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 vessels in the lungs from the main pulmonary artery down to subsegmental branches. 1
Technical Definition
CTPA is distinguished from standard contrast-enhanced chest CT by three essential elements 2:
- Precise timing: IV contrast administration is timed to coincide with peak arterial enhancement in the pulmonary circulation 2
- Thin-section acquisition: Modern protocols use 2-3 mm slice thickness with 2 mm reconstruction intervals to visualize vessels down to the subsegmental level 1, 3
- Advanced post-processing: Multiplanar reformations and 3D renderings are required components of interpretation, not optional 2, 1
Primary Clinical Applications
CTPA is the standard of care and first-line imaging test for diagnosing acute pulmonary embolism in hemodynamically stable patients. 2, 1, 4 The British Thoracic Society established this as the recommended initial lung imaging modality for non-massive PE 2.
Beyond pulmonary embolism, CTPA evaluates 2, 5:
- Pulmonary hypertension: Assesses main pulmonary artery diameter (≥29 mm suggests PH with 87% sensitivity and 89% specificity), PA-to-aorta ratio, and right ventricular changes 2
- Chronic thromboembolic disease: Detects eccentric thrombus, webs, bands, vessel narrowing, and mosaic lung perfusion with 83-100% sensitivity and 89-97% specificity 2
- Vascular abnormalities: Identifies pulmonary artery aneurysms, stenoses, intravascular tumors, and congenital heart disease 2, 5
Diagnostic Performance
CTPA demonstrates excellent accuracy for pulmonary embolism 1, 6:
- Sensitivity: 83-99% for segmental or larger emboli
- Specificity: 96-100%
- Negative predictive value: 96% in low-to-intermediate probability patients
- Inconclusive rate: Only 3-5% of studies
Patients with a good quality negative CTPA do not require further investigation or treatment for PE—it is safe to withhold anticoagulation with only 1.1% recurrence rate at 3 months. 2, 1
Key Advantages
CTPA offers multiple clinical benefits beyond other imaging modalities 2, 1, 6:
- 24/7 availability: Readily accessible in most medical centers at all hours
- Rapid acquisition: Short scan time (seconds) allows immediate diagnosis in emergency settings
- Alternative diagnoses: When PE is excluded, CTPA identifies the actual cause in many cases—pneumonia, pulmonary edema, aortic dissection, pericarditis, pneumothorax, or malignancy 2, 1
- Prognostic information: Provides assessment of right ventricular strain, RV/LV ratio, pulmonary artery diameter, and septal deviation that predict short-term outcomes 2, 1, 7
Important Limitations and Contraindications
Radiation exposure: CTPA delivers 3-10 mSv of radiation with significant breast tissue exposure, particularly concerning in young women due to breast cancer risk 1, 8
Contrast requirements limit use in 1:
- Iodine allergy or prior severe contrast reaction
- Hyperthyroidism (risk of thyroid storm)
- Severe renal failure (eGFR <30 mL/min)
- Pregnancy requires careful risk-benefit assessment
Subsegmental PE detection: While modern multidetector CTPA can visualize subsegmental vessels, the clinical significance of isolated subsegmental PE remains controversial, and some may be missed 2, 1
Comparison to Alternative Imaging
V/Q scanning is reserved for specific situations 1, 4, 8:
- Contraindications to CT contrast
- Younger patients where radiation minimization is critical
- Pregnancy (lower fetal radiation than CTPA)
- However, V/Q scans are non-diagnostic in 30-50% of cases, often requiring additional testing 4
MR angiography shows promise but has lower sensitivity for acute PE and limited availability, making it unsuitable as a routine alternative to CTPA 2, 6