Cardiac Imaging

Which Imaging Is Best for High-Probability Suspected Pulmonary Embolism? An ACR-Guided Workflow

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Which Imaging Is Best for High-Probability Suspected Pulmonary Embolism? An ACR-Guided Workflow

It’s 2 a.m. in the emergency department, and you are evaluating a 68-year-old man with acute-onset dyspnea and pleuritic chest pain. He underwent a hip replacement two weeks ago. His heart rate is 115, and his oxygen saturation is 91% on room air. His Wells’ score is 7.5, placing him in the high pretest probability category for pulmonary embolism (PE). The D-dimer is not indicated here; your next step is definitive imaging. This article provides a detailed clinical workflow for this specific scenario: selecting the initial imaging study for a patient with a high pretest probability of pulmonary embolism. According to the American College of Radiology (ACR) Appropriateness Criteria, the definitive study for this presentation, `CTA pulmonary arteries with IV contrast`, is rated Usually Appropriate.

Who Fits This Clinical Scenario?

This guidance is for patients where clinical suspicion for pulmonary embolism is high before any imaging is performed. High pretest probability is typically established using a validated clinical decision rule. This includes patients with:

  • A Wells’ score greater than 6.
  • A revised Geneva score of 11 or higher.
  • Strong gestalt clinical judgment from an experienced clinician, particularly when classic risk factors (e.g., recent major surgery, active malignancy, prolonged immobility) and symptoms (e.g., pleuritic chest pain, dyspnea, hemoptysis) are present.

This workflow is distinct from other common PE workups and does not apply to:

  • Patients with low or intermediate pretest probability: In these cases, a negative D-dimer test is typically sufficient to rule out PE, and imaging is only pursued if the D-dimer is positive. This article is for patients where the suspicion is too high to be ruled out by a lab test.
  • Pregnant patients: The diagnostic algorithm for suspected PE in pregnancy is different, prioritizing the reduction of radiation exposure to both the mother and fetus. Imaging choices like V/Q scans or specific low-dose CT protocols are considered differently.

Applying this high-probability pathway to a low-probability patient can lead to unnecessary radiation exposure and contrast administration. Correctly stratifying the patient first is the critical initial step.

What Diagnoses Are You Working Up in This Scenario?

While pulmonary embolism is the primary concern, the patient’s symptoms of acute chest pain and shortness of breath demand a broader differential. The choice of imaging is designed to definitively confirm or exclude PE while often providing clues to alternative diagnoses.

Pulmonary Embolism (PE): This is the leading diagnosis. A clot, typically from a deep vein thrombosis (DVT) in the legs, has traveled to the pulmonary arteries. The clinical picture of recent surgery, tachycardia, and hypoxia makes this the most likely and life-threatening possibility that must be addressed immediately.

Acute Coronary Syndrome (ACS): Myocardial infarction can present with chest pain and dyspnea. While the pleuritic nature of the pain makes PE more likely, ACS must be considered. A CTA of the pulmonary arteries is not designed to evaluate the coronary arteries, but it can sometimes show secondary signs of cardiac strain.

Aortic Dissection: This is a less common but catastrophic cause of acute chest pain. A standard PE-protocol CTA is not timed optimally to evaluate the aorta, but a large dissection flap may be incidentally identified, prompting a dedicated study. A “triple rule-out” CTA protocol exists to evaluate for PE, dissection, and ACS simultaneously but is rated Usually not appropriate for this focused clinical question due to higher radiation and contrast loads without proven benefit over targeted imaging.

Pneumonia or Pleuritis: An infection or inflammation of the lung parenchyma or pleura can cause pleuritic chest pain and hypoxia. The non-vascular lung windows of a CTA are excellent for identifying consolidation, infiltrates, or pleural effusions that would suggest an infectious or inflammatory cause.

Pneumothorax: A collapsed lung can cause sudden-onset pleuritic pain and dyspnea. This is easily and definitively identified on any CT scan of the chest, including a CTA performed for PE evaluation.

Why Is CTA Pulmonary Arteries with IV Contrast the Recommended Study?

For a patient with a high pretest probability of PE, the goal is rapid, definitive diagnosis. The ACR designates `CTA pulmonary arteries with IV contrast` as Usually appropriate because it directly visualizes the thrombus in the pulmonary vasculature, offering high sensitivity and specificity for the primary diagnosis.

The rationale for this choice over alternatives involves a balance of diagnostic accuracy, speed, and safety:

  • Diagnostic Power: CTA is highly effective at identifying clots down to the subsegmental level. It also provides crucial information about the clot burden and assesses for signs of right heart strain (e.g., right ventricle to left ventricle ratio > 1), which is a key prognostic indicator that guides the intensity of therapy.
  • Alternative Diagnoses: As noted above, the same scan provides excellent evaluation of the lung parenchyma, pleura, and mediastinum, often revealing an alternative diagnosis like pneumonia or pneumothorax if PE is absent.
  • Speed and Availability: In most hospitals, CT scanners are available 24/7, and a PE protocol can be completed in minutes, which is critical for a potentially unstable patient.

Why are other studies rated lower for this specific scenario?

  • V/Q Scan (Ventilation/Perfusion Scan): This nuclear medicine study is also rated Usually appropriate and is an excellent alternative, particularly in patients with severe IV contrast allergy or significant renal impairment. However, it is often less available emergently and can be indeterminate in patients with underlying lung disease (e.g., COPD, atelectasis), which is common in hospitalized populations. It provides no information on alternative diagnoses.
  • US Duplex Doppler Lower Extremity: This study is rated May be appropriate (Disagreement). While finding a DVT in a patient with suspected PE is sufficient to confirm the diagnosis of venous thromboembolism and start treatment, a negative ultrasound of the legs does not rule out PE. The clot may have already embolized entirely, or it may have originated from a different location (e.g., pelvic or upper extremity veins). It is not a rule-out test for PE.

