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Ventilation-perfusion (V/Q) scan
A pulmonary embolus blocks blood flow but not air, so its sign is a mismatch: a segmental perfusion defect in lung that is normally ventilated — and a normal perfusion scan excludes embolism.
Why it matters in the exam
The exam asks when to choose V/Q over CTPA: pregnancy (with a normal chest X-ray, sometimes perfusion only at reduced activity), contrast allergy, renal failure, and young women in whom breast radiation should be reduced. A second question: pulmonary hypertension — V/Q is the preferred screening test for CTEPH, because it is more sensitive than CTPA, and a normal scan excludes CTEPH. A patient whose breathlessness persists after 3 months of anticoagulation also needs V/Q, to look for defects that did not resolve and to refer in time to a pulmonary hypertension centre.
Technique: perfusion with Tc-99m MAA — particles of 10–90 micrometres that lodge in a small fraction of arterioles; in adults about 200,000–700,000 particles are injected. In pulmonary hypertension, right-to-left shunt, after pneumonectomy and in children the particle number is reduced (about 100,000–200,000). With a right-to-left shunt, uptake is seen in the brain and kidneys. Ventilation: Technegas, DTPA aerosol, Kr-81m or Xe-133. SPECT is more sensitive than planar imaging.
Interpretation under EANM 2019: embolism — a mismatch of at least one segment or two subsegments shaped to the anatomy; no embolism — normal perfusion, a match, or a mismatch that is not segmental in shape. The "stripe sign" — preserved perfusion at the periphery around the defect — argues against embolism. The older criteria (PIOPED) gave probability categories. V/Q is also used to calculate predicted lung function after resection: predicted FEV1 = preoperative FEV1 × (1 minus the perfusion fraction of the resected area).
Key points
- Segmental mismatch — embolism.
- Normal perfusion excludes embolism.
- V/Q — the screening test for CTEPH.
- Pregnancy, contrast allergy, renal failure — V/Q.
- Pulmonary hypertension and shunt — fewer MAA particles.
Summary table
| Pattern | Meaning | Note |
|---|---|---|
| Normal perfusion | Excludes embolism | The most important fact |
| Segmental mismatch | Embolism | At least one segment or 2 subsegments |
| Match (defect in both) | Lung disease, not embolism | COPD, pneumonia |
| Stripe sign | Not embolism | Peripheral perfusion preserved |
| Brain and kidney uptake | Right-to-left shunt | Reduce particles |
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Sources
- EANM Guidelines for Ventilation/Perfusion Scintigraphy, 2019 — Technique, SPECT and interpretation criteria
- ESC Guidelines for the Diagnosis and Management of Acute Pulmonary Embolism, 2019 — Diagnostic strategies including pregnancy
- ESC/ERS Guidelines for the Diagnosis and Treatment of Pulmonary Hypertension, 2022 — Screening for CTEPH
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.
See also: Pulmonary embolism