient cells | Never-event hemolytic/infectious risk—hard stop |
| Delayed reinjection | Poor viability → low sensitivity, abnormal lung uptake |
| Recent surgery | Nonspecific wound uptake |
| Steroids / immunosuppression | Altered migration |
| Ignoring marrow expansion | False + osteomyelitis near prosthesis without colloid comparison |
| Reading HMPAO bowel as abscess without timing | False positive abdomen |
Quick Selection Guide
| Clinical scenario | Often preferred infection tracer (teaching) |
|---|---|
| Abdominal abscess / IBD | In-111 WBC |
| Extremity osteomyelitis, need high resolution same day | Tc-99m HMPAO WBC |
| Leukopenic patient / selected chronic or spinal cases | Ga-67 (or other modality per protocol) |
| FUO survey | WBC or Ga-67 per institutional pathway |
Master In-111 vs HMPAO biodistribution and timing, plus antibiotic and chronic-disease sensitivity limits—those distinctions dominate CNMT infection procedure items.
Scheduling Notes for the Technologist
Infection studies are multi-step: blood draw, labeling time, reinjection, and delayed imaging returns. Explain the full timeline at booking so patients do not leave after injection thinking the study is finished. Coordinate Ga-67 multi-day visits and WBC same-day vs next-day imaging with staffing and camera energy windows. Always reconcile the ordered indication with the agent on hand before venipuncture—switching from HMPAO-WBC to In-111 mid-workup requires a new labeling plan, not a simple window change.
Compared with Tc-99m HMPAO-labeled leukocytes, In-111 oxine-labeled leukocytes are generally preferred for which situation?
Typical imaging times after reinjection of In-111 labeled WBCs for infection are closest to which schedule?
Which statement about pitfalls in labeled leukocyte infection imaging is most accurate?