## CT Signs of Acute Pulmonary Embolism **Key Point:** The filling defect (or thrombus sign) is the most specific CT finding for acute PE on CTPA — a low-attenuation clot surrounded by high-attenuation contrast within the pulmonary artery lumen. ### Classification of CTPA Findings | Finding | Specificity | Sensitivity | Comments | |---------|-------------|-------------|----------| | **Filling defect** | Very high (95–100%) | Moderate–High | Direct visualization of thrombus; gold standard | | Mosaic perfusion | Low | Moderate | Non-specific; seen in PE, COPD, asthma, ILD | | Hampton's hump | Low | Low | Wedge-shaped opacity; indicates infarction (rare) | | Enlarged PA (>29 mm) | Low | Moderate | Suggests chronic/massive PE; not specific | | Pulmonary artery sign | Moderate | Moderate | Abrupt vessel cutoff distal to clot | | Saddle embolus | High | Low | Clot straddling main PA bifurcation | **High-Yield:** The filling defect is the **direct sign** of PE — it visualizes the thrombus itself. All other signs are **indirect** and lack specificity. ### Why Filling Defect is Most Specific 1. **Direct visualization** of thrombus material within the arterial lumen 2. **Contrast pooling** around the clot creates the characteristic "halo" or "ring sign" 3. **Reproducible** across different observers and scanner protocols 4. **Pathognomonic** when present — no other condition mimics it **Clinical Pearl:** Central (main/lobar PA) filling defects carry higher clinical significance than subsegmental defects, which have lower positive predictive value and may represent artifacts or non-thromboembolic material. **Tip:** On exam, always distinguish between **direct signs** (filling defect, pulmonary artery sign) and **indirect signs** (mosaic perfusion, Hampton's hump, cardiomegaly) — examiners test this distinction frequently. 
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