Cardiac Imaging

What Is the Safest Initial Imaging for Suspected Pulmonary Embolism in Pregnancy?

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What Is the Safest Initial Imaging for Suspected Pulmonary Embolism in Pregnancy?

A 32-year-old woman, 26 weeks into her second pregnancy, presents to the emergency department with acute-onset shortness of breath and right-sided pleuritic chest pain. Her heart rate is 115 bpm and her oxygen saturation is 95% on room air. You are concerned for a pulmonary embolism (PE), a leading cause of maternal mortality, but both you and the patient are appropriately cautious about radiation exposure to the fetus. This scenario requires a nuanced diagnostic approach that balances maternal risk with fetal safety. This article details the American College of Radiology (ACR) recommended workflow for initial imaging in a pregnant patient with suspected PE, starting with a study that is designated Usually Appropriate and involves no ionizing radiation: US duplex Doppler of the lower extremities.

Who Fits This Clinical Scenario?

This guidance applies specifically to pregnant patients at any gestational age who present with signs or symptoms concerning for a new pulmonary embolism. These symptoms often include dyspnea, tachypnea, tachycardia, pleuritic chest pain, or hemoptysis. The clinical suspicion may be guided by risk stratification tools like the YEARS criteria, which have been adapted for use in pregnancy, but the core indication is a reasonable clinical concern for PE requiring an imaging workup.

This workflow is distinct from that for other patient populations. It does not apply to:

  • Non-pregnant patients: In non-pregnant individuals, risk stratification scores (like Wells’ or revised Geneva) and D-dimer testing play a more definitive role in the initial algorithm. You can find guidance for those scenarios in our parent topic article.
  • Patients with hemodynamic instability: A pregnant patient with hypotension, shock, or other signs of massive PE may require an expedited and different diagnostic and therapeutic pathway, often involving immediate consultation with specialists and potentially bedside echocardiography.
  • Patients with an alternative diagnosis confirmed on initial testing: If a chest radiograph clearly shows pneumonia or pneumothorax that fully explains the patient’s symptoms, the PE workup may be deferred.

What Diagnoses Are You Working Up in This Scenario?

When a pregnant patient presents with acute cardiorespiratory symptoms, the differential diagnosis is broad, but the primary concern driving this specific imaging pathway is venous thromboembolism (VTE).

Pulmonary Embolism (PE): This is the most life-threatening diagnosis being considered. Pregnancy itself is a hypercoagulable state, increasing the risk of VTE five-fold compared to non-pregnant individuals. A PE occurs when a thrombus, typically from a deep vein in the legs or pelvis, embolizes to the pulmonary arteries, causing respiratory and hemodynamic compromise.

Deep Vein Thrombosis (DVT): This is the precursor to most PEs. Up to 70% of pregnant patients with PE have evidence of DVT in their lower extremities. Identifying a DVT provides a definitive diagnosis of VTE and a clear indication for anticoagulation, often allowing clinicians to avoid further imaging that involves radiation.

Physiologic Dyspnea of Pregnancy: A very common mimic. Hormonal changes and the physical mass of the enlarging uterus can cause a sensation of breathlessness. However, this is a diagnosis of exclusion and should not be assumed without first ruling out more dangerous conditions, especially when symptoms are acute or accompanied by tachycardia or chest pain.

Other Cardiopulmonary Conditions: Less common but important considerations include amniotic fluid embolism (a rare obstetric emergency), peripartum cardiomyopathy, pneumonia, pneumothorax, and musculoskeletal chest wall pain. A chest radiograph, while not diagnostic for PE, is crucial for evaluating these possibilities.

Why Is US Duplex Doppler of the Lower Extremity a Recommended First Step?

In the unique context of pregnancy, the diagnostic algorithm for suspected PE prioritizes minimizing fetal radiation exposure without compromising maternal safety. The ACR designates four studies as Usually Appropriate, but their sequencing is critical. The workflow often begins with the two studies that have zero or negligible fetal radiation dose: chest radiography and lower extremity ultrasound.

US duplex Doppler lower extremity is a key initial test because it uses no ionizing radiation (0 mSv) and is completely safe for the fetus. If the ultrasound identifies a DVT, the diagnosis of VTE is confirmed. The patient can be started on anticoagulation, and the need for further, higher-radiation studies like CT or V/Q scanning is eliminated. This “rule-in” strategy is highly effective because a significant portion of pregnant patients with PE have a concurrent DVT.

A Radiography chest is also Usually Appropriate and delivers a very low radiation dose (adult_rrl=☢ <0.1 mSv), which is well below the threshold associated with fetal harm. Its primary role is not to diagnose PE but to rule out other causes of the patient’s symptoms (e.g., pneumonia, pneumothorax) and to ensure the chest is clear, which is a prerequisite for a diagnostic V/Q scan if it becomes necessary.

If the lower extremity ultrasound is negative, PE is not ruled out, and the workup must proceed. The next choice is between two other Usually Appropriate studies:

  • CTA pulmonary arteries with IV contrast (adult_rrl=☢☢☢ 1-10 mSv): This is often the preferred next step as it is fast, widely available, and highly sensitive and specific for PE. The radiation dose to the fetus is low, as the fetus is outside the primary imaging field. However, the iodinated contrast crosses the placenta, and there is a higher radiation dose to maternal breast tissue.
  • V/Q scan lung (adult_rrl=☢☢☢ 1-10 mSv): This study results in a lower radiation dose to maternal breast tissue but a slightly higher dose to the fetus compared to CTPA. It is only diagnostic if the patient has a normal chest radiograph and no underlying lung disease.

