V/Q Imaging in Pregnancy
Perfusion first, reduced activity, and why maternal breast dose tips the balance
Pulmonary embolism is a leading cause of maternal death in the developed world, so suspected PE in pregnancy has to be tested for. Both lung scintigraphy and CTPA are safe for the fetus. The choice turns on the chest radiograph, on the dose to the mother's breast, and on what is available at the time.
What the guidelines say
- ATS/STR: chest radiograph first (strong recommendation). If it is normal, lung scintigraphy rather than CTPA (strong). If it is abnormal, CTPA (weak). If V/Q is nondiagnostic, investigate further, with CTPA rather than catheter angiography.
- EANM: where both tests are available, V/Q is recommended. Perfusion-only imaging is the first step in the first trimester.
- Before imaging: the pregnancy-adapted YEARS algorithm (three clinical items plus D-dimer) avoided CTPA in 39% of 498 women, with one venous thrombosis (0.21%) at 3 months.
The perfusion-first protocol
- EANM: day 1, perfusion-only SPECT with 50 MBq ⁹⁹ᵐTc-MAA. A normal pattern excludes PE and the study ends.
- If perfusion is abnormal, anticoagulation can be started and ventilation added on day 2 (20–30 MBq deposited in the lung). After the first trimester a standard 1-day V/Q can be considered.
- ATS: halve the usual MAA activity and lengthen acquisition to recover counts. Hydration and frequent voiding lower fetal exposure.
- In two perfusion-only series, every completed scan that was not high probability had a 100% negative predictive value.
Pearl
Read the chest radiograph before booking. With a normal film, V/Q gave a definitive result in 94% and 96% of pregnant women in two series. With an abnormal film, 40% of V/Q studies were nondiagnostic, against 16.4% for CTPA.
Dose: the fetus and the breast
| Dose | V/Q | CTPA |
|---|---|---|
| Fetus (ATS) | 0.32–0.74 mGy | 0.03–0.66 mGy |
| Fetus (EANM, perfusion SPECT vs CTPA) | ≤0.12 mGy | ≤0.12 mGy |
| Maternal breast (ATS) | 0.98–1.07 mGy | 10–60 mGy |
| Maternal effective dose (ATS) | 1–2.5 mSv | 4–18 mSv |
- Fetal doses from both tests are similar to the 0.5–1 mGy the fetus receives from natural background over the whole pregnancy. The risk of abnormality is regarded as negligible at 50 mGy or less.
- The real difference is the maternal breast: in the ATS figures, CTPA delivers about 10 to 60 times the V/Q dose.
Nondiagnostic results
- A meta-analysis found pooled nondiagnostic rates of 14% for V/Q and 12% for CTPA, and a pooled false-negative rate of 0% for both.
- Pregnancy degrades CTPA: higher cardiac output and inflow of unopacified blood from the inferior vena cava. Nondiagnostic CTPA rates of 6–36% are reported.
- CTPA finds a clinically significant alternative, mostly pneumonia or pulmonary oedema, in 12–13% of pregnant patients. In one series, 9 of 14 such findings were already visible on the chest radiograph.
Take home
- Chest radiograph first. If it is normal, lung scintigraphy is the preferred test; if it is abnormal, go to CTPA.
- Start with a reduced-activity perfusion study. A normal perfusion scan ends the work-up; add ventilation only if perfusion is abnormal.
- Fetal doses are low and similar for both tests; the breast dose is what favours V/Q.
Sources
- Leung AN, Bull TM, Jaeschke R, et al. An official American Thoracic Society/Society of Thoracic Radiology clinical practice guideline: evaluation of suspected pulmonary embolism in pregnancy. Am J Respir Crit Care Med. 2011;184(10):1200-8.
- Bajc M, Schümichen C, Grüning T, et al. EANM guideline for ventilation/perfusion single-photon emission computed tomography (SPECT) for diagnosis of pulmonary embolism and beyond. Eur J Nucl Med Mol Imaging. 2019;46(12):2429-51.
- Tromeur C, van der Pol LM, Le Roux PY, et al. Computed tomography pulmonary angiography versus ventilation-perfusion lung scanning for diagnosing pulmonary embolism during pregnancy: a systematic review and meta-analysis. Haematologica. 2019;104(1):176-88.
- van der Pol LM, Tromeur C, Bistervels IM, et al. Pregnancy-adapted YEARS algorithm for diagnosis of suspected pulmonary embolism. N Engl J Med. 2019;380(12):1139-49.
- Hammache M, Simard C, Hamel S, et al. Diagnosing pulmonary embolism during pregnancy. Chest. 2025;168(4):1007-17.
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