A 58-year-old woman with acute dyspnea undergoes CT pulmonary angiography (CTPA). Images show a filling defect in the right lower lobe pulmonary artery with normal perfusion distal to the clot. Which imaging finding best distinguishes acute pulmonary embolism from chronic thromboembolic disease?

Explanation

## Distinguishing Acute from Chronic PE on CTPA ### Key Imaging Features **Key Point:** The acute angle sign (thrombus making an acute angle with the vessel wall) is the hallmark of acute PE and distinguishes it from chronic thromboembolic disease, which shows obtuse angles and recanalization. | Feature | Acute PE | Chronic PE | | --- | --- | --- | | **Clot-vessel angle** | Acute (< 90°) | Obtuse (> 90°) | | **Clot location** | Eccentric, mobile | Eccentric, adherent | | **Distal perfusion** | Normal (abrupt cutoff) | Abnormal (mosaic pattern) | | **Recanalization** | Absent | Present (channels through clot) | | **Vessel caliber** | Normal or dilated | Tapered, stenotic | | **Bronchial collaterals** | Absent | Prominent | ### Mechanism of Differentiation 1. **Acute PE**: Fresh thrombus is loosely adherent and makes an acute angle with the vessel wall; distal perfusion remains normal because the occlusion is recent. 2. **Chronic PE**: Organized thrombus becomes incorporated into the vessel wall (obtuse angle), develops recanalization channels, and causes downstream perfusion defects (mosaic perfusion) due to chronic hypoperfusion and collateral development. **High-Yield:** The **acute angle sign** is the single best discriminator between acute and chronic PE on CTPA. It reflects the geometry of fresh, non-adherent thrombus. **Clinical Pearl:** Patients with chronic thromboembolic pulmonary hypertension (CTEPH) show the mosaic perfusion pattern — alternating areas of normal and decreased attenuation reflecting areas of preserved and compromised perfusion. [cite:Harrison 21e Ch 297]