## CT Angiography Signs of Pulmonary Embolism **Key Point:** The most specific sign of acute PE on CT pulmonary angiography (CTPA) is a **filling defect** — an intraluminal thrombus surrounded by contrast material within the pulmonary artery lumen. This represents direct visualization of the clot. ### Central vs Peripheral PE | Feature | Central PE | Peripheral PE | |---------|-----------|---------------| | **Location** | Main, lobar, segmental arteries | Subsegmental arteries | | **Visibility** | Easily seen on standard CTPA | May require thin-slice reconstruction | | **Specificity** | Very high (>95%) | Lower; prone to motion artifact | | **Clinical significance** | Hemodynamically significant | Variable; many are clinically silent | **High-Yield:** The **contrast-surrounded filling defect** is the gold standard sign because it directly shows the thrombus. Other signs (RV enlargement, wedge infarcts) are secondary findings. ### Secondary Signs of PE - **Hampton's hump:** Peripheral wedge-shaped consolidation (infarction) — occurs in ~10% of PE cases, not specific for PE alone - **RV dilatation:** RV/LV ratio >0.9 suggests hemodynamic strain but is not diagnostic of PE - **Mosaic perfusion:** Patchy areas of hypoattenuation due to regional hypoperfusion — nonspecific **Clinical Pearl:** A negative CTPA with a filling defect sign rules out PE with >95% confidence. However, subsegmental PE detection remains controversial — isolated subsegmental PE is often managed conservatively unless there is clinical deterioration. [cite:Harrison 21e Ch 297] 
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