V/Q scan principles

V/Q Scan Principles - The Big Picture

• Core Principle: Compares regional airflow (Ventilation, V) to blood flow (Perfusion, Q) in the lungs.
  • Ventilation (V): Patient inhales a radiotracer (e.g., Xenon-133 gas, Technetium-99m DTPA aerosol).
  • Perfusion (Q): Patient receives an IV injection of Technetium-99m MAA, which lodges in pulmonary capillaries.
• Primary Indication: Diagnosing Pulmonary Embolism (PE) when CT Pulmonary Angiography (CTPA) is contraindicated (e.g., contrast allergy, severe renal insufficiency, pregnancy).

⭐ A "high probability" scan, showing two or more large segmental perfusion defects without corresponding ventilation defects (V/Q mismatch), is virtually diagnostic for PE.

Ventilation Scan - Radioactive Air

• Radiotracers: Patient inhales a radioactive agent to map airflow.

  • Gas: Xenon-133 ($^{133}$Xe), a low-energy gamma emitter.
  • Aerosol: Technetium-99m Diethylenetriaminepentaacetic acid ($^{99m}$Tc-DTPA), a nebulized particle.

• Procedure:

  • Patient breathes in the radiotracer through a mouthpiece.
  • A gamma camera captures sequential images, tracking the distribution of inhaled air.
  • This reveals which parts of the lung are being ventilated.

⭐ Exam Favorite: Xenon-133 is a gas that rapidly washes out of the lungs. Delayed washout from a lung region suggests air trapping, a key sign in obstructive lung diseases like COPD.

Perfusion Scan - Blood Flow Snapshot

• Radiotracer: Intravenous injection of Technetium-99m macroaggregated albumin ($Tc-99m$ MAA).
• Mechanism of Action:
  • MAA particles (10-100 µm) are larger than pulmonary capillaries (~7-10 µm).
  • They temporarily lodge in a fraction (~0.1%) of the pre-capillary arterioles, creating a safe micro-embolization.
• What it Visualizes: A map of pulmonary blood flow. "Cold spots" (areas with no tracer) indicate regions of ↓ or absent perfusion.

⭐ Exam Favourite: A perfusion defect is non-specific. It confirms reduced blood flow, which can be from a pulmonary embolism, but also from pneumonia, COPD, or tumor compression of a vessel.

Interpretation - Spot the Mismatch

• High Probability for PE: The hallmark is a V/Q mismatch.

  • A perfusion defect is found in an area with normal ventilation.
  • This signifies that a lung segment is getting air but not blood, the classic sign of an embolus.

• Low Probability for PE:

  • Features matched V/Q defects, where both ventilation and perfusion are abnormal in the same location (e.g., pneumonia, atelectasis).
  • A completely normal scan also falls into this category and effectively rules out PE.

• Interpretation is formally guided by the PIOPED II Criteria.

⭐ V/Q scans are a key diagnostic tool for suspected PE in patients with contraindications to CT angiography, such as significant renal impairment or severe contrast allergy.

High‑Yield Points - ⚡ Biggest Takeaways

• A V/Q scan is a nuclear medicine study primarily used to diagnose pulmonary embolism (PE).
• Ventilation is assessed using an inhaled radionuclide (e.g., Xenon-133).
• Perfusion is evaluated with IV injection of Technetium-99m-labeled macroaggregated albumin (MAA).
• The hallmark of PE is a V/Q mismatch: a perfusion defect in an area with normal ventilation.
• Matched defects, where both ventilation and perfusion are low, suggest non-embolic causes like pneumonia or atelectasis.