## CT Findings in Pulmonary Embolism ### Direct Signs of PE on CTPA **Key Point:** A filling defect within the pulmonary artery lumen surrounded by contrast material is the MOST specific direct sign of acute PE on CTPA. This finding represents the thrombus itself and is diagnostic. ### Classification of CT Findings | Finding | Specificity | Interpretation | |---------|-------------|----------------| | **Filling defect in PA** | Very High | Direct sign; diagnostic of PE | | **Central/eccentric defect** | Very High | Thrombus surrounded by contrast | | **Wedge-shaped opacity** | Low-Moderate | Infarction (indirect sign); occurs in ~10% of PE | | **Atelectasis/effusion** | Low | Non-specific; many etiologies | | **Ground-glass opacity** | Low | Hemorrhage or infarction; indirect | ### Direct vs. Indirect Signs **High-Yield:** CTPA distinguishes between: - **Direct signs** (diagnostic): Intraluminal filling defect, arterial cutoff - **Indirect signs** (suggestive but non-specific): Wedge-shaped infarction, pleural effusion, atelectasis, oligemia (Westermark sign) ### Clinical Correlation This patient has classic DVT/PE risk factors (recent long flight, acute dyspnea, elevated D-dimer). The filling defect sign on CTPA is pathognomonic for PE and would definitively establish the diagnosis, allowing immediate anticoagulation. **Clinical Pearl:** Wedge-shaped opacities (Hampton hump) represent pulmonary infarction and occur only when there is concurrent right heart failure or pre-existing cardiopulmonary disease; they are present in only ~10% of PE cases and are therefore NOT the most specific finding. [cite:Harrison 21e Ch 298] 
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