What is CT (Computed Tomography) PA (Pulmonary Angiography)?

What is CT PA (CTPA)?

CTPA (Computed Tomography Pulmonary Angiography) is a specialized CT scan that uses intravenous contrast material timed specifically to visualize the pulmonary arteries, and it is the imaging modality of choice for diagnosing pulmonary embolism. 1

Technical Definition

CTPA is a non-invasive imaging technique that involves:

• Intravenous contrast administration timed to achieve maximal opacification of the pulmonary arteries (typically requiring at least 210 Hounsfield units for optimal interpretation) 2
• Multidetector CT scanning that allows visualization of pulmonary vessels down to the subsegmental level 1
• Thin-slice imaging with 2-3 mm slice thickness and 2 mm reconstruction index for adequate analysis of segmental and subsegmental vessels 3
• Multiplanar reformations and 3D renderings as essential components of interpretation 3

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

The test directly visualizes:

• Intravascular thrombus appearing as filling defects with sharp interfaces against contrast material 4
• Pulmonary artery anatomy from main vessels to subsegmental branches 1
• Right ventricular strain and cardiac chamber dimensions that provide prognostic information 1

Diagnostic Performance

CTPA demonstrates excellent accuracy:

• Sensitivity of 83-99% and specificity of 96-100% for pulmonary embolism diagnosis 1, 5
• Low rate of inconclusive results (3-5% of studies) 1
• Negative predictive value of 96% in patients with low or intermediate clinical probability 1
• Safe to withhold anticoagulation after a good quality negative CTPA, with only 1.1% recurrence rate at 3 months 1

Key Advantages Over Other Imaging

CTPA offers several critical benefits:

• Readily available 24/7 in most medical centers 1
• Short acquisition time allowing rapid diagnosis in emergency settings 1
• Alternative diagnoses can be identified when PE is excluded (pneumonia, pulmonary edema, aortic dissection, pericarditis) 1, 6
• Prognostic information through assessment of RV/LV ratio, pulmonary artery diameter, and septal deviation 1, 6

Important Limitations and Contraindications

Be aware of these critical constraints:

• Radiation exposure of 3-10 mSv, with significant breast tissue exposure in young women 1
• Iodine contrast requirements limiting use 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

Distinction from Standard CT Chest Angiogram

CTPA differs from a standard CT chest angiogram in contrast timing: CTPA is optimized specifically for pulmonary artery opacification, while CT chest angiogram is timed for systemic arterial circulation (aorta and branches). 7 This timing difference is critical—using the wrong protocol will result in suboptimal vessel opacification and potential misdiagnosis.

Clinical Interpretation Pitfalls

Avoid these common errors:

• Motion artifacts from respiratory or cardiac motion can mimic filling defects 2
• Inadequate contrast timing (standard 15-second delay may need extension to 15-30 seconds in right ventricular failure or pulmonary hypertension) 3
• Lymph nodes adjacent to vessels can be mistaken for thrombus 4
• Flow-related artifacts at vessel bifurcations may simulate emboli 4, 8