The radiation dose for a CTA pulmonary arteries is moderate (ACR RRL ☢☢☢, 1-10 mSv), a necessary trade-off for the high diagnostic yield in this life-threatening situation. The use of iodinated IV contrast requires screening for renal dysfunction and potential allergies.

What’s Next After CTA Pulmonary Arteries? Downstream Workflow

The results of the CTA will guide your immediate next steps. The clinical workflow branches significantly based on the findings.

If the study is POSITIVE for pulmonary embolism:

The primary action is to initiate therapeutic anticoagulation without delay, unless a major contraindication exists. The next step is risk stratification. The radiologist’s report and your own review of the images should assess for signs of right heart strain. If present, or if the patient is hemodynamically unstable, this indicates a submassive or massive PE, respectively, and requires urgent consultation with a Pulmonary Embolism Response Team (PERT), critical care, or interventional radiology to consider advanced therapies like systemic thrombolysis or catheter-directed thrombectomy.

If the study is NEGATIVE for pulmonary embolism:

A technically adequate negative CTA effectively rules out clinically significant PE. The focus immediately shifts to the differential diagnoses. Carefully review the lung windows and other structures on the CT for an alternative cause of the patient’s symptoms, such as pneumonia, a small pneumothorax, or pericardial effusion. If no cause is identified on the CT, you must return to the bedside and reconsider non-thoracic and non-embolic causes for the patient’s presentation.

If the study is INDETERMINATE or suboptimal:

This can occur due to patient motion or poor contrast timing. The decision here depends on your continued clinical suspicion. If suspicion remains high, you may need to repeat the CTA or switch to an alternative modality like a V/Q scan. A lower extremity duplex ultrasound could also be considered to search for DVT as a proxy for VTE. Consultation with the radiologist is key to understanding the limitation of the study and planning the next best step.

Pitfalls to Avoid (and When to Get Help)

In the high-stakes workup of suspected PE, several common errors can delay diagnosis or lead to misinterpretation.

  • Ordering the Wrong CT: Ordering a “CT chest with contrast” is not the same as a “CTA pulmonary arteries.” The timing of the contrast bolus is completely different and a non-protocol scan will not opacify the pulmonary arteries, making the study non-diagnostic for PE.
  • Ignoring Contraindications: Failing to check for severe contrast allergy or acute kidney injury can lead to preventable adverse events. Always perform a safety check before sending the patient for a CTA.
  • Over-relying on a Negative Study with Persistently High Suspicion: While rare, a negative CTA is not perfect. If the clinical picture is overwhelmingly convincing for PE despite a negative scan, discuss the case with a senior colleague or a pulmonologist.
  • Misinterpreting Subsegmental PE: The clinical significance of an isolated subsegmental PE can be controversial. Management decisions should be made in consultation with pulmonology, especially in patients without DVT and with low cardiopulmonary reserve.

If the patient is hemodynamically unstable (hypotensive), escalate immediately to your senior resident, attending, or a rapid response team while arranging for a bedside echocardiogram and preparing for potential resuscitation and thrombolytics.

Related ACR Topics and Tools

This article covers one specific scenario in depth. For a broader view of all clinical variants in this topic, or to explore the technical details of the recommended imaging study, the following resources are essential.

Frequently Asked Questions

Why not just get a D-dimer test on every patient with suspected PE?

In a patient with a high pretest probability of PE, a D-dimer test is not helpful. The test has high sensitivity but low specificity, meaning it is good for ruling out a disease when negative, but a positive result is common in many conditions (including post-operative states, malignancy, and inflammation). A high-probability patient is very likely to have a positive D-dimer anyway, so the test does not change management and only delays definitive imaging.

In which high-probability patients should I choose a V/Q scan over a CTA?

A V/Q scan is the preferred alternative to CTA in two main situations: 1) Patients with a history of severe anaphylactic reaction to iodinated contrast media. 2) Patients with severe renal insufficiency (e.g., GFR < 30 mL/min) where contrast-induced nephropathy is a major concern. It may also be considered in very young patients, particularly women, to reduce radiation dose to breast tissue, though modern CTA techniques have significantly lowered radiation.

What is the role of bedside echocardiography in this scenario?

According to the ACR, a transthoracic echocardiogram is rated ‘May be appropriate.’ Its primary role is not to diagnose the PE itself (it can’t see the clot), but to assess for its hemodynamic consequences, specifically right ventricular (RV) strain or dysfunction. In an unstable patient, seeing RV strain on a bedside echo can increase suspicion for a massive PE and prompt life-saving therapies while awaiting a definitive CTA.

What should I do if the CTA is negative but my clinical suspicion remains extremely high?

This is a challenging clinical dilemma. First, re-confirm with the radiologist that the study was technically adequate. Second, thoroughly reconsider alternative diagnoses that could mimic a PE. If suspicion for PE is truly inescapable, further steps could include lower extremity duplex ultrasound to look for a DVT. In very rare and select cases, conventional catheter-based pulmonary angiography, the historical gold standard, might be considered, but this is an invasive procedure and is rarely performed today for PE diagnosis.

Does finding a ‘saddle embolus’ on CTA change immediate management?

Yes, significantly. A saddle embolus is a large clot that straddles the bifurcation of the main pulmonary artery. It represents a large clot burden and is often associated with hemodynamic instability (massive PE) or right heart strain (submassive PE). This finding is a critical result that should prompt immediate risk stratification and consideration of advanced therapies beyond standard anticoagulation, such as thrombolysis or mechanical thrombectomy.

Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 29, 2026