CT Pulmonary angiogram (CTPA)

Related Subjects: |Assessing Breathlessness |Pulmonary Embolism |Deep Vein Thrombosis |DVT/PE in pregnancy |CTPA

📖 Introduction

• CT pulmonary angiography (CTPA) is the first-line imaging test in most adults when pulmonary embolism (PE) is suspected and imaging is required after clinical risk stratification.
• The usual diagnostic pathway is: clinical probability assessment (for example Wells score) → D-dimer if low/intermediate probability → CTPA if D-dimer positive or clinical probability is high.
• CTPA uses a rapid intravenous bolus of iodinated contrast timed to opacify the pulmonary arterial tree.
• An acute PE appears as a low-attenuation intraluminal filling defect within a contrast-opacified pulmonary artery.
• CTPA can also reveal alternative diagnoses such as pneumonia, pleural effusion, pneumothorax, malignancy, or aortic pathology.

🖥️ What CTPA Shows

• After contrast injection, the normal pulmonary arteries should appear bright and uniformly opacified.
• An embolus is seen as a darker filling defect within the vessel lumen because contrast cannot occupy the space taken up by thrombus.
• Acute thrombus may be:
  • Central within the lumen, surrounded by contrast — producing the classic “polo mint” sign on axial views or “railway track” sign on longitudinal views.
  • Eccentric and wall-adherent, partially narrowing the lumen.
  • Occlusive, causing abrupt vessel cut-off.
• A large clot may straddle the bifurcation of the main pulmonary artery, forming a saddle embolus.
• More distal emboli may be seen in lobar, segmental, or subsegmental branches.
• Acute PE usually forms acute angles with the vessel wall, whereas chronic thrombus is more often eccentric, forms obtuse angles, and may be associated with webs, bands, or vessel narrowing.

📈 Why CTPA Matters Beyond Diagnosis

• CTPA not only confirms or excludes PE, but also provides important risk-stratification information.
• Features suggesting right ventricular (RV) strain include:
  • RV enlargement, often with an RV:LV ratio >1
  • Interventricular septal flattening or bowing towards the left ventricle
  • Reflux of contrast into the IVC or hepatic veins
  • Main pulmonary artery dilatation
• These signs indicate acute pressure overload and are associated with a worse short-term prognosis.
• CTPA may also show complications such as pulmonary infarction, pleural effusion, or atelectatic change.

🧭 Systematic Approach: How to Read a CTPA

1. ✅ Check technical quality first: are the pulmonary arteries bright and well-opacified? If contrast timing is poor, the study may be non-diagnostic.
2. ✅ Start centrally: inspect the main pulmonary artery, then the right and left pulmonary arteries.
3. ✅ Track distally: follow each vessel into the lobar → segmental → subsegmental branches.
4. ✅ Look for filling defects: sharply defined darker areas within contrast suggest thrombus.
   • Central clot surrounded by contrast = polo mint sign
   • Linear clot seen on sagittal/coronal reformats = railway track sign
   • Eccentric wall-adherent defect = partial obstruction
5. ✅ Assess the heart: look for RV enlargement, septal bowing, and contrast reflux into the IVC/hepatic veins.
6. ✅ Review the lung windows: look for wedge-shaped pleural-based infarcts, consolidation, collapse, masses, or emphysema.
7. ✅ Review mediastinum and pleura: do not miss pleural effusions, lymphadenopathy, pneumothorax, or aortic pathology.
8. ✅ Do a final top-to-bottom sweep: scroll from apices to bases to reduce the risk of missing peripheral emboli.

🫁 Pulmonary Artery Anatomy

• The main pulmonary artery arises from the right ventricle and bifurcates into:
  • Right pulmonary artery (RPA) — passes more horizontally towards the right lung
  • Left pulmonary artery (LPA) — arches posteriorly towards the left lung
• These vessels then divide into lobar, segmental, and subsegmental branches.
• A normal CTPA shows smooth, uninterrupted contrast opacification throughout the pulmonary arterial tree.

🩺 Common Secondary Findings

• Pulmonary infarction — often a peripheral, wedge-shaped pleural-based opacity.
• Pleural effusion — usually small and reactive.
• Atelectasis — common but non-specific.
• Alternative pathology — pneumonia, malignancy, pneumothorax, interstitial lung disease, or aortic disease.

⚠️ Contraindications / Limitations

• 🤰 Pregnancy: choice of imaging depends on gestation, chest X-ray findings, and local protocol; V/Q scanning may be preferred in some cases.
• 💧 Renal impairment: iodinated contrast may not be suitable in significant renal dysfunction; check renal function and weigh risk versus benefit.
• ⚠️ Contrast allergy: severe previous contrast reactions may preclude CTPA; alternatives include V/Q scanning.
• 💓 Haemodynamic instability: some patients are too unstable to leave the resuscitation area; bedside echo and emergency management may take priority.
• 🌫️ Suboptimal scan quality: motion artefact, poor breath-holding, and poor contrast bolus timing can limit interpretation.

📌 Practical Pitfalls

• ⏱️ Poor opacification: inadequate contrast timing can create a falsely reassuring scan or obscure small emboli.
• 💨 Respiratory motion artefact: may mimic filling defects, especially centrally.
• ❤️ Cardiac motion: can degrade assessment of vessels near the heart.
• 📍 Subsegmental PE: small peripheral emboli can be difficult to distinguish from artefact and must be interpreted in clinical context.
• 🧩 Non-thrombotic mimics: tumour emboli, flow artefact, mucus plugging adjacent to vessels, and beam-hardening artefact may confuse interpretation.
• 🔍 Satisfaction error: once one embolus is found, additional emboli and signs of RV strain may be overlooked.

💡 Teaching Pearls

1️⃣ Always assess scan quality before looking for PE.
2️⃣ Use a central → lobar → segmental → subsegmental search pattern every time.
3️⃣ A PE is a filling defect in a contrast-filled artery.
4️⃣ Do not stop at “PE present” — always assess for RV strain, as this changes risk.
5️⃣ Always review lung windows, pleura, and mediastinum for infarction or alternative diagnoses.
6️⃣ A negative CTPA does not overrule the clinical picture if the scan is poor quality or suspicion remains very high.

🎓 Reporting Language to Recognise

• “Central filling defect” = thrombus seen within the opacified artery.
• “Saddle embolus” = clot across the main pulmonary artery bifurcation.
• “RV strain” = imaging evidence of acute right heart pressure overload.
• “Peripheral wedge-shaped opacity” = likely pulmonary infarction.
• “No convincing PE, but limited by motion / poor opacification” = technically suboptimal scan; interpret with caution.

🎥 Online Learning

• Part 1: How to Read a CTPA
• Part 2: How to Read a CTPA

🩻 CTPA Images