What is the preferred imaging modality for suspected pulmonary embolism (PE)?

Preferred Imaging for Suspected Pulmonary Embolism

CT pulmonary angiography (CTPA) is the preferred initial imaging modality for suspected pulmonary embolism. 1

Primary Imaging Recommendation

• CTPA should be obtained in all patients with high pretest probability of PE, and in patients with low or intermediate pretest probability who have elevated D-dimer levels. 1

• CTPA has become the de facto clinical gold standard, replacing catheter pulmonary angiography and ventilation-perfusion scanning as first-line imaging. 2

• The American College of Physicians explicitly recommends CTPA as the imaging modality of choice, with ventilation-perfusion scans reserved only for patients who have contraindications to CTPA or when CTPA is unavailable. 1

• The British Thoracic Society guidelines similarly designate CTPA as the recommended initial lung imaging modality for non-massive PE. 1

Diagnostic Performance

• Patients with a good quality negative CTPA do not require further investigation or treatment for PE. 1

• Modern multidetector CTPA demonstrates sensitivities of 99-100% and specificities of 100% for detecting thromboembolic disease at the segmental level. 3

• Clinical outcome studies demonstrate it is safe to withhold anticoagulation when PE is excluded on CTPA, with subsequent PE occurring in only 1.1% of patients at 3 months—comparable to the 0.9% recurrence rate after negative conventional angiography. 1

• CTPA is clearly superior in specificity to ventilation-perfusion isotope scanning and allows quantitative assessment that correlates well with clinical severity. 1

Technical Advantages

• CTPA enables direct visualization of intravascular thrombus, webs, bands, and vessel occlusion while simultaneously evaluating alternative diagnoses when PE is excluded. 1

• Modern multidetector technology with thin-section collimation (2-3 mm slice thickness) provides excellent visualization of segmental and subsegmental vessels. 3, 4

• CTPA demonstrates secondary effects including wedge-shaped opacities, right ventricular changes, and can assess for right ventricular dysfunction for prognostication. 1, 2

• Dual-energy CT techniques improve detection through perfusion mapping and can rescue suboptimal contrast studies using monoenergetic images. 1

Alternative Imaging Modalities

Ventilation-Perfusion Scanning

• V/Q scanning should be used only when CTPA is unavailable or contraindicated (e.g., contrast allergy, renal insufficiency). 1

• V/Q scanning remains a valid alternative but has been largely supplanted by CTPA due to superior specificity and diagnostic clarity. 1

MR Angiography

• MRA has limited utility for acute PE diagnosis, with the PIOPED III trial showing technically inadequate studies in 25% of patients and sensitivity of only 78% in adequate studies. 1

• MRA combined with perfusion imaging can diagnose chronic thromboembolic disease with 83-100% sensitivity but has lower sensitivity for acute PE compared to CTPA. 1, 3

Echocardiography

• Echocardiography (or CTPA) will reliably diagnose clinically massive PE and should be performed within 1 hour in massive PE. 1

• Echocardiography allows firm diagnosis in only a minority of non-massive PE cases and is primarily useful for prognostic information rather than definitive diagnosis. 1

Critical Timing Considerations

• Imaging should be performed within 1 hour in massive PE, and ideally within 24 hours in non-massive PE. 1

• CTPA is now widely available in most hospitals and may be quicker to arrange than echocardiography, even out of hours. 1

Important Caveats

• Adequate contrast timing is essential—standard scan delay is 15 seconds, but may require 15-30 seconds in patients with right ventricular failure or pulmonary hypertension. 3

• Meticulous attention to technique is necessary to achieve contrast opacification of at least 210 Hounsfield units for optimal embolus detection. 5

• A small proportion of examinations will be technically unsatisfactory; quality control is critical for reliable interpretation. 1

• Subsegmental clot may be less reliably detected than proximal clot, though most patients with subsegmental PE also have more proximal clot that can be identified. 1

• Interobserver agreement is good even with relatively inexperienced assessors, but familiarity with interpretation pitfalls and artifacts is essential to avoid false positives. 1, 5