What is CTPA (Computed Tomography Pulmonary Angiography)?
CTPA is a specialized CT scan that uses intravenous contrast timed to opacify the pulmonary arteries, allowing direct visualization of blood clots from the main pulmonary vessels down to subsegmental branches—it is the gold standard first-line imaging test for diagnosing suspected pulmonary embolism. 1, 2
Technical Definition
CTPA is a multidetector CT scanning technique with specific technical requirements:
- Uses thin-slice imaging (2-3 mm slice thickness with 2 mm reconstruction index) to adequately visualize segmental and subsegmental pulmonary vessels 1
- Requires precise timing of iodinated contrast bolus to optimally opacify the pulmonary arterial tree 3, 1
- Includes multiplanar reformations and 3D renderings as essential components of interpretation 1
- Images must be viewed at workstations with familiarity of interpretation pitfalls to achieve optimal accuracy 3
Primary Clinical Role
CTPA is now the recommended initial lung imaging modality for non-massive pulmonary embolism when PE cannot be excluded by clinical decision rules (Wells criteria, Geneva score) and D-dimer testing 3, 1
The test serves multiple diagnostic functions:
- Directly demonstrates intravascular thrombus in pulmonary arteries 3
- Shows secondary effects such as wedge-shaped opacities and right ventricular changes 3
- Identifies alternative diagnoses when PE is excluded, including pneumonia, pulmonary edema, aortic dissection, and pericarditis 3, 1
- Provides prognostic information through assessment of right ventricular dysfunction, RV/LV ratio, pulmonary artery diameter, and septal deviation 3, 1
Diagnostic Performance
CTPA demonstrates excellent accuracy for PE diagnosis:
- Sensitivity of 83-99% and specificity of 96-100% for pulmonary embolism 1
- Negative predictive value of 96% in patients with low or intermediate clinical probability 1
- Only 3-5% inconclusive studies, far fewer than ventilation-perfusion scanning 1
- Patients with good quality negative CTPA do not require further investigation or treatment for PE, with only 1.1% recurrence rate at 3 months compared to 0.9% for negative conventional angiography 3, 1
The test reliably identifies proximal clot in 94-96% of cases, though subsegmental clot detection remains a limitation 3
Key Advantages
CTPA has become the dominant imaging modality due to several practical advantages:
- Readily available 24/7 in most medical centers 1
- Short acquisition time allowing rapid diagnosis in emergency settings 1
- Superior specificity compared to ventilation-perfusion scanning 3
- Good interobserver agreement even with relatively inexperienced assessors and in patients with co-existing cardiorespiratory disease 3
- Can assess right ventricular function to guide treatment decisions (thrombolysis versus anticoagulation alone) 3
Important Limitations and Contraindications
Several factors limit CTPA use in certain populations:
- Radiation exposure of 3-10 mSv with significant breast tissue exposure in young women 1
- Requires iodinated contrast, contraindicated in patients with iodine allergy, hyperthyroidism, or severe renal failure 1
- Pregnancy and breastfeeding require careful risk-benefit consideration 1
- Subsegmental PE detection remains controversial regarding clinical significance 1
- Meticulous attention to technique is necessary, as a small proportion of examinations are technically unsatisfactory 3
Common Pitfalls to Avoid
- Do not perform CTPA in low or intermediate probability patients with negative D-dimer—imaging is not indicated in this scenario 3, 4
- Ensure proper contrast timing to avoid false-negative studies from suboptimal opacification 3
- Recognize that subsegmental clot is less reliably detected compared to conventional angiography, though most patients have more proximal clot that can be identified 3