What is CTPA (Computed Tomography Pulmonary Angiography)?
CTPA is a specialized CT scan that uses intravenous contrast timed to visualize the pulmonary arteries from the main vessels down to the subsegmental branches, and is the gold standard imaging test for diagnosing pulmonary embolism. 1, 2
Technical Definition
CTPA is distinguished from standard contrast-enhanced CT by three essential elements 1:
- Precise timing of contrast administration to coincide with peak pulmonary arterial enhancement (typically 15-30 seconds after injection) 3
- Thin-section acquisition using 2-3 mm slice thickness with 2 mm reconstruction intervals to adequately visualize segmental and subsegmental vessels 2, 3
- Multiplanar reformations and 3D renderings as required components of interpretation, not just axial images 1, 2
The scan directly visualizes the pulmonary vascular tree, allowing detection of thrombus, vascular abnormalities, and associated cardiac and lung parenchymal changes 2, 4.
Primary Clinical Use
CTPA is the recommended initial lung imaging modality for suspected acute pulmonary embolism when PE cannot be ruled out by clinical decision tools and D-dimer testing. 1, 2
The test serves multiple diagnostic purposes 2, 5:
- Direct visualization of intravascular thrombus in pulmonary arteries
- Assessment of right ventricular strain and cardiac chamber dimensions for prognostic information
- Evaluation of pulmonary hypertension through measurement of vessel diameters and cardiac morphology
- Detection of chronic thromboembolic disease
Diagnostic Performance
CTPA demonstrates excellent accuracy for pulmonary embolism 1, 2, 6:
- Sensitivity: 83-99% for detecting PE
- Specificity: 96-100% for PE diagnosis
- Negative predictive value: 96% in patients with low or intermediate clinical probability
- Low inconclusive rate: 3-5% of studies
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 when anticoagulation is withheld. 1, 2 This compares favorably to the 0.9% recurrence rate after negative conventional pulmonary angiography 1.
Key Advantages
CTPA has largely replaced conventional pulmonary angiography and ventilation-perfusion scanning as first-line imaging because 1, 2, 6:
- 24/7 availability in most medical centers
- Rapid acquisition time allowing emergency diagnosis
- Ability to identify alternative diagnoses when PE is excluded, such as pneumonia, pulmonary edema, aortic dissection, pleural effusion, or pericarditis 1, 2
- Superior specificity compared to V/Q scanning 1
- Prognostic information through RV/LV ratio, pulmonary artery diameter, and septal deviation assessment 2, 5
Important Limitations and Contraindications
Radiation exposure: 3-10 mSv effective dose, with significant breast tissue exposure in young women requiring careful risk-benefit consideration 1, 2
Iodine contrast requirements limit use in 1, 2:
- Iodine allergy
- Hyperthyroidism
- Severe renal failure
- Pregnancy (requires careful consideration)
Subsegmental PE detection: While modern multidetector CTPA can visualize subsegmental vessels, the clinical significance of isolated subsegmental PE remains controversial 1, 2
Critical Technical Pitfalls
Meticulous attention to technique is essential to achieve diagnostic quality 1, 3:
- Contrast timing must be optimized—standard 15-second delay may need extension to 15-30 seconds in patients with right ventricular failure or pulmonary hypertension
- Thin collimation (2 mm) is required for adequate subsegmental vessel evaluation
- Proper injection rates are necessary, though diagnostic studies can be achieved even with low flow rates (2.0-2.5 mL/s) when necessary 7
A small proportion of examinations will be technically unsatisfactory despite optimal technique 1.