A 52-year-old woman presents to the emergency department with acute onset dyspnea and right-sided pleuritic chest pain. She had undergone total abdominal hysterectomy 5 days ago. Vital signs: BP 118/76 mmHg, HR 102/min, RR 22/min, SpO₂ 94% on room air. D-dimer is elevated at 2.8 μg/mL. CTPA (CT pulmonary angiography) is performed. The radiologist reports a filling defect in the right lower lobe pulmonary artery with failure of opacification of the distal branches. What is the most specific CT finding for acute pulmonary embolism?

Explanation

## CT Pulmonary Angiography (CTPA) Findings in Acute PE ### Most Specific Finding: Central Filling Defect **Key Point:** A filling defect within the pulmonary artery lumen with failure of opacification of distal branches is the most specific and direct CT sign of acute pulmonary embolism on CTPA. ### Classification of CTPA Signs | Finding | Specificity | Timing | Description | |---------|-------------|--------|-------------| | **Central filling defect** | Highest | Acute | Thrombus within PA lumen; non-opacification of distal vessels | | **Mosaic perfusion** | Moderate | Acute/chronic | Patchy areas of oligemia due to reduced perfusion | | **Wedge-shaped opacity** | Lower | Subacute (days) | Hampton's hump; represents infarction, not PE itself | | **Pulmonary infarction** | Variable | Subacute | Peripheral wedge-shaped consolidation with pleural base | **High-Yield:** The filling defect is the **direct visualization of thrombus** and is pathognomonic for PE. It appears as: - A low-attenuation clot surrounded by contrast-enhanced blood - Partial or complete obstruction of the PA lumen - Failure of distal branch opacification ### Why Other Findings Are Less Specific **Mosaic Perfusion:** Reflects regional perfusion mismatch but can occur in other conditions (chronic thromboembolic disease, emphysema, bronchiectasis). **Hampton's Hump:** A wedge-shaped peripheral opacity with rounded apex—this is a sign of **pulmonary infarction** (tissue necrosis), which occurs days after PE in only ~10% of cases. It is a secondary finding, not the PE itself. **Clinical Pearl:** CTPA has become the gold standard for PE diagnosis because it directly visualizes the thrombus in the pulmonary arterial tree, achieving sensitivity and specificity both >95% for central and lobar PE. **Tip:** When reading CTPA, look for the thrombus *within the vessel lumen*, not downstream complications. The presence of a filling defect with non-opacification of distal branches is diagnostic; you do not need to wait for infarction to confirm PE.