Pulmonary Nuclear Medicine

Pulmonary Radiopharmaceuticals - Glowing Guides

• Perfusion Agents:
  • Tc-99m MAA (Macroaggregated Albumin)
    • Mechanism: Capillary blockade (microembolization) in pulmonary arterioles.
    • Particle size: 10-90 μm (ideal 20-50 μm for even distribution).
    • No. of particles: 200k-700k (adults). ↓ for PHT, R-L shunt (to 100k-200k), peds, pregnancy.
    • Contraindications (relative): Severe PHT, R-L shunt.
    • T½_bio: 2-8 hrs (enzymatic breakdown, RES clearance).

⭐ Tc-99m MAA: Particle size (10-90 μm) and number (200,000-700,000 for adults) are critical for safety and efficacy, preventing large emboli and ensuring proper distribution.

• Ventilation Agents:
  • Gases:
    • $^{ ext{133}}$Xe: 81 keV (low E, poor images), T½ 5.2d. 3 phases (breath, equil, washout). Fat trapping (liver), requires negative pressure room.
    • $^{ ext{81m}}$Kr: 190 keV (good E), T½ 13s (ultra-short). From $^{ ext{81}}$Rb/$^{ ext{81m}}$Kr generator. Continuous flow, matches perfusion timing.
  • Aerosols:
    • Tc-99m DTPA: Nebulized (0.5-2 μm for alveolar deposition). Airway deposition. Risk of central clumping, patient cooperation needed.
    • Tc-99m Technegas: Ultrafine carbon particles (<0.2 μm). Gas-like, superior alveolar penetration. Specialized generator.

V/Q Scintigraphy - Breath vs Blood

Compares alveolar air entry (Ventilation) with blood flow (Perfusion) to detect pulmonary embolism (PE).

• Ventilation (V) Scan:
  • Radiopharmaceutical: $^{99m}$Tc-Technegas or $^{99m}$Tc-DTPA aerosol.
  • Method: Inhalation.
  • Assesses: Alveolar air entry.
• Perfusion (Q) Scan:
  • Radiopharmaceutical: $^{99m}$Tc-MAA (Macroaggregated Albumin).
  • Method: IV injection; particles lodge in capillaries.
  • Assesses: Pulmonary arterial perfusion.

Key Interpretation Patterns:

• Normal: Homogeneous ventilation & perfusion.

  ⭐ A normal perfusion scan effectively rules out acute PE (NPV >99%).

• Mismatch (V normal, Q ↓): Suggests PE. Classic: wedge-shaped, pleural-based.
• Match (V ↓, Q ↓): Non-embolic (COPD, pneumonia, atelectasis).
• Stripe Sign: Perfusion defect with preserved peripheral perfusion. Low PE probability.

PIOPED Criteria (Simplified PE Probability):

• 📌 Mnemonic: "Two large mismatches point to High PE chance."
• High Probability: ≥ 2 large (≥75% of a segment) V/Q mismatches.
• Low Probability: Matched defects, stripe sign, < 1 moderate V/Q mismatch.
• Intermediate: Not high or low.

Pulmonary PET & Other Scans - Hotspot Hunters

• $^{18}$F-FDG PET/CT (Fluorodeoxyglucose):

  • Mechanism: Glucose analog; accumulates in cells with high glucose metabolism (malignancy, inflammation, infection).
  • Key Pulmonary Uses:
    • Oncology: NSCLC staging (nodal/distant mets), SPN characterization (SUVmax >2.5 suggests malignancy), therapy response, recurrence detection.
    • Inflammation/Infection: Identifies active processes; less specific for infection/inflammation.

• $^{67}$Ga-Citrate (Gallium) Scan:

  • Mechanism: Iron analog; binds transferrin. Accumulates in inflammation, infection, some tumors (e.g., lymphoma).
  • Key Pulmonary Uses:
    • Sarcoidosis: Assessing disease activity. 📌 Panda sign (lacrimal, parotid, salivary glands) & Lambda sign (bilateral hilar, R paratracheal LNs).
    • Interstitial Lung Diseases (ILDs): Monitoring activity.
    • Pneumocystis jirovecii pneumonia (PJP) in HIV: Diffuse bilateral uptake.

⭐ FDG PET/CT is superior for NSCLC staging and crucial for characterizing indeterminate SPNs.

High‑Yield Points - ⚡ Biggest Takeaways

• V/Q scan: Primary for PE with contrast allergy, pregnancy, or renal failure.
• High probability PE (V/Q): ≥2 large mismatched segmental perfusion defects.
• Perfusion: ^99mTc-MAA. Ventilation: ^99mTc-DTPA aerosol, ¹³³Xe, ^81mKr gas.
• Stripe sign: Preserved peripheral perfusion around defect, indicates low PE probability.
• Gallium-67 (⁶⁷Ga) scan: Panda sign & Lambda sign characteristic for sarcoidosis.
• ¹⁸F-FDG PET-CT: Essential for lung cancer staging & SPN (Solitary Pulmonary Nodule) assessment.