A 52-year-old woman with acute dyspnoea and chest pain undergoes CTPA for suspected pulmonary embolism. The radiologist identifies a filling defect in the right lower lobe pulmonary artery. Which CT finding best distinguishes acute PE from chronic thromboembolic disease?

Explanation

## Distinguishing Acute from Chronic PE on CTPA ### The Key Discriminator: Thrombus Morphology **Key Point:** The stem asks which CT finding **best distinguishes acute PE from chronic thromboembolic disease**. The answer is the finding most characteristic of **acute** PE — eccentric, low-attenuation thrombus forming **acute angles** at the vessel wall (Option A). | Feature | Acute PE | Chronic PE | |---------|----------|------------| | **Thrombus shape** | Eccentric, irregular | Concentric, smooth, adherent | | **Vessel wall angles** | **Acute angles** (hallmark) | Obtuse angles, tapering | | **Attenuation** | Low (20–40 HU), homogeneous | Calcified (>100 HU) or mixed | | **Recanalization** | Absent | Present (highly specific for chronic) | | **Vessel diameter** | Normal or acutely dilated | Chronically dilated, pruned | | **Associated findings** | Wedge infarcts, pleural effusion | Mosaic perfusion, bronchial dilatation | ### Why Option A is Correct **High-Yield:** Acute thrombi are composed of fresh fibrin and red blood cells, making them **low attenuation (20–40 HU)** on CT. They sit eccentrically within the vessel lumen and form **acute angles** where they contact the vessel wall — this is the classic CTPA appearance of acute PE (Harrison's Principles of Internal Medicine, 21e, Ch. 298). ### Why Option B is Incorrect as the "Best Distinguisher" Option B describes chronic thromboembolic disease (concentric, calcified thrombus with recanalization). While these are valid features of chronic PE, the question asks which finding **best distinguishes** acute from chronic PE. The eccentric low-attenuation thrombus with acute wall angles is the **primary discriminating feature on CTPA** for acute PE, and is the finding the radiologist would use first to make this distinction. **Clinical Pearl:** Recanalization channels within a thrombus are pathognomonic for chronic PE, but calcification alone is not — older acute thrombi can occasionally calcify. The acute-angle eccentric morphology is the most reliable acute PE discriminator. ### Options C and D - **Option C** (peripheral wedge consolidation + pleural effusion) = pulmonary infarction, a complication of acute PE, not a primary discriminator. - **Option D** (mosaic perfusion + bronchial dilatation) = secondary findings more pronounced in chronic PE due to vascular remodeling, but not the primary discriminating feature. **Tip:** Eccentric low-attenuation thrombus with acute vessel-wall angles → Acute PE. Concentric calcified thrombus with recanalization → Chronic PE. [cite:Harrison 21e Ch 298; Hansell et al., Imaging of Diseases of the Chest, 5e]