Imaging Modality of Choice in Pulmonary Embolism
CT pulmonary angiography (CTPA) is the imaging modality of choice for diagnosing pulmonary embolism. 1
Primary Recommendation
CTPA should be obtained as the initial imaging test in patients with suspected PE who have intermediate to high pretest probability or when clinical decision tools indicate imaging is warranted. 1 The American College of Physicians explicitly recommends obtaining CTPA in patients with high pretest probability of PE, while reserving ventilation-perfusion scans only for patients who have contraindications to CTPA or when CTPA is unavailable 1.
Why CTPA is Superior
CTPA offers several critical advantages that make it the definitive first-line test:
Excellent diagnostic accuracy: CTPA demonstrates sensitivity of 83-99% and specificity of 96-100% for PE diagnosis, with a negative predictive value of 96% in patients with low or intermediate clinical probability 2, 3, 4.
Safety of negative results: Patients with a good quality negative CTPA do not require further investigation or treatment for PE, with only 1.1% recurrence rate at 3 months—comparable to the 0.9% recurrence rate after negative conventional angiography 1, 2.
Comprehensive evaluation: CTPA provides detailed visualization of pulmonary vasculature from main vessels down to subsegmental branches (2-3 mm), while simultaneously evaluating lung parenchyma and mediastinal structures 1, 2, 5.
Alternative diagnoses: When PE is excluded, CTPA frequently identifies other causes of symptoms such as pneumonia, pulmonary edema, heart failure, malignancy, aortic dissection, or pericarditis 1, 2, 6.
Prognostic information: CTPA assesses right ventricular strain through RV/LV ratio, pulmonary artery diameter, and septal deviation—parameters that predict short-term mortality and need for ICU-level care 7, 2, 4.
Availability and speed: CTPA is readily available 24/7 in most medical centers with short acquisition times, allowing rapid diagnosis in emergency settings 2, 4.
Technical Considerations for Optimal Results
To achieve diagnostic quality comparable to published studies, meticulous attention to technique is essential:
Thin-slice imaging: Use 2-3 mm slice thickness with 2 mm reconstruction index for adequate visualization of segmental and subsegmental vessels 2.
Contrast optimization: Achieve pulmonary artery contrast opacification of at least 210 Hounsfield units through proper timing and injection rates of 4.0-5.0 mL/s 5, 6.
Bolus timing: Utilize automated bolus tracking ("smart-prep" method) to ensure optimal contrast enhancement in the main pulmonary artery 6.
Multiplanar reconstructions: Include multiplanar reformations and 3D renderings as essential components of interpretation 2.
When to Use Alternative Imaging
Reserve ventilation-perfusion (V/Q) scanning for patients with absolute contraindications to CTPA, including severe iodine allergy, hyperthyroidism, or severe renal failure 1, 2. V/Q scanning is less useful in patients with abnormal chest radiographs or underlying lung disease 7.
Consider compression ultrasonography in patients with concomitant symptomatic deep vein thrombosis as an adjunct to diagnosis 3.
Magnetic resonance angiography (MRA) is not currently a suitable alternative for routine PE diagnosis due to limited sensitivity for peripheral PE (78% sensitivity in technically adequate studies) and high rates of technically inadequate examinations (25% in the PIOPED III trial) 1, 3.
Critical Pitfalls to Avoid
Do not obtain D-dimer testing in high pretest probability patients—proceed directly to CTPA as D-dimer will not change management 1.
Recognize artifacts that mimic PE: Flow-related artifacts, respiratory motion, cardiac pulsation artifacts, and lymph nodes can be mistaken for emboli, requiring careful interpretation 5.
Ensure adequate image quality: A small proportion of examinations (3-5%) may be technically unsatisfactory; suboptimal studies should not be used to exclude PE 1, 2.
Do not delay imaging while awaiting other tests in patients with suspected PE, as this is a potentially fatal condition requiring prompt diagnosis 7.
Special Populations
In patients with known history of PE and suspected recurrent/residual disease, CTPA remains the initial imaging modality of choice 1. Catheter-based pulmonary angiography with or without right heart catheterization is reserved for confirmation when CTPA is negative but clinical suspicion remains high 1.
For pregnant patients, careful risk-benefit consideration is required, though CTPA remains the preferred test when imaging is indicated 2.