What is Computed Tomography Pulmonary Angiography (CTPA)?

What is CTPA (Computed Tomography Pulmonary Angiography)?

CTPA is a specialized CT imaging technique using intravenous contrast with optimized timing for peak pulmonary arterial enhancement, designed to directly visualize the pulmonary arterial tree from main vessels down to subsegmental branches. 1, 2

Technical Definition and Key Distinguishing Features

CTPA differs fundamentally from standard CT chest with contrast through several critical technical specifications:

• Contrast timing is optimized specifically for pulmonary arterial phase, with scan delay typically 15 seconds (or 15-30 seconds in patients with right ventricular failure or pulmonary hypertension), ensuring peak enhancement of pulmonary vessels 3
• Thin-slice acquisition with 2-3 mm slice thickness and 2 mm reconstruction index is essential for adequate visualization of segmental and subsegmental vessels 2, 3
• Specialized reconstructions including multiplanar reformations and 3D renderings are essential components that distinguish CTPA from standard CT protocols 1, 2
• Collimation of 2 mm improves subsegmental vessel visualization when needed for detailed evaluation 3

Primary Clinical Applications

CTPA serves as the first-line imaging test for diagnosing acute pulmonary embolism in patients who cannot have PE ruled out by clinical decision tools and D-dimer testing 4, 1, 2:

• Demonstrates excellent diagnostic accuracy with sensitivity of 83-99% and specificity of 96-100% for pulmonary embolism 2, 5, 6
• The negative predictive value is 96% in patients with low or intermediate clinical probability, making it safe to withhold anticoagulation after a negative study with only 1.1% recurrence rate at 3 months 1, 2
• Provides prognostic information through assessment of right ventricular strain, cardiac chamber dimensions, RV/LV ratio, pulmonary artery diameter, and septal deviation 4, 2

Additional Diagnostic Capabilities

Beyond pulmonary embolism diagnosis, CTPA offers comprehensive cardiovascular and pulmonary assessment:

• Identifies alternative diagnoses when PE is excluded, including pneumonia, pulmonary edema, aortic dissection, pericarditis, and other thoracic pathology 4, 2
• Evaluates for pulmonary hypertension with MPA diameter sensitivity of 87%, specificity of 89%, and positive predictive value of 97% 4
• Characterizes chronic thromboembolic disease with findings including eccentric thrombus, abrupt vessel cutoff, linear webs, bronchial artery dilation, and mosaic lung attenuation 4
• Assesses segmental pulmonary arteries with sensitivities of 99-100% and specificities of 100% for chronic thromboembolic disease at the segmental level 3

Important Limitations and Contraindications

CTPA involves significant radiation exposure of 3-10 mSv, which is particularly concerning for young women due to breast tissue exposure 1, 2:

• Requires iodinated contrast, limiting use in patients with severe renal dysfunction, iodine allergy, or hyperthyroidism 1, 2
• Pregnancy and breastfeeding require careful risk-benefit consideration before proceeding with the examination 2
• Detection of subsegmental PE remains controversial regarding clinical significance and treatment necessity 1, 2
• Low rate of inconclusive results (3-5% of studies) but quality is operator and technique-dependent 2

Advantages Over Alternative Imaging

CTPA has largely replaced ventilation-perfusion scanning and catheter pulmonary angiography as the diagnostic standard 5, 7, 6:

• Readily available 24/7 in most medical centers with short acquisition time allowing rapid diagnosis in emergency settings 2
• Superior to other imaging methods in diagnosing alternative conditions that mimic PE symptoms in a single examination 6
• Modern multidetector CTPA has achieved equivalent sensitivity (99-100%) to V/Q scanning for chronic thromboembolic disease 3
• Traditional catheter angiography is now reserved almost exclusively for presurgical planning or therapeutic interventions 3