What is CTPA (Computed Tomography Pulmonary Angiography)?

What is CTPA (Computed Tomography Pulmonary Angiography)?

CTPA is the gold standard imaging test for diagnosing pulmonary embolism, using timed intravenous contrast to visualize the pulmonary vasculature down to the subsegmental level with excellent accuracy and low rates of inconclusive results. 1

Definition and Technical Aspects

CTPA is a specialized CT scan that:

• Uses timed intravenous contrast administration to coincide with peak enhancement of the pulmonary arteries
• Employs thin-section CT acquisition techniques
• Produces images that are interpreted using transverse reconstructions, multiplanar reformations, and 3D renderings 1
• Allows adequate visualization of the pulmonary vasculature down to the subsegmental level 1

Diagnostic Performance

CTPA offers superior diagnostic capabilities compared to many other imaging modalities:

• Excellent accuracy with sensitivity of 83% and specificity of 96% for pulmonary embolism 1
• Strong validation in prospective management outcome studies 1
• Low rate of inconclusive results (3-5%) 1
• High negative predictive value (96% in patients with low clinical probability, 89% in intermediate probability) 1

Clinical Applications

CTPA has become the first-line imaging modality for suspected pulmonary embolism for several reasons:

• Readily available around the clock in most centers 1
• Short acquisition time, making it suitable for unstable patients 1
• Provides alternative diagnoses when pulmonary embolism is excluded 1
• Can assess right ventricular function, which has prognostic value 1
• Can identify signs of pulmonary hypertension and chronic thromboembolic disease 1

Advantages Over Other Imaging Modalities

CTPA has largely replaced conventional pulmonary angiography and offers several advantages over V/Q scanning:

• Higher specificity than ventilation-perfusion isotope scanning 1
• Better interobserver agreement, even with relatively inexperienced assessors 1
• Reliable in patients with co-existing cardiorespiratory disease 1
• Allows quantitative assessment that correlates with clinical severity 1

Limitations and Considerations

Despite its advantages, CTPA has some limitations:

• Radiation exposure (effective dose 3-10 mSv), with significant exposure to young female breast tissue 1
• Requires iodinated contrast, limiting use in patients with iodine allergy or hyperthyroidism 1
• May be less sensitive for subsegmental pulmonary emboli, though most patients also have more proximal clot that can be reliably identified 1
• Tendency toward overuse due to easy accessibility 1

Role in Diagnostic Algorithms

CTPA has a well-established place in diagnostic algorithms:

• Recommended as the initial lung imaging modality for non-massive pulmonary embolism 1
• Patients with a good quality negative CTPA do not require further investigation or treatment for pulmonary embolism 1
• Should be used after clinical probability assessment and D-dimer testing in appropriate patients 2

Technical Considerations

For optimal results, CTPA requires:

• Meticulous attention to technique, particularly in the timing of contrast administration 1
• Thin section collimation and proper viewing at workstations 1
• Familiarity with potential pitfalls in interpretation 1

CTPA has revolutionized the diagnosis of pulmonary embolism, providing rapid, accurate results that guide clinical management and improve patient outcomes.