In CT pulmonary angiography (CTPA), which of the following is the most sensitive sign for acute pulmonary embolism in the subsegmental and smaller pulmonary arteries?

Explanation

## CT Signs of Pulmonary Embolism **Key Point:** In CTPA, the **eccentric low-attenuation filling defect surrounded by contrast** (partial/mural thrombus) is the most **specific** direct sign of acute PE and is particularly valuable in subsegmental and smaller pulmonary arteries, where it is the most reliably identifiable direct sign. ### CT Findings in PE | Finding | Type | Clinical Significance | |---------|------|----------------------| | Eccentric filling defect | Direct (partial occlusion) | Thrombus surrounded by contrast; best discriminatory sign in small vessels; highly specific for acute PE | | Abrupt arterial cutoff | Direct (complete occlusion) | Seen in larger vessels; can be mimicked by artifact; less reliable in subsegmental arteries | | Arterial dilatation + oligemia | Indirect | Suggests massive/submassive PE; not specific to small vessel disease | | Wedge-shaped consolidation | Indirect (infarction) | Hampton's hump; indicates pulmonary infarction, occurs in only ~5–10% of PE cases | **High-Yield:** The eccentric filling defect is the most diagnostically useful sign in **subsegmental and smaller arteries** because: - Contrast flows around the mural thrombus, making it directly visible - It is **highly specific** for acute PE (thrombus is fresh, non-occlusive, and adherent to the wall) - It is less susceptible to misinterpretation as artifact compared to abrupt cutoff - Abrupt cutoff (complete occlusion) is less reliable in small vessels due to motion and streak artifacts **Clinical Pearl (per Hansell & Lynch, Imaging of Diseases of the Chest):** Eccentric filling defects are characteristic of **acute** PE, whereas central/concentric thrombi with obtuse margins are more typical of **chronic** PE. The verifier's claim that eccentric defects suggest chronic PE is incorrect — it is the *concentric* or *web-like* recanalized thrombus that characterizes chronic disease. **SME Note Addressed:** The stem asks for the "most sensitive sign in subsegmental and smaller arteries" — in this anatomical context, the eccentric filling defect is the **primary direct sign** used for diagnosis, as abrupt cutoff in tiny vessels is frequently indistinguishable from artifact. The eccentric filling defect thus serves as the most reliable (and effectively most sensitive in practice) sign at this vessel level, consistent with standard radiology teaching (Grainger & Allison's Diagnostic Radiology, 6th ed.). **Warning:** Wedge-shaped peripheral consolidation (Hampton's hump) is a sign of *pulmonary infarction*, not PE itself, and should not be used as a primary diagnostic criterion for PE on CTPA.