Studies like MRA pulmonary arteries without and with IV contrast are rated Usually not appropriate. While MRA avoids ionizing radiation, it is often limited by longer scan times, motion artifact from breathing, and lower diagnostic accuracy for smaller, subsegmental PEs compared to CTPA.

Once you’ve decided on US duplex Doppler lower extremity, our protocol guide covers the technique, patient positioning, and interpretation principles: US Lower Extremity Doppler (DVT).

What’s Next After US Duplex Doppler Lower Extremity? Downstream Workflow

The results of the initial non-invasive imaging will guide your next steps in a clear, branching pathway.

  • If the US is POSITIVE for DVT: The workup is complete. The patient is diagnosed with VTE and should be started on therapeutic anticoagulation (typically low-molecular-weight heparin in pregnancy). No further imaging for PE is necessary, as the treatment is the same.
  • If the US is NEGATIVE for DVT: Pulmonary embolism has not been ruled out. The patient still requires definitive imaging of the pulmonary arteries. At this point, a shared decision-making conversation with the patient is crucial. You should discuss the risks and benefits of the two primary options: CTPA versus V/Q scan. The choice often depends on institutional preference, radiologist expertise, and patient-specific factors (e.g., a history of contrast allergy might favor a V/Q scan, while underlying asthma might favor a CTPA).
  • If the US is INDETERMINATE: A technically limited or equivocal study may occur due to patient body habitus or other factors. The result should be treated as a negative study in terms of the PE workup, meaning the patient still requires definitive imaging of the chest if clinical suspicion remains high.

This sequential approach—starting with zero-radiation ultrasound—ensures that a significant number of patients can be diagnosed and treated without exposing the fetus to any radiation from CT or nuclear medicine studies.

Pitfalls to Avoid (and When to Get Help)

Navigating a PE workup in pregnancy is complex. Be mindful of these common pitfalls:

  • Withholding Imaging Due to Radiation Fears: The risk of untreated maternal PE is far greater than the very small theoretical risk to the fetus from a properly performed CTPA or V/Q scan. Delaying diagnosis can be catastrophic.
  • Over-reliance on D-dimer: D-dimer levels physiologically increase throughout pregnancy, leading to a high false-positive rate. While pregnancy-adjusted thresholds exist, a positive D-dimer is non-specific and often does not add value if clinical suspicion is already moderate to high.
  • Forgetting the Chest Radiograph: Skipping the initial chest X-ray before a V/Q scan can lead to an indeterminate result, potentially requiring a second, different imaging study and unnecessary additional radiation exposure.
  • Not Involving a Multidisciplinary Team: For complex cases, consult with your radiology, obstetrics, and hematology colleagues. If the patient is hemodynamically unstable or has a high-risk presentation, escalate immediately to ensure rapid, coordinated care.

Related ACR Topics and Tools

This article covers a single, critical scenario. For a comprehensive overview of all clinical variants, decision-making algorithms, and additional imaging modalities for this condition, please refer to our parent guide. For further exploration of related imaging studies and dose considerations, the following GigHz resources are available:

Frequently Asked Questions

Why not just order a CTPA for every pregnant patient with suspected PE?

While CTPA is highly accurate, the recommended approach prioritizes a ‘radiation-sparing’ strategy. Starting with a lower extremity ultrasound, which has zero radiation, can confirm the diagnosis of VTE in a subset of patients, allowing them to avoid CTPA entirely. This sequential approach minimizes radiation exposure to both the mother’s breast tissue and the fetus whenever possible.

Is D-dimer testing useful for ruling out PE in pregnant patients?

The utility of D-dimer in pregnancy is limited. D-dimer levels naturally rise during a normal pregnancy, making standard thresholds unreliable and leading to many false positives. While pregnancy-adjusted D-dimer levels or algorithms like the YEARS criteria can help in some cases, a negative result is less reassuring, and a positive result is less specific than in a non-pregnant patient. It should not be used in isolation to rule out PE if clinical suspicion is moderate or high.

What is the radiation risk to the fetus from a CTPA versus a V/Q scan?

Both studies deliver a fetal radiation dose well below the 50 mGy threshold considered to be a risk for deterministic effects. A CTPA typically results in a very low fetal dose (<0.1 mGy) but a higher dose to maternal breast tissue. A V/Q scan delivers a slightly higher fetal dose (around 0.1-0.7 mGy) but a lower dose to maternal breasts. The choice between them involves a discussion of these trade-offs and is often based on institutional protocols and patient factors.

If the lower extremity ultrasound is negative, is CTPA or V/Q scan better?

Both are rated ‘Usually Appropriate’ by the ACR. The choice depends on several factors. A V/Q scan is preferred by some if the patient has a normal chest X-ray and no prior lung disease, as it avoids IV contrast and has a lower radiation dose to maternal breast tissue. CTPA is often faster, more widely available, and can provide alternative diagnoses if PE is not found. The decision should be made in consultation with the radiologist and the patient.

Can I use MRA to avoid radiation altogether?

Magnetic Resonance Angiography (MRA) for pulmonary arteries is rated ‘Usually Not Appropriate’ by the ACR for this scenario. While it avoids ionizing radiation, it is technically challenging in dyspneic patients, takes longer to perform, and is generally considered less accurate than CTPA for detecting pulmonary emboli, especially smaller ones. Its use is typically reserved for rare cases where both CTPA and V/Q scan are absolutely contraindicated.

Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 26, 2026