Introduction
The CT Pulmonary Angiogram (CTPA) is one of the most frequently requested urgent studies, and correctly interpreting it for Pulmonary Embolism (PE) is non-negotiable. A missed PE is a clinical disaster; overcalling a PE is a therapeutic mistake.
To ensure accuracy and speed, you need a systematic approach. This guide provides the CTPA Interpretation Checklist, a reliable, step-by-step protocol for radiology residents that covers the diagnosis of PE, assessment of severity, and identification of technical pitfalls.
The 4-Point CTPA Interpretation Checklist
Always follow this sequence to avoid anchoring bias and ensure all critical findings are reported.
1. Confirm Technical Adequacy (The Image Quality Check)
Before looking for a clot, check your image quality. The most common pitfall is inadequate contrast timing.
- Vessel Opacification: The pulmonary arteries, from the main pulmonary artery down to the segmental level, should be densely opacified.
- Contrast Density: Look for contrast layering or streaky/suboptimal opacification. If the main PA is not bright, your ability to exclude PE is severely limited.
- Motion Artifact: Significant respiratory motion can mimic or obscure small emboli.
2. Systematic Search for Pulmonary Embolism (PE)
Search from the center to the periphery, paying attention to the most common locations.
- Central PE: Look first in the main pulmonary artery (MPA) and the proximal right and left pulmonary arteries (RPA/LPA). A saddle embolus straddles the MPA bifurcation—a true emergency.
- Segmental/Subsegmental: Systematically trace all segmental and subsegmental arteries. PE typically presents as:
- Polo-Mint Sign (Cross-section): A central filling defect surrounded by contrast.
- Railroad Track Sign (Long-axis): Contrast flow around a parallel-oriented filling defect.
- Chronic vs. Acute:
- Acute: Centrally located, free-floating, forming an acute angle with the vessel wall.
- Chronic: Peripheral, eccentric, retracting from the vessel wall, and forming obtuse angles.
3. Assess PE Severity (RV Strain)
The presence of a clot is only half the report; determining its hemodynamic significance is the most critical step for risk stratification (Massive, Submassive, or Non-massive PE). This is assessed by looking for Right Ventricular (RV) Strain.
- RV/LV Diameter Ratio: Measure the maximum short-axis diameter of the RV and LV at the level of the papillary muscles or above. An RV/LV ratio >0.9 (ideally >1.0) indicates RV dilation and acute strain.
- IVC Reflux: Look for contrast reflux into the inferior vena cava (IVC) or hepatic veins.
- Septal Flattening: Look for flattening or bowing of the interventricular septum toward the left ventricle.
4. Evaluate Extrapulmonary Findings (The Alternative Diagnosis)
Always scroll through the entire study to look for the cause of the PE or an alternative cause for the patient’s symptoms (chest pain/shortness of breath).
- Parenchyma: Look for consolidation (pneumonia), pleural effusion, or ground-glass opacities (CHF).
- Aorta/Mediastinum: Rule out Aortic Dissection.
- Deep Vein Thrombosis (DVT): Check the abdomen and pelvis for a potential source (e.g., iliac vein thrombus).
Common CTPA Pitfalls to Avoid
- Artifact: Beam-hardening artifact from the superior vena cava (SVC) catheter or streak artifact can mimic PE. Look for a true endoluminal defect.
- Flow Motion: Pulsatile flow in the MPA can sometimes appear as a faint filling defect. Always confirm the defect is in multiple planes and persistent.
- Lymph Nodes: Hilar or mediastinal lymph nodes should not be mistaken for subsegmental PE.
🚨 On-Call Action Point: If you diagnose a PE with signs of acute RV strain (RV/LV ratio >1.0), the patient is at higher risk. Communicate this to the referring service immediately and recommend escalation of care, possibly including IR for catheter-directed thrombolysis.
Conclusion
Mastering the CTPA is a cornerstone of being a competent radiology resident. By consistently applying this CTPA Interpretation Checklist, you’ll increase your speed, boost your diagnostic accuracy, and provide the crucial clinical context needed for excellent patient care.
