V/Q Scan & Pulmonary Embolism⁹⁹ᵐTc-MAA (Q) · ⁹⁹ᵐTc-Technegas / ¹³³Xe (V)

Quick Answer

The ventilation–perfusion (V/Q) scan diagnoses pulmonary embolism by finding perfusion defects (Tc-99m-MAA) in regions that ventilate normally — a mismatch. It is the preferred alternative to CT pulmonary angiography when CT is contraindicated (contrast allergy, renal impairment, pregnancy/young women, to reduce breast dose). Quantitative perfusion also predicts postoperative lung function before resection.

Clinical Importance

V/Q remains valuable precisely where CTPA is least desirable, and SPECT V/Q has improved accuracy and reduced non-diagnostic studies compared with older planar techniques. In pregnancy and in young women, the favorable radiation profile is a specific advantage.

Technique

• Perfusion (Q): IV Tc-99m-MAA microemboli lodge in pulmonary capillaries proportional to blood flow.
• Ventilation (V): inhaled Tc-99m-Technegas (or aerosol/DTPA, or Xe-133) maps regional ventilation.
• Compare the two: PE produces perfusion defects with preserved ventilation (mismatch).

Interpretation frameworks

• PIOPED / modified PIOPED II: probability-based (normal, low, intermediate, high) using mismatch pattern and segment size.
• PISAPED / perfusion-only criteria: a perfusion scan with chest radiograph, read as PE-present/absent.
• EANM favors SPECT V/Q with a mismatch-based binary read and a reduced non-diagnostic rate.

The hallmark of PE is segmental or larger mismatched perfusion defects; matched defects (perfusion and ventilation both abnormal) suggest parenchymal disease instead.

Mismatch means PE

The V/Q scan diagnoses pulmonary embolism by finding perfusion defects (Tc-99m-MAA) in normally-ventilated lung — a mismatch; matched defects (both abnormal) point to parenchymal disease instead. It is the preferred alternative when CTPA is contraindicated — contrast allergy, renal impairment, and especially pregnancy/young women (lower breast dose). A normal perfusion scan effectively excludes PE.

Reading frameworks and the shunt/labeling traps

Several frameworks coexist: PIOPED / modified PIOPED II (probability bands — normal/low/intermediate/high by mismatch pattern and segment size), PISAPED / perfusion-only (perfusion + chest radiograph read as PE present/absent), and EANM, which favors SPECT V/Q with a binary mismatch read and a lower non-diagnostic rate than planar. Technical pitfalls to recognize: poor MAA labeling/clumping creating artifactual hot spots (free pertechnetate if labeling fails), and right-to-left shunt sending MAA to the brain/kidneys (systemic activity) — a finding, not an artifact. Quantitative perfusion also predicts postoperative lung function before resection.

High-Yield Pearls

• V/Q is the go-to when CTPA is contraindicated — notably pregnancy and young women (lower breast dose).
• Mismatch = PE pattern; match = parenchymal disease (COPD, pneumonia, effusion).
• SPECT V/Q lowers the non-diagnostic rate versus planar.

Common Pitfalls

• A normal perfusion scan effectively excludes PE — but matched defects and intermediate-probability planar reads cause overcalling.
• Poor MAA labeling/clumping producing artifactual hot spots; free pertechnetate if labeling fails.
• Right-to-left shunt causing systemic (brain/kidney) MAA activity.

Related Pages

• Perfusion agent: Tc-99m-MAA; ventilation agents: Technegas/aerosol/Xenon; pitfalls: Pearls, pitfalls & normal variants.

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