6.4 Tc-99m CNS, Infection, and Labeled Cells

6.4 Tc-99m CNS, Infection, and Labeled Cells

Quick Answer: HMPAO/ECD → lipophilic rCBF brain agents retained after BBB crossing. HMPAO-WBC → labeled leukocytes for infection. MAA → mechanical capillary blockade in lung (watch particle count). Tilmanocept (Lymphoseek®) → CD206 mannose-receptor binding for sentinel nodes. Cell labeling and particle limits are high-stakes safety topics.

Brain Perfusion: HMPAO and ECD

Mechanism

Tc-99m exametazime (HMPAO, Ceretec®) and Tc-99m bicisate (ECD, Neurolite®) are small lipophilic molecules that cross the intact blood–brain barrier. Inside the brain they convert to more hydrophilic species and are retained in proportion to regional cerebral blood flow (rCBF) at injection. Imaging thus maps perfusion patterns—not a specific “Alzheimer tracer” chemistry like amyloid PET, though patterns can support dementia workups and other neurologic indications.

Indications and Prep

Clinical uses include stroke and other cerebrovascular perfusion patterns, dementia pattern support (e.g., temporoparietal decreases in Alzheimer-type patterns versus more frontal patterns in some frontotemporal cases—always interpret with neurology), epilepsy (ictal vs interictal injections—timing is everything), and brain-death adjunct imaging (absent intracranial uptake with proper technique and quality control). Inject IV in a quiet, low-stimulation environment; keep interaction minimal so cognitive or sensory activation does not create artifactual focal cortical “hot spots.” Keep eyes open/closed consistent with the lab protocol. Image typically 30–90 minutes after injection. Traps: injecting hours-old unstable HMPAO, patient talking or reading during uptake, and mistaking scalp or sinus activity for cortex.

Infection Imaging: HMPAO-Labeled Leukocytes

Mechanism

Autologous white blood cells labeled with Tc-99m HMPAO migrate to sites of infection/inflammation. This is a cell trafficking mechanism—not simple free-tracer leakage. In-111 oxine WBCs are an alternative (longer half-life, dual-isotope with sulfur colloid for marrow)—Domain III expects you to know Tc-99m HMPAO is a common Tc label for WBCs.

Critical Labeling Quality Steps

1. Positive patient identification and a dedicated blood-labeling workflow.
2. Adequate blood volume with proper sedimentation/separation to harvest leukocytes.
3. Correct HMPAO kit preparation and timed incubation with cells.
4. Washing steps to remove unbound technetium.
5. Assay labeling efficiency; inspect for clumping or damaged cells.
6. Reinject promptly only into the donor patient (never another patient).

Typical administered activity is often ~5–10+ mCi of labeled cells (lab-dependent). Image at about 1–4 hours and sometimes at ~24 hours depending on body region. Indications include osteomyelitis, infected prostheses (often paired with marrow mapping), abdominal abscess, inflammatory bowel disease, and fever of unknown origin. Sterile inflammation, drains, and recent surgery can false-positive; poor viability or chronic low-grade infection can false-negative. Free Tc or damaged cells distort liver, spleen, and lung patterns.

Lung Perfusion: Tc-99m MAA

Mechanism

Tc-99m macroaggregated albumin (MAA) particles (typically 10–90 µm) lodge mechanically in pulmonary arterioles/capillaries—capillary blockade. Distribution maps regional pulmonary arterial perfusion. Combined with ventilation (Xe-133, Tc-99m DTPA aerosol, or Technegas where available) for PE workup (V/Q).

Particle Limits and Safety

Right-to-left shunt: particles enter systemic circulation → brain/kidney visualization. May be diagnostic but requires reduced particle load when shunt is known/suspected.

Hot spots on perfusion images: clumped MAA or blood clots in syringe—technical, not PE.

Lymphoscintigraphy: Tilmanocept (Lymphoseek®)

Mechanism

Tc-99m tilmanocept is a CD206 mannose receptor–targeted agent. It binds RES cells in lymph nodes, designed for rapid clearance from injection site and persistent node retention—used for sentinel lymph node mapping (breast, melanoma, and other approved/protocol uses).

Unlike filtered sulfur colloid, which depends mainly on particle size and phagocytosis, tilmanocept uses specific receptor binding. Injection routes are intradermal, subcutaneous, or peritumoral/periareolar per surgical protocol—not an IV liver-spleen dose. Administered activity is in a low mCi range per package insert; map drainage with a gamma camera and localize nodes with an intraoperative probe. Traps include skin contamination mistaken for nodes, wrong injection depth, and failure to image all relevant drainage basins (for example, internal mammary or epitrochlear pathways when clinically relevant).

Master Comparison Table

PE perfusion → MAA. Osteomyelitis cells → WBC labeling QC. Sentinel receptor agent → tilmanocept. Particle limits and blood-label identity are non-negotiable safety checks.