When to prefer V/Q over CTPA: renal insufficiency · contrast allergy · pregnancy (lower fetal dose) · young women (lower breast dose)
PIOPED II Interpretation Categories
| Category |
Scintigraphic Criteria |
PE Probability |
Next Step |
| Normal |
No perfusion defects; perfusion outlines normal lung borders |
<1% |
PE excluded; no further workup |
| Very Low |
Non-segmental defects only (pleural effusion, cardiomegaly, enlarged hilum, aortic impression); ≤3 small (<25% segment) matched defects with normal CXR |
~3% |
Clinical correlation; CTPA if high pre-test probability |
| Low Probability |
Small segmental perfusion defects (<25% of segment); any matched V/Q defects with/without CXR abnormality; non-segmental defects with corresponding CXR changes |
~14% |
Low pre-test prob → PE unlikely; high pre-test prob → CTPA |
| Intermediate |
Single moderate (25–75% of segment) unmatched defect without CXR abnormality; neither clearly high nor low |
~30% |
CTPA or bilateral lower extremity Doppler; do not treat on V/Q alone |
| High Probability |
≥2 large (>75% of segment) unmatched segmental perfusion defects without corresponding ventilation or CXR abnormality |
>85% |
Treat for PE; confirmatory CTPA only if anticoagulation very high-risk |
Pre-Test Probability + V/Q Result Matrix (PIOPED II)
| Pre-test Probability |
High V/Q |
Intermediate V/Q |
Low / Very Low V/Q |
Normal V/Q |
| High |
~96% |
~66% — image further |
~40% — CTPA required |
<5% |
| Intermediate |
~88% |
~30% — image further |
~16% |
<2% |
| Low |
~56% — CTPA required |
~16% |
~4% |
<1% |
Intermediate V/Q + high pre-test probability = ~66% PE probability. Do not withhold further imaging based on an intermediate scan alone — proceed to CTPA or bilateral lower extremity Doppler.
V/Q Pitfalls
| Pitfall |
Explanation |
| COPD / bullous disease |
Creates matched defects → pushed to low probability; CTPA preferred in severe obstructive disease |
| PE with infarction (Hampton's hump) |
CXR consolidation matches perfusion defect → "triple match" → falsely low probability; look for additional unmatched defects elsewhere |
| Large pleural effusion |
Compresses lung → multiple non-segmental defects → very low probability; but large effusion can also obscure underlying segmental defects |
| Prior PE |
Old defects indistinguishable from acute; baseline V/Q or prior CTPA for comparison is critical |
| MAA particle clumping |
Can create apparent focal perfusion defects; inject slowly with patient supine; use filtered preparation |
| Right-to-left shunt |
MAA particles reach systemic circulation → brain and kidney uptake; reduce dose in known shunt |
"Triple match" pattern (perfusion defect + matched ventilation defect + corresponding CXR opacity) pushes the study toward low probability — but do not stop there. Scan the rest of the lung for additional unmatched defects before finalizing the report.
Radiopharmaceuticals
| Agent |
Use |
Dose |
| Tc-99m MAA (macroaggregated albumin) |
Perfusion |
2–4 mCi |
| Tc-99m DTPA aerosol |
Ventilation |
Standard dose |
| Xe-133 gas |
Ventilation (alternative) |
Standard dose |
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