CTA PE Protocol: What It Is and Where to Find It
CTPA (CT Pulmonary Angiography) is a specialized CT protocol that requires three essential elements: (1) precise timing to capture peak pulmonary arterial enhancement, (2) multiplanar reconstructions, and (3) 3D renderings—this is fundamentally different from a standard "CT chest with contrast" which is inadequate for PE diagnosis. 1
Critical Technical Specifications
The CTPA protocol differs from other CTA protocols in several key ways:
- Timing and injection parameters: High injection rates with scan durations under 1 second are required to achieve homogenous opacification of pulmonary arteries specifically optimized for detecting acute PE 1
- Voltage settings: Low-kilovoltage scanning (80-100 kV) is the standard to maximize contrast enhancement while reducing radiation dose by up to 63.6% 1, 2
- Anatomic coverage: Focused exclusively on the chest with thin-section acquisition (typically 1mm collimation) to visualize pulmonary vessels down to subsegmental branches 1
- Field of view: Specifically tailored for pulmonary vascular assessment with reconstructions optimized for vessel visualization 1
Where to Find the Protocol Quickly
In your hospital's radiology ordering system, you must specifically order "CTPA" or "CT Pulmonary Angiography"—never order "CT chest with contrast" as this will not be protocoled correctly and is inadequate for PE diagnosis. 1
- The protocol is pre-programmed in your institution's CT scanner under "CTPA" or "PE protocol" 1
- Contact your radiology department directly if you cannot locate the specific order—they maintain standardized protocols based on ACR and ESC guidelines 3, 1
- Most electronic ordering systems have CTPA as a distinct option separate from routine chest CT 1
Common Ordering Pitfalls to Avoid
- Do not order "CT chest with contrast": This lacks the precise timing, reconstructions, and 3D rendering required for PE diagnosis and will result in an inadequate study 1
- Ensure adequate contrast delivery: Poor opacification leads to false-negative studies—the protocol depends on patient weight, cardiac output, and scan duration 1
- Specify clinical indication: Write "rule out PE" or "suspected pulmonary embolism" to ensure radiology applies the correct protocol 3
Special Populations
- Pregnant patients: Modified protocol scanning from diaphragm to top of aorta (not full chest) reduces radiation by 70% while maintaining diagnostic accuracy, using 80-100 kV and fixed mAs around 80-100 1
- Patients with recent ACS: Consider triple rule-out CT protocol with ECG-gating to simultaneously evaluate coronary arteries, thoracic aorta, and pulmonary arteries 4
Diagnostic Performance
- CTPA has 96-99% sensitivity and specificity for PE, making it the definitive diagnostic test 1, 5
- Modern multidetector CTPA detects emboli down to 2-3 mm in subsegmental arteries 5
- Clinical outcome studies show it is safe to withhold anticoagulation when PE is excluded on CTPA, with subsequent PE occurring in only 1.1% at 3 months 4
Beyond Diagnosis: Prognostic Information
The CTPA provides critical prognostic markers that guide treatment decisions:
- RV/LV ratio: Predicts short-term mortality and need for ICU-level care 4, 5
- Pulmonary artery diameter and septal deviation: Indicators of right ventricular strain that may warrant thrombolytic therapy or mechanical thrombectomy 3, 5
- Alternative diagnoses: Frequently reveals other causes of symptoms such as pulmonary edema, pneumonia, pericarditis, or aortic dissection 3, 5